高效的AWK编程第四版


TEXT PROCESSING Effective awk Programming ISBN: 978-1-491-90461-9 US $44.99 CAN $51.99 “Arnold has distilled over two and a half decades of experience writing and using awk programs, and developing gawk, into this book. If you use awk or want to learn how, then read this book.”—Michael Brennan author of mawk Twitter: @oreillymedia facebook.com/oreilly When processing text files, the awk language is ideal for handling data extraction, reporting, and data-reformatting jobs. This practical guide serves as both a reference and tutorial for POSIX-standard awk and for the GNU implementation, called gawk. This book is useful for novices and awk experts alike. In this thoroughly revised edition, author and gawk lead developer Arnold Robbins describes the awk language and gawk program in detail, shows you how to use awk and gawk for problem solving, and then dives into specific features of gawk. System administrators, programmers, webmasters, and other power users will find everything they need to know about awk and gawk. You will learn how to: ■■ Format text and use regular expressions in awk and gawk ■■ Process data using awk's operators and built-in functions ■■ Manage data relationships using associative arrays ■■ Define your own functions ■■ “Think in awk” with two full chapters of sample functions and programs ■■ Take advantage of gawk's many advanced features ■■ Debug awk programs with the gawk built-in debugger This book is published under the terms of the GNU Free Documentation License. You have the freedom to copy and modify this GNU manual. Royalties from the sales of this book go to the Free Software Foundation and to the author. Arnold Robbins, a professional programmer and technical author, has worked with Unix systems since 1980 and has used awk since 1987. Arnold is the maintainer of gawk and its documentation. As a member of the POSIX 1003.2 balloting group, he helped shape the POSIX standard for awk. Arnold Robbins 4th Edition A GNU Manual Effective awkProgramming UNIVERSAL TEXT PROCESSING AND PATTERN MATCHING Effective awk Programming FOURTH EDITION Robbins TEXT PROCESSING Effective awk Programming ISBN: 978-1-491-90461-9 US $44.99 CAN $51.99 “Arnold has distilled over two and a half decades of experience writing and using awk programs, and developing gawk, into this book. If you use awk or want to learn how, then read this book.”—Michael Brennan author of mawk Twitter: @oreillymedia facebook.com/oreilly When processing text files, the awk language is ideal for handling data extraction, reporting, and data-reformatting jobs. This practical guide serves as both a reference and tutorial for POSIX-standard awk and for the GNU implementation, called gawk. This book is useful for novices and awk experts alike. In this thoroughly revised edition, author and gawk lead developer Arnold Robbins describes the awk language and gawk program in detail, shows you how to use awk and gawk for problem solving, and then dives into specific features of gawk. System administrators, programmers, webmasters, and other power users will find everything they need to know about awk and gawk. You will learn how to: ■■ Format text and use regular expressions in awk and gawk ■■ Process data using awk's operators and built-in functions ■■ Manage data relationships using associative arrays ■■ Define your own functions ■■ “Think in awk” with two full chapters of sample functions and programs ■■ Take advantage of gawk's many advanced features ■■ Debug awk programs with the gawk built-in debugger This book is published under the terms of the GNU Free Documentation License. You have the freedom to copy and modify this GNU manual. Royalties from the sales of this book go to the Free Software Foundation and to the author. Arnold Robbins, a professional programmer and technical author, has worked with Unix systems since 1980 and has used awk since 1987. Arnold is the maintainer of gawk and its documentation. As a member of the POSIX 1003.2 balloting group, he helped shape the POSIX standard for awk. Arnold Robbins 4th Edition A GNU Manual Effective awkProgramming UNIVERSAL TEXT PROCESSING AND PATTERN MATCHING Effective awk Programming FOURTH EDITION Robbins Arnold Robbins FOURTH EDITION Effective awk Programming Effective awk Programming, Fourth Edition by Arnold Robbins Copyright © 1989, 1991, 1992, 1993, 1996–2005, 2007, 2009–2015 Free Software Foundation, Inc. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://safaribooksonline.com). 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Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While the publisher and the author have used good faith efforts to ensure that the information and instruc‐ tions contained in this work are accurate, the publisher and the author disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any code samples or other technology this work contains or describes is subject to open source licenses or the intel‐ lectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights. ISBN: 978-1-491-90461-9 [LSI] To my parents, for their love, and for the wonderful example they set for me. To my wife Miriam, for making me complete. Thank you for building your life together with me. To our children Chana, Rivka, Nachum, and Malka, for enrichening our lives in innumerable ways. Table of Contents Foreword to the Third Edition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv Foreword to the Fourth Edition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix Part I. The awk Language 1. Getting Started with awk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 How to Run awk Programs 3 One-Shot Throwaway awk Programs 4 Running awk Without Input Files 4 Running Long Programs 5 Executable awk Programs 6 Comments in awk Programs 7 Shell Quoting Issues 8 Datafiles for the Examples 11 Some Simple Examples 12 An Example with Two Rules 14 A More Complex Example 15 awk Statements Versus Lines 16 Other Features of awk 18 When to Use awk 18 Summary 19 2. Running awk and gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Invoking awk 21 Command-Line Options 22 Other Command-Line Arguments 29 Naming Standard Input 30 The Environment Variables gawk Uses 30 v The AWKPATH Environment Variable 31 The AWKLIBPATH Environment Variable 32 Other Environment Variables 32 gawk’s Exit Status 34 Including Other Files into Your Program 35 Loading Dynamic Extensions into Your Program 36 Obsolete Options and/or Features 37 Undocumented Options and Features 37 Summary 37 3. Regular Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 How to Use Regular Expressions 39 Escape Sequences 40 Regular Expression Operators 43 Using Bracket Expressions 46 How Much Text Matches? 48 Using Dynamic Regexps 49 gawk-Specific Regexp Operators 50 Case Sensitivity in Matching 52 Summary 54 4. Reading Input Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 How Input Is Split into Records 55 Record Splitting with Standard awk 55 Record Splitting with gawk 58 Examining Fields 60 Nonconstant Field Numbers 61 Changing the Contents of a Field 62 Specifying How Fields Are Separated 65 Whitespace Normally Separates Fields 66 Using Regular Expressions to Separate Fields 66 Making Each Character a Separate Field 67 Setting FS from the Command Line 68 Making the Full Line Be a Single Field 69 Field-Splitting Summary 70 Reading Fixed-Width Data 71 Defining Fields by Content 73 Multiple-Line Records 75 Explicit Input with getline 77 Using getline with No Arguments 78 Using getline into a Variable 79 Using getline from a File 80 vi | Table of Contents Using getline into a Variable from a File 80 Using getline from a Pipe 81 Using getline into a Variable from a Pipe 82 Using getline from a Coprocess 83 Using getline into a Variable from a Coprocess 83 Points to Remember About getline 83 Summary of getline Variants 85 Reading Input with a Timeout 85 Directories on the Command Line 87 Summary 87 5. Printing Output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 The print Statement 89 print Statement Examples 90 Output Separators 91 Controlling Numeric Output with print 92 Using printf Statements for Fancier Printing 93 Introduction to the printf Statement 93 Format-Control Letters 94 Modifiers for printf Formats 96 Examples Using printf 99 Redirecting Output of print and printf 100 Special Files for Standard Preopened Data Streams 103 Special Filenames in gawk 104 Accessing Other Open Files with gawk 104 Special Files for Network Communications 104 Special Filename Caveats 105 Closing Input and Output Redirections 105 Summary 108 6. Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Constants, Variables, and Conversions 111 Constant Expressions 111 Using Regular Expression Constants 114 Variables 115 Conversion of Strings and Numbers 117 Operators: Doing Something with Values 119 Arithmetic Operators 119 String Concatenation 121 Assignment Expressions 122 Increment and Decrement Operators 125 Truth Values and Conditions 127 Table of Contents | vii True and False in awk 127 Variable Typing and Comparison Expressions 128 Boolean Expressions 132 Conditional Expressions 134 Function Calls 134 Operator Precedence (How Operators Nest) 136 Where You Are Makes a Difference 138 Summary 139 7. Patterns, Actions, and Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Pattern Elements 141 Regular Expressions as Patterns 142 Expressions as Patterns 142 Specifying Record Ranges with Patterns 143 The BEGIN and END Special Patterns 145 The BEGINFILE and ENDFILE Special Patterns 147 The Empty Pattern 148 Using Shell Variables in Programs 148 Actions 149 Control Statements in Actions 150 The if-else Statement 150 The while Statement 151 The do-while Statement 151 The for Statement 152 The switch Statement 154 The break Statement 155 The continue Statement 156 The next Statement 157 The nextfile Statement 158 The exit Statement 159 Predefined Variables 160 Built-in Variables That Control awk 160 Built-in Variables That Convey Information 163 Using ARGC and ARGV 170 Summary 172 8. Arrays in awk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 The Basics of Arrays 173 Introduction to Arrays 173 Referring to an Array Element 175 Assigning Array Elements 176 Basic Array Example 177 viii | Table of Contents Scanning All Elements of an Array 178 Using Predefined Array Scanning Orders with gawk 179 Using Numbers to Subscript Arrays 182 Using Uninitialized Variables as Subscripts 183 The delete Statement 184 Multidimensional Arrays 185 Scanning Multidimensional Arrays 187 Arrays of Arrays 187 Summary 190 9. Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Built-in Functions 191 Calling Built-in Functions 191 Numeric Functions 192 String-Manipulation Functions 194 Input/Output Functions 208 Time Functions 211 Bit-Manipulation Functions 217 Getting Type Information 219 String-Translation Functions 220 User-Defined Functions 220 Function Definition Syntax 221 Function Definition Examples 223 Calling User-Defined Functions 224 The return Statement 229 Functions and Their Effects on Variable Typing 230 Indirect Function Calls 231 Summary 236 Part II. Problem Solving with awk 10. A Library of awk Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Naming Library Function Global Variables 242 General Programming 243 Converting Strings to Numbers 244 Assertions 245 Rounding Numbers 247 The Cliff Random Number Generator 247 Translating Between Characters and Numbers 248 Merging an Array into a String 250 Managing the Time of Day 250 Table of Contents | ix Reading a Whole File at Once 252 Quoting Strings to Pass to the Shell 253 Datafile Management 254 Noting Datafile Boundaries 254 Rereading the Current File 256 Checking for Readable Datafiles 257 Checking for Zero-Length Files 257 Treating Assignments as Filenames 258 Processing Command-Line Options 259 Reading the User Database 264 Reading the Group Database 269 Traversing Arrays of Arrays 273 Summary 275 11. Practical awk Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Running the Example Programs 277 Reinventing Wheels for Fun and Profit 278 Cutting Out Fields and Columns 278 Searching for Regular Expressions in Files 283 Printing Out User Information 287 Splitting a Large File into Pieces 289 Duplicating Output into Multiple Files 290 Printing Nonduplicated Lines of Text 292 Counting Things 296 A Grab Bag of awk Programs 299 Finding Duplicated Words in a Document 299 An Alarm Clock Program 300 Transliterating Characters 302 Printing Mailing Labels 304 Generating Word-Usage Counts 306 Removing Duplicates from Unsorted Text 308 Extracting Programs from Texinfo Source Files 309 A Simple Stream Editor 312 An Easy Way to Use Library Functions 314 Finding Anagrams from a Dictionary 321 And Now for Something Completely Different 323 Summary 323 x | Table of Contents Part III. Moving Beyond Standard awk with gawk 12. Advanced Features of gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Allowing Nondecimal Input Data 327 Controlling Array Traversal and Array Sorting 328 Controlling Array Traversal 328 Sorting Array Values and Indices with gawk 333 Two-Way Communications with Another Process 334 Using gawk for Network Programming 337 Profiling Your awk Programs 338 Summary 342 13. Internationalization with gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 Internationalization and Localization 345 GNU gettext 346 Internationalizing awk Programs 348 Translating awk Programs 350 Extracting Marked Strings 350 Rearranging printf Arguments 351 awk Portability Issues 352 A Simple Internationalization Example 353 gawk Can Speak Your Language 355 Summary 355 14. Debugging awk Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 Introduction to the gawk Debugger 357 Debugging in General 357 Debugging Concepts 358 awk Debugging 359 Sample gawk Debugging Session 359 How to Start the Debugger 359 Finding the Bug 360 Main Debugger Commands 363 Control of Breakpoints 364 Control of Execution 365 Viewing and Changing Data 367 Working with the Stack 369 Obtaining Information About the Program and the Debugger State 369 Miscellaneous Commands 372 Readline Support 374 Limitations 374 Summary 375 Table of Contents | xi 15. Arithmetic and Arbitrary-Precision Arithmetic with gawk. . . . . . . . . . . . . . . . . . . . . . . 377 A General Description of Computer Arithmetic 377 Other Stuff to Know 379 Arbitrary-Precision Arithmetic Features in gawk 381 Floating-Point Arithmetic: Caveat Emptor! 382 Floating-Point Arithmetic Is Not Exact 382 Getting the Accuracy You Need 384 Try a Few Extra Bits of Precision and Rounding 385 Setting the Precision 385 Setting the Rounding Mode 387 Arbitrary-Precision Integer Arithmetic with gawk 388 Standards Versus Existing Practice 389 Summary 391 16. Writing Extensions for gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Introduction 393 Extension Licensing 394 How It Works at a High Level 394 API Description 396 Introduction 397 General-Purpose Data Types 399 Memory Allocation Functions and Convenience Macros 402 Constructor Functions 403 Registration Functions 404 Printing Messages 413 Updating ERRNO 413 Requesting Values 413 Accessing and Updating Parameters 414 Symbol Table Access 414 Array Manipulation 419 API Variables 428 Boilerplate Code 429 How gawk Finds Extensions 431 Example: Some File Functions 431 Using chdir() and stat() 431 C Code for chdir() and stat() 434 Integrating the Extensions 440 The Sample Extensions in the gawk Distribution 442 File-Related Functions 442 Interface to fnmatch() 445 Interface to fork(), wait(), and waitpid() 446 Enabling In-Place File Editing 447 xii | Table of Contents Character and Numeric values: ord() and chr() 448 Reading Directories 448 Reversing Output 449 Two-Way I/O Example 449 Dumping and Restoring an Array 450 Reading an Entire File 450 Extension Time Functions 451 API Tests 452 The gawkextlib Project 452 Summary 453 Part IV. Appendices A. The Evolution of the awk Language. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 B. Installing gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 C. GNU General Public License. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 Table of Contents | xiii Foreword to the Third Edition Arnold Robbins and I are good friends. We were introduced in 1990 by circumstances —and our favorite programming language, awk. The circumstances started a couple of years earlier. I was working at a new job and noticed an unplugged Unix computer sitting in the corner. No one knew how to use it, and neither did I. However, a couple of days later, it was running, and I was root and the one-and-only user. That day, I began the transition from statistician to Unix programmer. On one of many trips to the library or bookstore in search of books on Unix, I found the gray awk book, a.k.a. Alfred V. Aho, Brian W. Kernighan, and Peter J. Weinberger’s The AWK Programming Language (Addison-Wesley, 1988). awk’s simple programming paradigm—find a pattern in the input and then perform an action—often reduced complex or tedious data manipulations to a few lines of code. I was excited to try my hand at programming in awk. Alas, the awk on my computer was a limited version of the language described in the gray book. I discovered that my computer had “old awk” and the book described “new awk.” I learned that this was typical; the old version refused to step aside or relinquish its name. If a system had a new awk, it was invariably called nawk, and few systems had it. The best way to get a new awk was to ftp the source code for gawk from prep.ai.mit.edu. gawk was a version of new awk written by David Trueman and Arnold, and available under the GNU General Public License. (Incidentally, it’s no longer difficult to find a new awk. gawk ships with GNU/Linux, and you can download binaries or source code for almost any system; my wife uses gawk on her VMS box.) My Unix system started out unplugged from the wall; it certainly was not plugged into a network. So, oblivious to the existence of gawk and the Unix community in general, and desiring a new awk, I wrote my own, called mawk. Before I was finished, I knew xv about gawk, but it was too late to stop, so I eventually posted to a comp.sources news‐ group. A few days after my posting, I got a friendly email from Arnold introducing himself. He suggested we share design and algorithms and attached a draft of the POSIX standard so that I could update mawk to support language extensions added after publication of The AWK Programming Language. Frankly, if our roles had been reversed, I would not have been so open and we probably would have never met. I’m glad we did meet. He is an awk expert’s awk expert and a genuinely nice person. Arnold contributes significant amounts of his expertise and time to the Free Software Foundation. This book is the gawk reference manual, but at its core it is a book about awk program‐ ming that will appeal to a wide audience. It is a definitive reference to the awk language as defined by the 1987 Bell Laboratories release and codified in the 1992 POSIX Utilities standard. On the other hand, the novice awk programmer can study a wealth of practical programs that emphasize the power of awk’s basic idioms: data-driven control flow, pattern matching with regular expressions, and associative arrays. Those looking for something new can try out gawk’s interface to network protocols via special /inet files. The programs in this book make clear that an awk program is typically much smaller and faster to develop than a counterpart written in C. Consequently, there is often a payoff to prototyping an algorithm or design in awk to get it running quickly and expose problems early. Often, the interpreted performance is adequate and the awk prototype becomes the product. The new pgawk (profiling gawk) produces program execution counts. I recently ex‐ perimented with an algorithm that for n lines of input exhibited ∼ Cn2 performance, while theory predicted ∼ Cn log n behavior. A few minutes poring over the awk prof.out profile pinpointed the problem to a single line of code. pgawk is a welcome addition to my programmer’s toolbox. Arnold has distilled over a decade of experience writing and using awk programs, and developing gawk, into this book. If you use awk or want to learn how, then read this book. —Michael Brennan Author of mawk March 2001 xvi | Foreword to the Third Edition Foreword to the Fourth Edition Some things don’t change. Thirteen years ago I wrote: “If you use awk or want to learn how, then read this book.” True then, and still true today. Learning to use a programming language is about more than mastering the syntax. One needs to acquire an understanding of how to use the features of the language to solve practical programming problems. A focus of this book is many examples that show how to use awk. Some things do change. Our computers are much faster and have more memory. Con‐ sequently, speed and storage inefficiencies of a high-level language matter less. Proto‐ typing in awk and then rewriting in C for performance reasons happens less, because more often the prototype is fast enough. Of course, there are computing operations that are best done in C or C++. With gawk 4.1 and later, you do not have to choose between writing your program in awk or in C/ C++. You can write most of your program in awk and the aspects that require C/C++ capabilities can be written in C/C++, and then the pieces glued together when the gawk module loads the C/C++ module as a dynamic plug-in. Chapter 16 has all the details, and, as expected, many examples to help you learn the ins and outs. I enjoy programming in awk and had fun (re)reading this book. I think you will, too. —Michael Brennan Author of mawk October 2014 xvii 1. The 2008 POSIX standard is accessible online. Preface Several kinds of tasks occur repeatedly when working with text files. You might want to extract certain lines and discard the rest. Or you may need to make changes wherever certain patterns appear, but leave the rest of the file alone. Such jobs are often easy with awk. The awk utility interprets a special-purpose programming language that makes it easy to handle simple data-reformatting jobs. The GNU implementation of awk is called gawk; if you invoke it with the proper options or environment variables, it is fully compatible with the POSIX1 specification of the awk language and with the Unix version of awk maintained by Brian Kernighan. This means that all properly written awk programs should work with gawk. So most of the time, we don’t distinguish between gawk and other awk implementations. Using awk you can: • Manage small, personal databases • Generate reports • Validate data • Produce indexes and perform other document-preparation tasks • Experiment with algorithms that you can adapt later to other computer languages In addition, gawk provides facilities that make it easy to: • Extract bits and pieces of data for processing • Sort data • Perform simple network communications xix 2. These utilities are available on POSIX-compliant systems, as well as on traditional Unix-based systems. If you are using some other operating system, you still need to be familiar with the ideas of I/O redirection and pipes. 3. Some other, obsolete systems to which gawk was once ported are no longer supported and the code for those systems has been removed. • Profile and debug awk programs • Extend the language with functions written in C or C++ This book teaches you about the awk language and how you can use it effectively. You should already be familiar with basic system commands, such as cat and ls,2 as well as basic shell facilities, such as input/output (I/O) redirection and pipes. Implementations of the awk language are available for many different computing envi‐ ronments. This book, while describing the awk language in general, also describes the particular implementation of awk called gawk (which stands for “GNU awk”). gawk runs on a broad range of Unix systems, ranging from Intel-architecture PC-based computers up through large-scale systems. gawk has also been ported to Mac OS X, Microsoft Windows (all versions), and OpenVMS.3 History of awk and gawk Recipe for a Programming Language 1 part egrep 1 part snobol 2 parts ed 3 parts C Blend all parts well using lex and yacc. Document minimally and release. After eight years, add another part egrep and two more parts C. Document very well and release. The name awk comes from the initials of its designers: Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version of awk was written in 1977 at AT&T Bell Laboratories. In 1985, a new version made the programming language more powerful, introducing user-defined functions, multiple input streams, and computed regular ex‐ pressions. This new version became widely available with Unix System V Release 3.1 (1987). The version in System V Release 4 (1989) added some new features and cleaned up the behavior in some of the “dark corners” of the language. The specification for awk in the POSIX Command Language and Utilities standard further clarified the language. xx | Preface 4. Only Solaris systems still use an old awk for the default awk utility. A more modern awk lives in /usr/xpg6/ bin on these systems. Both the gawk designers and the original awk designers at Bell Laboratories provided feedback for the POSIX specification. Paul Rubin wrote gawk in 1986. Jay Fenlason completed it, with advice from Richard Stallman. John Woods contributed parts of the code as well. In 1988 and 1989, David Trueman, with help from me, thoroughly reworked gawk for compatibility with the newer awk. Circa 1994, I became the primary maintainer. Current development focuses on bug fixes, performance improvements, standards compliance, and, occasionally, new features. In May 1997, Jürgen Kahrs felt the need for network access from awk, and with a little help from me, set about adding features to do this for gawk. At that time, he also wrote the bulk of TCP/IP Internetworking with gawk (a separate document, available as part of the gawk distribution). His code finally became part of the main gawk distribution with gawk version 3.1. John Haque rewrote the gawk internals, in the process providing an awk-level debugger. This version became available as gawk version 4.0 in 2011. See “Major Contributors to gawk” on page 465 for a full list of those who have made important contributions to gawk. A Rose by Any Other Name The awk language has evolved over the years. Full details are provided in Appendix A. The language described in this book is often referred to as “new awk.” By analogy, the original version of awk is referred to as “old awk.” On most current systems, when you run the awk utility you get some version of new awk.4 If your system’s standard awk is the old one, you will see something like this if you try the test program: $ awk 1 /dev/null error→ awk: syntax error near line 1 error→ awk: bailing out near line 1 In this case, you should find a version of new awk, or just install gawk! Throughout this book, whenever we refer to a language feature that should be available in any complete implementation of POSIX awk, we simply use the term awk. When referring to a feature that is specific to the GNU implementation, we use the term gawk. Preface | xxi Using This Book The term awk refers to a particular program as well as to the language you use to tell this program what to do. When we need to be careful, we call the language “the awk language,” and the program “the awk utility.” This book explains both how to write programs in the awk language and how to run the awk utility. The term “awk program” refers to a program written by you in the awk programming language. Primarily, this book explains the features of awk as defined in the POSIX standard. It does so in the context of the gawk implementation. While doing so, it also attempts to describe important differences between gawk and other awk implementations. Finally, it notes any gawk features that are not in the POSIX standard for awk. This book has the difficult task of being both a tutorial and a reference. If you are a novice, feel free to skip over details that seem too complex. You should also ignore the many cross-references; they are for the expert user and for the online Info and HTML versions of the book. There are sidebars scattered throughout the book. They add a more complete explan‐ ation of points that are relevant, but not likely to be of interest on first reading. Most of the time, the examples use complete awk programs. Some of the more advanced sections show only the part of the awk program that illustrates the concept being described. Although this book is aimed principally at people who have not been exposed to awk, there is a lot of information here that even the awk expert should find useful. In partic‐ ular, the description of POSIX awk and the example programs in Chapter 10 and Chap‐ ter 11 should be of interest. This book is split into several parts, as follows: • Part I, The awk Language, describes the awk language and the gawk program in detail. It starts with the basics, and continues through all of the features of awk. It contains the following chapters: — Chapter 1, Getting Started with awk, provides the essentials you need to know to begin using awk. — Chapter 2, Running awk and gawk, describes how to run gawk, the meaning of its command-line options, and how it finds awk program source files. — Chapter 3, Regular Expressions, introduces regular expressions in general, and in particular the flavors supported by POSIX awk and gawk. xxii | Preface — Chapter 4, Reading Input Files, describes how awk reads your data. It introduces the concepts of records and fields, as well as the getline command. I/O redi‐ rection is first described here. Network I/O is also briefly introduced here. — Chapter 5, Printing Output, describes how awk programs can produce output with print and printf. — Chapter 6, Expressions, describes expressions, which are the basic building blocks for getting most things done in a program. — Chapter 7, Patterns, Actions, and Variables, describes how to write patterns for matching records, actions for doing something when a record is matched, and the predefined variables awk and gawk use. — Chapter 8, Arrays in awk, covers awk’s one and only data structure: the associative array. Deleting array elements and whole arrays is described, as well as sorting arrays in gawk. The chapter also describes how gawk provides arrays of arrays. — Chapter 9, Functions, describes the built-in functions awk and gawk provide, as well as how to define your own functions. It also discusses how gawk lets you call functions indirectly. • Part II, Problem Solving with awk, shows how to use awk and gawk for problem solving. There is lots of code here for you to read and learn from. This part contains the following chapters: — Chapter 10, A Library of awk Functions, provides a number of functions meant to be used from main awk programs. — Chapter 11, Practical awk Programs, provides many sample awk programs. Reading these two chapters allows you to see awk solving real problems. • Part III, Moving Beyond Standard awk with gawk, focuses on features specific to gawk. It contains the following chapters: — Chapter 12, Advanced Features of gawk, describes a number of advanced fea‐ tures. Of particular note are the abilities to control the order of array traversal, have two-way communications with another process, perform TCP/IP net‐ working, and profile your awk programs. — Chapter 13, Internationalization with gawk, describes special features for trans‐ lating program messages into different languages at runtime. — Chapter 14, Debugging awk Programs, describes the gawk debugger. — Chapter 15, Arithmetic and Arbitrary-Precision Arithmetic with gawk, describes advanced arithmetic facilities. — Chapter 16, Writing Extensions for gawk, describes how to add new variables and functions to gawk by writing extensions in C or C++. Preface | xxiii • Part IV, Appendices, provides the following appendices, including the GNU General Public License: — Appendix A, The Evolution of the awk Language, describes how the awk language has evolved since its first release to the present. It also describes how gawk has acquired features over time. — Appendix B, Installing gawk, describes how to get gawk, how to compile it on POSIX-compatible systems, and how to compile and use it on different non- POSIX systems. It also describes how to report bugs in gawk and where to get other freely available awk implementations. — Appendix C, GNU General Public License, presents the license that covers the gawk source code. The version of this book distributed with gawk contains additional appendices and other end material. To save space, we have omitted them from the printed edition. You may find them online, as follows: • The appendix on implementation notes describes how to disable gawk’s extensions, how to contribute new code to gawk, where to find information on some possible future directions for gawk development, and the design decisions behind the ex‐ tension API. • The appendix on basic concepts provides some very cursory background material for those who are completely unfamiliar with computer programming. • The glossary defines most, if not all, of the significant terms used throughout the book. If you find terms that you aren’t familiar with, try looking them up here. • The GNU FDL is the license that covers this book. Some of the chapters have exercise sections; these have also been omitted from the print edition but are available online. Typographical Conventions This book is written in Texinfo, the GNU documentation formatting language. A single Texinfo source file is used to produce both the printed and online versions of the doc‐ umentation. Because of this, the typographical conventions are slightly different than in other books you may have read. Examples you would type at the command line are preceded by the common shell primary and secondary prompts, ‘$’ and ‘>’. Input that you type is shown like this. Output from the command, usually its standard output, appears like this. Error messages and other output on the command’s standard error are preceded by the glyph “error→”. For example: xxiv | Preface $ echo hi on stdout hi on stdout $ echo hello on stderr 1>&2 error→ hello on stderr In the text, almost anything related to programming, such as command names, variable and function names, and string, numeric and regexp constants appear in this font. Code fragments appear in the same font and quoted, ‘like this’. Things that are re‐ placed by the user or programmer appear in this font. Options look like this: -f. Filenames are indicated like this: /path/to/ourfile. The first occurrence of a new term is usually its definition and appears in the same font as the previous occurrence of “definition” in this sentence. Characters that you type at the keyboard look like this. In particular, there are special characters called “control characters.” These are characters that you type by holding down both the CONTROL key and another key, at the same time. For example, a Ctrl-d is typed by first pressing and holding the CONTROL key, next pressing the d key, and finally releasing both keys. For the sake of brevity, throughout this book, we refer to Brian Kernighan’s version of awk as “BWK awk.” (See “Other Freely Available awk Implementations” on page 485 for information on his and other versions.) Notes of interest look like this. Cautionary or warning notes look like this. Dark Corners Dark corners are basically fractal—no matter how much you illuminate, there’s always a smaller but darker one. —Brian Kernighan Until the POSIX standard (and Effective awk Programming), many features of awk were either poorly documented or not documented at all. Descriptions of such features (often called “dark corners”) are noted in this book with “(d.c.).” Preface | xxv 5. GNU stands for “GNU’s Not Unix.” But, as noted by the opening quote, any coverage of dark corners is by definition incomplete. Extensions to the standard awk language that are supported by more than one awk implementation are marked “(c.e.)” for “common extension.” The GNU Project and This Book The Free Software Foundation (FSF) is a nonprofit organization dedicated to the pro‐ duction and distribution of freely distributable software. It was founded by Richard M. Stallman, the author of the original Emacs editor. GNU Emacs is the most widely used version of Emacs today. The GNU5 Project is an ongoing effort on the part of the Free Software Foundation to create a complete, freely distributable, POSIX-compliant computing environment. The FSF uses the GNU General Public License (GPL) to ensure that its software’s source code is always available to the end user. The GPL applies to the C language source code for gawk. To find out more about the FSF and the GNU Project online, see the GNU Project’s home page. This book may also be read from GNU’s website. The book you are reading is actually free—at least, the information in it is free to anyone. The machine-readable source code for the book comes with gawk. The book itself has gone through multiple previous editions. Paul Rubin wrote the very first draft of The GAWK Manual; it was around 40 pages long. Diane Close and Richard Stallman improved it, yielding a version that was around 90 pages and barely described the original, “old” version of awk. I started working with that version in the fall of 1988. As work on it progressed, the FSF published several preliminary versions (numbered 0.x). In 1996, edition 1.0 was released with gawk 3.0.0. The FSF published the first two editions under the title The GNU Awk User’s Guide. SSC published two editions of the book under the title Effective awk Pro‐ gramming, and O’Reilly published the third edition in 2001. This edition maintains the basic structure of the previous editions. For FSF edition 4.0, the content was thoroughly reviewed and updated. All references to gawk versions prior to 4.0 were removed. Of significant note for that edition was the addition of Chapter 14. For FSF edition 4.1 (the fourth edition as published by O’Reilly), the content has been reorganized into parts, and the major new additions are Chapter 15 and Chapter 16. xxvi | Preface This book will undoubtedly continue to evolve. If you find an error in the book, please report it! See “Reporting Problems and Bugs” on page 484 for information on submitting problem reports electronically. How to Stay Current You may have a newer version of gawk than the one described here. To find out what has changed, you should first look at the NEWS file in the gawk distribution, which pro‐ vides a high-level summary of the changes in each release. You can then look at the online version of this book to read about any new features. Using Code Examples This book is here to help you get your job done. Most of the example programs in this book come in the gawk distribution and are marked in the files as being in the public domain. So, in general, you may use the code in this book in your programs and doc‐ umentation. Incorporating a significant amount of prose or example code from this book into your product’s documentation requires compliance with the GNU FDL. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Effective awk Programming, Fourth Edition, by Arnold Robbins (O’Reilly). Copyright 2015 Free Software Foundation, 978-1-491-90461-9.” If you feel your use of code examples falls outside fair use or the permission given here, feel free to contact us at permissions@oreilly.com. Safari® Books Online Safari Books Online (www.safaribooksonline.com) is an on-demand digital library that delivers expert content in both book and video form from the world’s leading authors in technology and business. Technology professionals, software developers, web designers, and business and crea‐ tive professionals use Safari Books Online as their primary resource for research, prob‐ lem solving, learning, and certification training. Safari Books Online offers a range of product mixes and pricing programs for organi‐ zations, government agencies, and individuals. Subscribers have access to thousands of books, training videos, and prepublication manuscripts in one fully searchable database from publishers like O’Reilly Media, Prentice Hall Professional, Addison-Wesley Pro‐ fessional, Microsoft Press, Sams, Que, Peachpit Press, Focal Press, Cisco Press, John Preface | xxvii Wiley & Sons, Syngress, Morgan Kaufmann, IBM Redbooks, Packt, Adobe Press, FT Press, Apress, Manning, New Riders, McGraw-Hill, Jones & Bartlett, Course Technol‐ ogy, and dozens more. For more information about Safari Books Online, please visit us online. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, examples, and any additional information. You can access this page at http://bit.ly/effective-awk-programming-4e. To comment or ask technical questions about this book, send email to bookques tions@oreilly.com. For more information about our books, courses, conferences, and news, see our website at http://www.oreilly.com. Find us on Facebook: http://facebook.com/oreilly Follow us on Twitter: http://twitter.com/oreillymedia Watch us on YouTube: http://www.youtube.com/oreillymedia Acknowledgments The initial draft of The GAWK Manual had the following acknowledgments: Many people need to be thanked for their assistance in producing this manual. Jay Fen‐ lason contributed many ideas and sample programs. Richard Mlynarik and Robert Chas‐ sell gave helpful comments on drafts of this manual. The paper A Supplemental Document for awk by John W. Pierce of the Chemistry Department at UC San Diego, pinpointed several issues relevant both to awk implementation and to this manual, that would other‐ wise have escaped us. I would like to acknowledge Richard M. Stallman, for his vision of a better world and for his courage in founding the FSF and starting the GNU Project. The previous edition of this book had the following acknowledgments: xxviii | Preface The following people (in alphabetical order) provided helpful comments on various ver‐ sions of this book: Rick Adams, Dr. Nelson H.F. Beebe, Karl Berry, Dr. Michael Brennan, Rich Burridge, Claire Cloutier, Diane Close, Scott Deifik, Christopher (“Topher”) Eliot, Jeffrey Friedl, Dr. Darrel Hankerson, Michal Jaegermann, Dr. Richard J. LeBlanc, Michael Lijewski, Pat Rankin, Miriam Robbins, Mary Sheehan, and Chuck Toporek. Robert J. Chassell provided much valuable advice on the use of Texinfo. He also deserves special thanks for convincing me not to title this book How to Gawk Politely. Karl Berry helped significantly with the TeX part of Texinfo. I would like to thank Marshall and Elaine Hartholz of Seattle and Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet vacation time in their homes, which al‐ lowed me to make significant progress on this book and on gawk itself. Phil Hughes of SSC contributed in a very important way by loaning me his laptop GNU/ Linux system, not once, but twice, which allowed me to do a lot of work while away from home. David Trueman deserves special credit; he has done a yeoman job of evolving gawk so that it performs well and without bugs. Although he is no longer involved with gawk, working with him on this project was a significant pleasure. The intrepid members of the GNITS mailing list, and most notably Ulrich Drepper, provided invaluable help and feedback for the design of the internationalization features. Chuck Toporek, Mary Sheehan, and Claire Cloutier of O’Reilly & Associates contributed significant editorial help for this book for the 3.1 release of gawk. Dr. Nelson Beebe, Andreas Buening, Dr. Manuel Collado, Antonio Colombo, Stephen Davies, Scott Deifik, Akim Demaille, Darrel Hankerson, Michal Jaegermann, Jürgen Kahrs, Stepan Kasal, John Malmberg, Dave Pitts, Chet Ramey, Pat Rankin, Andrew Schorr, Corinna Vinschen, and Eli Zaretskii (in alphabetical order) make up the current gawk “crack portability team.” Without their hard work and help, gawk would not be nearly the robust, portable program it is today. It has been and continues to be a pleasure working with this team of fine people. Notable code and documentation contributions were made by a number of people. See “Major Contributors to gawk” on page 465 for the full list. Thanks to Andy Oram of O’Reilly Media for initiating the fourth edition and for his support during the work. Thanks to Jasmine Kwityn for her copyediting work. Thanks to Michael Brennan for the Forewords. Thanks to Patrice Dumas for the new makeinfo program. Thanks to Karl Berry, who continues to work to keep the Texinfo markup language sane. Robert P.J. Day, Michael Brennan, and Brian Kernighan kindly acted as reviewers for the 2015 edition of this book. Their feedback helped improve the final work. I would also like to thank Brian Kernighan for his invaluable assistance during the testing and debugging of gawk, and for his ongoing help and advice in clarifying nu‐ Preface | xxix merous points about the language. We could not have done nearly as good a job on either gawk or its documentation without his help. Brian is in a class by himself as a programmer and technical author. I have to thank him (yet again) for his ongoing friendship and for being a role model to me for close to 30 years! Having him as a reviewer is an exciting privilege. It has also been extremely humbling... I must thank my wonderful wife, Miriam, for her patience through the many versions of this project, for her proofreading, and for sharing me with the computer. I would like to thank my parents for their love, and for the grace with which they raised and educated me. Finally, I also must acknowledge my gratitude to G-d, for the many opportunities He has sent my way, as well as for the gifts He has given me with which to take advantage of those opportunities. —Arnold Robbins Nof Ayalon Israel February, 2015 xxx | Preface PART I The awk Language Part I describes the awk language and gawk program in detail. It starts with the basics, and continues through all of the features of awk. Included also are many, but not all, of the features of gawk. This part contains the following chapters: • Chapter 1, Getting Started with awk • Chapter 2, Running awk and gawk • Chapter 3, Regular Expressions • Chapter 4, Reading Input Files • Chapter 5, Printing Output • Chapter 6, Expressions • Chapter 7, Patterns, Actions, and Variables • Chapter 8, Arrays in awk • Chapter 9, Functions CHAPTER 1 Getting Started with awk The basic function of awk is to search files for lines (or other units of text) that contain certain patterns. When a line matches one of the patterns, awk performs specified actions on that line. awk continues to process input lines in this way until it reaches the end of the input files. Programs in awk are different from programs in most other languages, because awk programs are data driven (i.e., you describe the data you want to work with and then what to do when you find it). Most other languages are procedural; you have to describe, in great detail, every step the program should take. When working with procedural languages, it is usually much harder to clearly describe the data your program will pro‐ cess. For this reason, awk programs are often refreshingly easy to read and write. When you run awk, you specify an awk program that tells awk what to do. The program consists of a series of rules (it may also contain function definitions, an advanced feature that we will ignore for now; see “User-Defined Functions” on page 220). Each rule specifies one pattern to search for and one action to perform upon finding the pattern. Syntactically, a rule consists of a pattern followed by an action. The action is enclosed in braces to separate it from the pattern. Newlines usually separate rules. Therefore, an awk program looks like this: pattern { action } pattern { action } … How to Run awk Programs There are several ways to run an awk program. If the program is short, it is easiest to include it in the command that runs awk, like this: awk 'program' input-file1 input-file2 … 3 When the program is long, it is usually more convenient to put it in a file and run it with a command like this: awk -f program-file input-file1 input-file2 … This section discusses both mechanisms, along with several variations of each. One-Shot Throwaway awk Programs Once you are familiar with awk, you will often type in simple programs the moment you want to use them. Then you can write the program as the first argument of the awk command, like this: awk 'program' input-file1 input-file2 … where program consists of a series of patterns and actions, as described earlier. This command format instructs the shell, or command interpreter, to start awk and use the program to process records in the input file(s). There are single quotes around program so the shell won’t interpret any awk characters as special shell characters. The quotes also cause the shell to treat all of program as a single argument for awk, and allow program to be more than one line long. This format is also useful for running short or medium-sized awk programs from shell scripts, because it avoids the need for a separate file for the awk program. A self- contained shell script is more reliable because there are no other files to misplace. Later in this chapter, in the section “Some Simple Examples” on page 12, we’ll see examples of several short, self-contained programs. Running awk Without Input Files You can also run awk without any input files. If you type the following command line: awk 'program' awk applies the program to the standard input, which usually means whatever you type on the keyboard. This continues until you indicate end-of-file by typing Ctrl-d. (On non-POSIX operating systems, the end-of-file character may be different.) As an example, the following program prints a friendly piece of advice (from Douglas Adams’s The Hitchhiker’s Guide to the Galaxy), to keep you from worrying about the complexities of computer programming: $ awk 'BEGIN { print "Don\47t Panic!" }' Don't Panic! awk executes statements associated with BEGIN before reading any input. If there are no other statements in your program, as is the case here, awk just stops, instead of 4 | Chapter 1: Getting Started with awk trying to read input it doesn’t know how to process. The ‘\47’ is a magic way (explained later) of getting a single quote into the program, without having to engage in ugly shell quoting tricks. If you use Bash as your shell, you should execute the command ‘set +H’ before running this program interactively, to disable the C shell- style command history, which treats ‘!’ as a special character. We recommend putting this command into your personal startup file. This next simple awk program emulates the cat utility; it copies whatever you type on the keyboard to its standard output (why this works is explained shortly): $ awk '{ print }' Now is the time for all good men Now is the time for all good men to come to the aid of their country. to come to the aid of their country. Four score and seven years ago, ... Four score and seven years ago, ... What, me worry? What, me worry? Ctrl-d Running Long Programs Sometimes awk programs are very long. In these cases, it is more convenient to put the program into a separate file. In order to tell awk to use that file for its program, you type: awk -f source-file input-file1 input-file2 … The -f instructs the awk utility to get the awk program from the file source-file (see “Command-Line Options” on page 22). Any filename can be used for source-file. For example, you could put the program: BEGIN { print "Don't Panic!" } into the file advice. Then this command: awk -f advice does the same thing as this one: awk 'BEGIN { print "Don\47t Panic!" }' This was explained earlier (see “Running awk Without Input Files” on page 4). Note that you don’t usually need single quotes around the filename that you specify with -f, because most filenames don’t contain any of the shell’s special characters. Notice that in advice, the awk program did not have single quotes around it. The quotes are only needed for programs that are provided on the awk command line. (Also, placing the How to Run awk Programs | 5 1. The ‘#!’ mechanism works on GNU/Linux systems, BSD-based systems, and commercial Unix systems. program in a file allows us to use a literal single quote in the program text, instead of the magic ‘\47’.) If you want to clearly identify an awk program file as such, you can add the exten‐ sion .awk to the filename. This doesn’t affect the execution of the awk program, but it does make “housekeeping” easier. Executable awk Programs Once you have learned awk, you may want to write self-contained awk scripts, using the ‘#!’ script mechanism. You can do this on many systems.1 For example, you could update the file advice to look like this: #! /bin/awk -f BEGIN { print "Don't Panic!" } After making this file executable (with the chmod utility), simply type ‘advice’ at the shell and the system arranges to run awk as if you had typed ‘awk -f advice’: $ chmod +x advice $ advice Don't Panic! (We assume you have the current directory in your shell’s search path variable [typically $PATH]. If not, you may need to type ‘./advice’ at the shell.) Self-contained awk scripts are useful when you want to write a program that users can invoke without their having to know that the program is written in awk. Understanding ‘#!’ awk is an interpreted language. This means that the awk utility reads your program and then processes your data according to the instructions in your program. (This is different from a compiled language such as C, where your program is first compiled into machine code that is executed directly by your system’s processor.) The awk utility is thus termed an interpreter. Many modern languages are interpreted. The line beginning with ‘#!’ lists the full filename of an interpreter to run and a single optional initial command-line argument to pass to that interpreter. The operating sys‐ tem then runs the interpreter with the given argument and the full argument list of the executed program. The first argument in the list is the full filename of the awk program. The rest of the argument list contains either options to awk, or datafiles, or both. (Note that on many systems awk may be found in /usr/bin instead of in /bin.) 6 | Chapter 1: Getting Started with awk Some systems limit the length of the interpreter name to 32 characters. Often, this can be dealt with by using a symbolic link. You should not put more than one argument on the ‘#!’ line after the path to awk. It does not work. The operating system treats the rest of the line as a single argument and passes it to awk. Doing this leads to confusing behavior—most likely a usage diagnostic of some sort from awk. Finally, the value of ARGV[0] (see “Predefined Variables” on page 160) varies depending upon your operating system. Some systems put ‘awk’ there, some put the full pathname of awk (such as /bin/awk), and some put the name of your script (‘advice’). (d.c.) Don’t rely on the value of ARGV[0] to provide your script name. Comments in awk Programs A comment is some text that is included in a program for the sake of human readers; it is not really an executable part of the program. Comments can explain what the program does and how it works. Nearly all programming languages have provisions for com‐ ments, as programs are typically hard to understand without them. In the awk language, a comment starts with the number sign character (‘#’) and continues to the end of the line. The ‘#’ does not have to be the first character on the line. The awk language ignores the rest of a line following a number sign. For example, we could have put the following into advice: # This program prints a nice, friendly message. It helps # keep novice users from being afraid of the computer. BEGIN { print "Don't Panic!" } You can put comment lines into keyboard-composed throwaway awk programs, but this usually isn’t very useful; the purpose of a comment is to help you or another person understand the program when reading it at a later time. How to Run awk Programs | 7 As mentioned in “One-Shot Throwaway awk Programs” on page 4, you can enclose short to medium-sized programs in single quotes, in order to keep your shell scripts self-contained. When doing so, don’t put an apostrophe (i.e., a single quote) into a comment (or anywhere else in your program). The shell interprets the quote as the closing quote for the entire program. As a result, usually the shell prints a message about mismatched quotes, and if awk actually runs, it will probably print strange messages about syntax errors. For ex‐ ample, look at the following: $ awk 'BEGIN { print "hello" } # let's be cute' > The shell sees that the first two quotes match, and that a new quot‐ ed object begins at the end of the command line. It therefore prompts with the secondary prompt, waiting for more input. With Unix awk, closing the quoted string produces this result: $ awk '{ print "hello" } # let's be cute' > ' error→ awk: can't open file be error→ source line number 1 Putting a backslash before the single quote in ‘let's’ wouldn’t help, because backslashes are not special inside single quotes. The next subsection describes the shell’s quoting rules. Shell Quoting Issues For short to medium-length awk programs, it is most convenient to enter the program on the awk command line. This is best done by enclosing the entire program in single quotes. This is true whether you are entering the program interactively at the shell prompt, or writing it as part of a larger shell script: awk 'program text' input-file1 input-file2 … Once you are working with the shell, it is helpful to have a basic knowledge of shell quoting rules. The following rules apply only to POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again Shell). If you use the C shell, you’re on your own. Before diving into the rules, we introduce a concept that appears throughout this book, which is that of the null, or empty, string. The null string is character data that has no value. In other words, it is empty. It is written in awk programs like this: "". In the shell, it can be written using single or double quotes: "" or ''. Although the null string has no characters in it, it does exist. For example, consider this command: $ echo "" 8 | Chapter 1: Getting Started with awk Here, the echo utility receives a single argument, even though that argument has no characters in it. In the rest of this book, we use the terms null string and empty string interchangeably. Now, on to the quoting rules: • Quoted items can be concatenated with nonquoted items as well as with other quoted items. The shell turns everything into one argument for the command. • Preceding any single character with a backslash (‘\’) quotes that character. The shell removes the backslash and passes the quoted character on to the command. • Single quotes protect everything between the opening and closing quotes. The shell does no interpretation of the quoted text, passing it on verbatim to the command. It is impossible to embed a single quote inside single-quoted text. Refer back to “Comments in awk Programs” on page 7 for an example of what happens if you try. • Double quotes protect most things between the opening and closing quotes. The shell does at least variable and command substitution on the quoted text. Different shells may do additional kinds of processing on double-quoted text. Because certain characters within double-quoted text are processed by the shell, they must be escaped within the text. Of note are the characters ‘$’, ‘`’, ‘\’, and ‘"’, all of which must be preceded by a backslash within double-quoted text if they are to be passed on literally to the program. (The leading backslash is stripped first.) Thus, the example seen previously in “Running awk Without Input Files” on page 4: awk 'BEGIN { print "Don\47t Panic!" }' could instead be written this way: $ awk "BEGIN { print \"Don't Panic!\" }" Don't Panic! Note that the single quote is not special within double quotes. • Null strings are removed when they occur as part of a non-null command-line argument, while explicit null objects are kept. For example, to specify that the field separator FS should be set to the null string, use: awk -F "" 'program' files # correct Don’t use this: awk -F"" 'program' files # wrong! In the second case, awk attempts to use the text of the program as the value of FS, and the first filename as the text of the program! This results in syntax errors at best, and confusing behavior at worst. How to Run awk Programs | 9 Mixing single and double quotes is difficult. You have to resort to shell quoting tricks, like this: $ awk 'BEGIN { print "Here is a single quote <'"'"'>" }' Here is a single quote <'> This program consists of three concatenated quoted strings. The first and the third are single-quoted, and the second is double-quoted. This can be “simplified” to: $ awk 'BEGIN { print "Here is a single quote <'\''>" }' Here is a single quote <'> Judge for yourself which of these two is the more readable. Another option is to use double quotes, escaping the embedded, awk-level double quotes: $ awk "BEGIN { print \"Here is a single quote <'>\" }" Here is a single quote <'> This option is also painful, because double quotes, backslashes, and dollar signs are very common in more advanced awk programs. A third option is to use the octal escape sequence equivalents (see “Escape Sequences” on page 40) for the single- and double-quote characters, like so: $ awk 'BEGIN { print "Here is a single quote <\47>" }' Here is a single quote <'> $ awk 'BEGIN { print "Here is a double quote <\42>" }' Here is a double quote <"> This works nicely, but you should comment clearly what the escapes mean. A fourth option is to use command-line variable assignment, like this: $ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }' Here is a single quote <'> (Here, the two string constants and the value of sq are concatenated into a single string that is printed by print.) If you really need both single and double quotes in your awk program, it is probably best to move it into a separate file, where the shell won’t be part of the picture and you can say what you mean. Quoting in MS-Windows batch files Although this book generally only worries about POSIX systems and the POSIX shell, the following issue arises often enough for many users that it is worth addressing. 10 | Chapter 1: Getting Started with awk The “shells” on Microsoft Windows systems use the double-quote character for quoting, and make it difficult or impossible to include an escaped double-quote character in a command-line script. The following example, courtesy of Jeroen Brink, shows how to print all lines in a file surrounded by double quotes: gawk "{ print \"\042\" $0 \"\042\" }" file Datafiles for the Examples Many of the examples in this book take their input from two sample datafiles. The first, mail-list, represents a list of peoples’ names together with their email addresses and information about those people. The second datafile, called inventory-shipped, con‐ tains information about monthly shipments. In both files, each line is considered to be one record. In mail-list, each record contains the name of a person, his/her phone number, his/her email address, and a code for his/her relationship with the author of the list. The columns are aligned using spaces. An ‘A’ in the last column means that the person is an acquaintance. An ‘F’ in the last column means that the person is a friend. An ‘R’ means that the person is a relative: Amelia 555-5553 amelia.zodiacusque@gmail.com F Anthony 555-3412 anthony.asserturo@hotmail.com A Becky 555-7685 becky.algebrarum@gmail.com A Bill 555-1675 bill.drowning@hotmail.com A Broderick 555-0542 broderick.aliquotiens@yahoo.com R Camilla 555-2912 camilla.infusarum@skynet.be R Fabius 555-1234 fabius.undevicesimus@ucb.edu F Julie 555-6699 julie.perscrutabor@skeeve.com F Martin 555-6480 martin.codicibus@hotmail.com A Samuel 555-3430 samuel.lanceolis@shu.edu A Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R The datafile inventory-shipped represents information about shipments during the year. Each record contains the month, the number of green crates shipped, the number of red boxes shipped, the number of orange bags shipped, and the number of blue packages shipped, respectively. There are 16 entries, covering the 12 months of last year and the first four months of the current year. An empty line separates the data for the two years: Jan 13 25 15 115 Feb 15 32 24 226 Mar 15 24 34 228 Apr 31 52 63 420 May 16 34 29 208 Jun 31 42 75 492 Jul 24 34 67 436 Aug 15 34 47 316 Sep 13 55 37 277 Datafiles for the Examples | 11 Oct 29 54 68 525 Nov 20 87 82 577 Dec 17 35 61 401 Jan 21 36 64 620 Feb 26 58 80 652 Mar 24 75 70 495 Apr 21 70 74 514 The sample files are included in the gawk distribution, in the directory awklib/eg/data. Some Simple Examples The following command runs a simple awk program that searches the input file mail- list for the character string ‘li’ (a grouping of characters is usually called a string; the term string is based on similar usage in English, such as “a string of pearls” or “a string of cars in a train”): awk '/li/ { print $0 }' mail-list When lines containing ‘li’ are found, they are printed because ‘print $0’ means print the current line. (Just ‘print’ by itself means the same thing, so we could have written that instead.) You will notice that slashes (‘/’) surround the string ‘li’ in the awk program. The slashes indicate that ‘li’ is the pattern to search for. This type of pattern is called a regular expression, which is covered in more detail later (see Chapter 3). The pattern is allowed to match parts of words. There are single quotes around the awk program so that the shell won’t interpret any of it as special shell characters. Here is what this program prints: $ awk '/li/ { print $0 }' mail-list Amelia 555-5553 amelia.zodiacusque@gmail.com F Broderick 555-0542 broderick.aliquotiens@yahoo.com R Julie 555-6699 julie.perscrutabor@skeeve.com F Samuel 555-3430 samuel.lanceolis@shu.edu A In an awk rule, either the pattern or the action can be omitted, but not both. If the pattern is omitted, then the action is performed for every input line. If the action is omitted, the default action is to print all lines that match the pattern. Thus, we could leave out the action (the print statement and the braces) in the previous example and the result would be the same: awk prints all lines matching the pattern ‘li’. By comparison, omitting the print statement but retaining the braces makes an empty action that does nothing (i.e., no lines are printed). Many practical awk programs are just a line or two long. Following is a collection of useful, short programs to get you started. Some of these programs contain constructs 12 | Chapter 1: Getting Started with awk that haven’t been covered yet. (The description of the program will give you a good idea of what is going on, but you’ll need to read the rest of the book to become an awk expert!) Most of the examples use a datafile named data. This is just a placeholder; if you use these programs yourself, substitute your own filenames for data. For future reference, note that there is often more than one way to do things in awk. At some point, you may want to look back at these examples and see if you can come up with different ways to do the same things shown here: • Print every line that is longer than 80 characters: awk 'length($0) > 80' data The sole rule has a relational expression as its pattern and has no action—so it uses the default action, printing the record. • Print the length of the longest input line: awk '{ if (length($0) > max) max = length($0) } END { print max }' data The code associated with END executes after all input has been read; it’s the other side of the coin to BEGIN. • Print the length of the longest line in data: expand data | awk '{ if (x < length($0)) x = length($0) } END { print "maximum line length is " x }' This example differs slightly from the previous one: the input is processed by the expand utility to change TABs into spaces, so the widths compared are actually the right-margin columns, as opposed to the number of input characters on each line. • Print every line that has at least one field: awk 'NF > 0' data This is an easy way to delete blank lines from a file (or rather, to create a new file similar to the old file but from which the blank lines have been removed). • Print seven random numbers from 0 to 100, inclusive: awk 'BEGIN { for (i = 1; i <= 7; i++) print int(101 * rand()) }' • Print the total number of bytes used by files: ls -l files | awk '{ x += $5 } END { print "total bytes: " x }' • Print the total number of kilobytes used by files: ls -l files | awk '{ x += $5 } END { print "total K-bytes:", x / 1024 }' • Print a sorted list of the login names of all users: Some Simple Examples | 13 awk -F: '{ print $1 }' /etc/passwd | sort • Count the lines in a file: awk 'END { print NR }' data • Print the even-numbered lines in the datafile: awk 'NR % 2 == 0' data If you used the expression ‘NR % 2 == 1’ instead, the program would print the odd- numbered lines. An Example with Two Rules The awk utility reads the input files one line at a time. For each line, awk tries the patterns of each rule. If several patterns match, then several actions execute in the order in which they appear in the awk program. If no patterns match, then no actions run. After processing all the rules that match the line (and perhaps there are none), awk reads the next line. (However, see “The next Statement” on page 157 and “The nextfile State‐ ment” on page 158.) This continues until the program reaches the end of the file. For example, the following awk program contains two rules: /12/ { print $0 } /21/ { print $0 } The first rule has the string ‘12’ as the pattern and ‘print $0’ as the action. The second rule has the string ‘21’ as the pattern and also has ‘print $0’ as the action. Each rule’s action is enclosed in its own pair of braces. This program prints every line that contains the string ‘12’ or the string ‘21’. If a line contains both strings, it is printed twice, once by each rule. This is what happens if we run this program on our two sample datafiles, mail-list and inventory-shipped: $ awk '/12/ { print $0 } > /21/ { print $0 }' mail-list inventory-shipped Anthony 555-3412 anthony.asserturo@hotmail.com A Camilla 555-2912 camilla.infusarum@skynet.be R Fabius 555-1234 fabius.undevicesimus@ucb.edu F Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R Jan 21 36 64 620 Apr 21 70 74 514 Note how the line beginning with ‘Jean-Paul’ in mail-list was printed twice, once for each rule. 14 | Chapter 1: Getting Started with awk A More Complex Example Now that we’ve mastered some simple tasks, let’s look at what typical awk programs do. This example shows how awk can be used to summarize, select, and rearrange the output of another utility. It uses features that haven’t been covered yet, so don’t worry if you don’t understand all the details: ls -l | awk '$6 == "Nov" { sum += $5 } END { print sum }' This command prints the total number of bytes in all the files in the current directory that were last modified in November (of any year). The ‘ls -l’ part of this example is a system command that gives you a listing of the files in a directory, including each file’s size and the date the file was last modified. Its output looks like this: -rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile -rw-r--r-- 1 arnold user 10809 Nov 7 13:03 awk.h -rw-r--r-- 1 arnold user 983 Apr 13 12:14 awk.tab.h -rw-r--r-- 1 arnold user 31869 Jun 15 12:20 awkgram.y -rw-r--r-- 1 arnold user 22414 Nov 7 13:03 awk1.c -rw-r--r-- 1 arnold user 37455 Nov 7 13:03 awk2.c -rw-r--r-- 1 arnold user 27511 Dec 9 13:07 awk3.c -rw-r--r-- 1 arnold user 7989 Nov 7 13:03 awk4.c The first field contains read-write permissions, the second field contains the number of links to the file, and the third field identifies the file’s owner. The fourth field identifies the file’s group. The fifth field contains the file’s size in bytes. The sixth, seventh, and eighth fields contain the month, day, and time, respectively, that the file was last modi‐ fied. Finally, the ninth field contains the filename. The ‘$6 == "Nov"’ in our awk program is an expression that tests whether the sixth field of the output from ‘ls -l’ matches the string ‘Nov’. Each time a line has the string ‘Nov’ for its sixth field, awk performs the action ‘sum += $5’. This adds the fifth field (the file’s size) to the variable sum. As a result, when awk has finished reading all the input lines, sum is the total of the sizes of the files whose lines matched the pattern. (This works because awk variables are automatically initialized to zero.) After the last line of output from ls has been processed, the END rule executes and prints the value of sum. In this example, the value of sum is 80600. These more advanced awk techniques are covered in later sections (see “Actions” on page 149). Before you can move on to more advanced awk programming, you have to know how awk interprets your input and displays your output. By manipulating fields and using print statements, you can produce some very useful and impressive- looking reports. A More Complex Example | 15 2. The ‘?’ and ‘:’ referred to here is the three-operand conditional expression described in “Conditional Ex‐ pressions” on page 134. Splitting lines after ‘?’ and ‘:’ is a minor gawk extension; if --posix is specified (see “Command-Line Options” on page 22), then this extension is disabled. awk Statements Versus Lines Most often, each line in an awk program is a separate statement or separate rule, like this: awk '/12/ { print $0 } /21/ { print $0 }' mail-list inventory-shipped However, gawk ignores newlines after any of the following symbols and keywords: , { ? : || && do else A newline at any other point is considered the end of the statement.2 If you would like to split a single statement into two lines at a point where a newline would terminate it, you can continue it by ending the first line with a backslash character (‘\’). The backslash must be the final character on the line in order to be recognized as a continuation character. A backslash is allowed anywhere in the statement, even in the middle of a string or regular expression. For example: awk '/This regular expression is too long, so continue it\ on the next line/ { print $1 }' We have generally not used backslash continuation in our sample programs. gawk places no limit on the length of a line, so backslash continuation is never strictly necessary; it just makes programs more readable. For this same reason, as well as for clarity, we have kept most statements short in the programs presented throughout the book. Backslash continuation is most useful when your awk program is in a separate source file instead of entered from the command line. You should also note that many awk implementations are more particular about where you may use backslash continuation. For example, they may not allow you to split a string constant using backslash continuation. Thus, for maximum portability of your awk programs, it is best not to split your lines in the middle of a regular expression or a string. 16 | Chapter 1: Getting Started with awk Backslash continuation does not work as described with the C shell. It works for awk programs in files and for one-shot programs, provid‐ ed you are using a POSIX-compliant shell, such as the Unix Bourne shell or Bash. But the C shell behaves differently! There you must use two backslashes in a row, followed by a newline. Note also that when using the C shell, every newline in your awk program must be escaped with a backslash. To illustrate: % awk 'BEGIN { \ ? print \\ ? "hello, world" \ ? }' hello, world Here, the ‘%’ and ‘?’ are the C shell’s primary and secondary prompts, analogous to the standard shell’s ‘$’ and ‘>’. Compare the previous example to how it is done with a POSIX- compliant shell: $ awk 'BEGIN { > print \ > "hello, world" > }' hello, world awk is a line-oriented language. Each rule’s action has to begin on the same line as the pattern. To have the pattern and action on separate lines, you must use backslash con‐ tinuation; there is no other option. Another thing to keep in mind is that backslash continuation and comments do not mix. As soon as awk sees the ‘#’ that starts a comment, it ignores everything on the rest of the line. For example: $ gawk 'BEGIN { print "dont panic" # a friendly \ > BEGIN rule > }' error→ gawk: cmd. line:2: BEGIN rule error→ gawk: cmd. line:2: ^ syntax error In this case, it looks like the backslash would continue the comment onto the next line. However, the backslash-newline combination is never even noticed because it is “hidden” inside the comment. Thus, the BEGIN is noted as a syntax error. When awk statements within one rule are short, you might want to put more than one of them on a line. This is accomplished by separating the statements with a semicolon (‘;’). This also applies to the rules themselves. Thus, the program shown at the start of this section could also be written this way: /12/ { print $0 } ; /21/ { print $0 } awk Statements Versus Lines | 17 The requirement that states that rules on the same line must be sep‐ arated with a semicolon was not in the original awk language; it was added for consistency with the treatment of statements within an action. Other Features of awk The awk language provides a number of predefined, or built-in, variables that your programs can use to get information from awk. There are other variables your program can set as well to control how awk processes your data. In addition, awk provides a number of built-in functions for doing common computa‐ tional and string-related operations. gawk provides built-in functions for working with timestamps, performing bit manipulation, for runtime string translation (internation‐ alization), determining the type of a variable, and array sorting. As we develop our presentation of the awk language, we will introduce most of the variables and many of the functions. They are described systematically in “Predefined Variables” on page 160 and in “Built-in Functions” on page 191. When to Use awk Now that you’ve seen some of what awk can do, you might wonder how awk could be useful for you. By using utility programs, advanced patterns, field separators, arithmetic statements, and other selection criteria, you can produce much more complex output. The awk language is very useful for producing reports from large amounts of raw data, such as summarizing information from the output of other utility programs like ls. (See “A More Complex Example” on page 15.) Programs written with awk are usually much smaller than they would be in other lan‐ guages. This makes awk programs easy to compose and use. Often, awk programs can be quickly composed at your keyboard, used once, and thrown away. Because awk programs are interpreted, you can avoid the (usually lengthy) compilation part of the typical edit-compile-test-debug cycle of software development. Complex programs have been written in awk, including a complete retargetable assem‐ bler for eight-bit microprocessors, and a microcode assembler for a special-purpose Prolog computer. The original awk’s capabilities were strained by tasks of such com‐ plexity, but modern versions are more capable. If you find yourself writing awk scripts of more than, say, a few hundred lines, you might consider using a different programming language. The shell is good at string and pattern 18 | Chapter 1: Getting Started with awk 3. Other popular scripting languages include Ruby and Perl. matching; in addition, it allows powerful use of the system utilities. Python offers a nice balance between high-level ease of programming and access to system facilities.3 Summary • Programs in awk consist of pattern–action pairs. • An action without a pattern always runs. The default action for a pattern without one is ‘{ print $0 }’. • Use either ‘awk 'program' files’ or ‘awk -f program-file files’ to run awk. • You may use the special ‘#!’ header line to create awk programs that are directly executable. • Comments in awk programs start with ‘#’ and continue to the end of the same line. • Be aware of quoting issues when writing awk programs as part of a larger shell script (or MS-Windows batch file). • You may use backslash continuation to continue a source line. Lines are automat‐ ically continued after a comma, open brace, question mark, colon ‘||’, ‘&&’, do, and else. Summary | 19 CHAPTER 2 Running awk and gawk This chapter covers how to run awk, both POSIX-standard and gawk-specific command-line options, and what awk and gawk do with nonoption arguments. It then proceeds to cover how gawk searches for source files, reading standard input along with other files, gawk’s environment variables, gawk’s exit status, using include files, and obsolete and undocumented options and/or features. Many of the options and features described here are discussed in more detail later in the book; feel free to skip over things in this chapter that don’t interest you right now. Invoking awk There are two ways to run awk—with an explicit program or with one or more program files. Here are templates for both of them; items enclosed in [...] in these templates are optional: awk [options] -f progfile [--] file … awk [options] [--] 'program' file … In addition to traditional one-letter POSIX-style options, gawk also supports GNU long options. It is possible to invoke awk with an empty program: awk '' datafile1 datafile2 Doing so makes little sense, though; awk exits silently when given an empty program. (d.c.) If --lint has been specified on the command line, gawk issues a warning that the program is empty. 21 Command-Line Options Options begin with a dash and consist of a single character. GNU-style long options consist of two dashes and a keyword. The keyword can be abbreviated, as long as the abbreviation allows the option to be uniquely identified. If the option takes an argument, either the keyword is immediately followed by an equals sign (‘=’) and the argument’s value, or the keyword and the argument’s value are separated by whitespace. If a par‐ ticular option with a value is given more than once, it is the last value that counts. Each long option for gawk has a corresponding POSIX-style short option. The long and short options are interchangeable in all contexts. The following list describes options mandated by the POSIX standard: -F fs --field-separator fs Set the FS variable to fs (see “Specifying How Fields Are Separated” on page 65). -f source-file --file source-file Read the awk program source from source-file instead of in the first nonoption argument. This option may be given multiple times; the awk program consists of the concatenation of the contents of each specified source-file. -v var=val --assign var=val Set the variable var to the value val before execution of the program begins. Such variable values are available inside the BEGIN rule (see “Other Command-Line Ar‐ guments” on page 29). The -v option can only set one variable, but it can be used more than once, setting another variable each time, like this: ‘awk -v foo=1 -v bar=2 …’. Using -v to set the values of the built-in variables may lead to surprising results. awk will reset the values of those variables as it needs to, possibly ignoring any initial value you may have given. -W gawk-opt Provide an implementation-specific option. This is the POSIX convention for pro‐ viding implementation-specific options. These options also have corresponding GNU-style long options. Note that the long options may be abbreviated, as long as the abbreviations remain unique. The full list of gawk-specific options is provided next. 22 | Chapter 2: Running awk and gawk -- Signal the end of the command-line options. The following arguments are not treated as options even if they begin with ‘-’. This interpretation of -- follows the POSIX argument parsing conventions. This is useful if you have filenames that start with ‘-’, or in shell scripts, if you have filenames that will be specified by the user that could start with ‘-’. It is also useful for passing options on to the awk program; see “Processing Command-Line Op‐ tions” on page 259. The following list describes gawk-specific options: -b --characters-as-bytes Cause gawk to treat all input data as single-byte characters. In addition, all output written with print or printf is treated as single-byte characters. Normally, gawk follows the POSIX standard and attempts to process its input data according to the current locale (see “Where You Are Makes a Difference” on page 138). This can often involve converting multibyte characters into wide characters (internally), and can lead to problems or confusion if the input data does not contain valid multibyte characters. This option is an easy way to tell gawk, “Hands off my data!” -c --traditional Specify compatibility mode, in which the GNU extensions to the awk language are disabled, so that gawk behaves just like BWK awk. See “Extensions in gawk Not in POSIX awk” on page 460, which summarizes the extensions. -C --copyright Print the short version of the General Public License and then exit. -d[file] --dump-variables[=file] Print a sorted list of global variables, their types, and final values to file. If no file is provided, print this list to a file named awkvars.out in the current directory. No space is allowed between the -d and file, if file is supplied. Having a list of all global variables is a good way to look for typographical errors in your programs. You would also use this option if you have a large program with a lot of functions, and you want to be sure that your functions don’t inadvertently use global variables that you meant to be local. (This is a particularly easy mistake to make with simple variable names like i, j, etc.) Command-Line Options | 23 1. For more detail, please see Section 4.4 of RFC 3875. Also see the explanatory note sent to the gawk bug mailing list. -D[file] --debug[=file] Enable debugging of awk programs (see “Introduction to the gawk Debugger” on page 357). By default, the debugger reads) commands interactively from the keyboard (standard input). The optional file argument allows you to specify a file with a list of commands for the debugger to execute noninteractively. No space is allowed between the -D and file, if file is supplied. -e program-text --source program-text Provide program source code in the program-text. This option allows you to mix source code in files with source code that you enter on the command line. This is particularly useful when you have library functions that you want to use from your command-line programs (see “The AWKPATH Environment Variable” on page 31). -E file --exec file Similar to -f, read awk program text from file. There are two differences from -f: • This option terminates option processing; anything else on the command line is passed on directly to the awk program. • Command-line variable assignments of the form ‘var=value’ are disallowed. This option is particularly necessary for World Wide Web CGI applications that pass arguments through the URL; using this option prevents a malicious (or other) user from passing in options, assignments, or awk source code (via -e) to the CGI application.1 This option should be used with ‘#!’ scripts (see “Executable awk Pro‐ grams” on page 6), like so: #! /usr/local/bin/gawk -E awk program here … -g --gen-pot Analyze the source program and generate a GNU gettext portable object template file on standard output for all string constants that have been marked for translation. See Chapter 13 for information about this option. 24 | Chapter 2: Running awk and gawk -h --help Print a “usage” message summarizing the short- and long-style options that gawk accepts and then exit. -i source-file --include source-file Read an awk source library from source-file. This option is completely equivalent to using the @include directive inside your program. It is very similar to the -f option, but there are two important differences. First, when -i is used, the program source is not loaded if it has been previously loaded, whereas with -f, gawk always loads the file. Second, because this option is intended to be used with code libraries, gawk does not recognize such files as constituting main program input. Thus, after processing an -i argument, gawk still expects to find the main source code via the -f option or on the command line. -l ext --load ext Load a dynamic extension named ext. Extensions are stored as system shared li‐ braries. This option searches for the library using the AWKLIBPATH environment variable. The correct library suffix for your platform will be supplied by default, so it need not be specified in the extension name. The extension initialization routine should be named dl_load(). An alternative is to use the @load keyword inside the program to load a shared library. This advanced feature is described in detail in Chapter 16. -L[value] --lint[=value] Warn about constructs that are dubious or nonportable to other awk implementa‐ tions. No space is allowed between the -L and value, if value is supplied. Some warnings are issued when gawk first reads your program. Others are issued at run‐ time, as your program executes. With an optional argument of ‘fatal’, lint warnings become fatal errors. This may be drastic, but its use will certainly encourage the development of cleaner awk programs. With an optional argument of ‘invalid’, only warnings about things that are actually invalid are issued. (This is not fully implemented yet.) Some warnings are only printed once, even if the dubious constructs they warn about occur multiple times in your awk program. Thus, when eliminating problems pointed out by --lint, you should take care to search for all occurrences of each inappropriate construct. As awk programs are usually short, doing so is not burdensome. Command-Line Options | 25 -M --bignum Force arbitrary-precision arithmetic on numbers. This option has no effect if gawk is not compiled to use the GNU MPFR and MP libraries (see Chapter 15). -n --non-decimal-data Enable automatic interpretation of octal and hexadecimal values in input data (see “Allowing Nondecimal Input Data” on page 327). This option can severely break old programs. Use with care. Also note that this option may disappear in a future version of gawk. -N --use-lc-numeric Force the use of the locale’s decimal point character when parsing numeric input data (see “Where You Are Makes a Difference” on page 138). -o[file] --pretty-print[=file] Enable pretty-printing of awk programs. By default, the output program is created in a file named awkprof.out (see “Profiling Your awk Programs” on page 338). The optional file argument allows you to specify a different filename for the output. No space is allowed between the -o and file, if file is supplied. Due to the way gawk has evolved, with this option your pro‐ gram still executes. This will change in the next major release, such that gawk will only pretty-print the program and not run it. -O --optimize Enable some optimizations on the internal representation of the program. At the moment, this includes just simple constant folding. -p[file] --profile[=file] Enable profiling of awk programs (see “Profiling Your awk Programs” on page 338). By default, profiles are created in a file named awkprof.out. The optional file 26 | Chapter 2: Running awk and gawk argument allows you to specify a different filename for the profile file. No space is allowed between the -p and file, if file is supplied. The profile contains execution counts for each statement in the program in the left margin, and function call counts for each function. -P --posix Operate in strict POSIX mode. This disables all gawk extensions (just like --traditional) and disables all extensions not allowed by POSIX. See “Common Extensions Summary” on page 463 for a summary of the extensions in gawk that are disabled by this option. Also, the following additional restrictions apply: • Newlines do not act as whitespace to separate fields when FS is equal to a single space (see “Examining Fields” on page 60). • Newlines are not allowed after ‘?’ or ‘:’ (see “Conditional Expressions” on page 134). • Specifying ‘-Ft’ on the command line does not set the value of FS to be a single TAB character (see “Specifying How Fields Are Separated” on page 65). • The locale’s decimal point character is used for parsing input data (see “Where You Are Makes a Difference” on page 138). If you supply both --traditional and --posix on the command line, --posix takes precedence. gawk issues a warning if both options are supplied. -r --re-interval Allow interval expressions (see “Regular Expression Operators” on page 43) in regexps. This is now gawk’s default behavior. Nevertheless, this option remains (both for backward compatibility and for use in combination with --traditional). -S --sandbox Disable the system() function, input redirections with getline, output redirec‐ tions with print and printf, and dynamic extensions. This is particularly useful when you want to run awk scripts from questionable sources and need to make sure the scripts can’t access your system (other than the specified input datafile). -t --lint-old Warn about constructs that are not available in the original version of awk from Version 7 Unix (see “Major Changes Between V7 and SVR3.1” on page 457). Command-Line Options | 27 -V --version Print version information for this particular copy of gawk. This allows you to de‐ termine if your copy of gawk is up to date with respect to whatever the Free Software Foundation is currently distributing. It is also useful for bug reports (see “Reporting Problems and Bugs” on page 484). As long as program text has been supplied, any other options are flagged as invalid with a warning message but are otherwise ignored. In compatibility mode, as a special case, if the value of fs supplied to the -F option is ‘t’, then FS is set to the TAB character ("\t"). This is true only for --traditional and not for --posix (see “Specifying How Fields Are Separated” on page 65). The -f option may be used more than once on the command line. If it is, awk reads its program source from all of the named files, as if they had been concatenated together into one big file. This is useful for creating libraries of awk functions. These functions can be written once and then retrieved from a standard place, instead of having to be included in each individual program. The -i option is similar in this regard. (As men‐ tioned in “Function Definition Syntax” on page 221, function names must be unique.) With standard awk, library functions can still be used, even if the program is entered at the keyboard, by specifying ‘-f /dev/tty’. After typing your program, type Ctrl-d (the end-of-file character) to terminate it. (You may also use ‘-f -’ to read program source from the standard input, but then you will not be able to also use the standard input as a source of data.) Because it is clumsy using the standard awk mechanisms to mix source file and command-line awk programs, gawk provides the -e option. This does not require you to preempt the standard input for your source code; it allows you to easily mix command-line and library source code (see “The AWKPATH Environment Variable” on page 31). As with -f, the -e and -i options may also be used multiple times on the command line. If no -f or -e option is specified, then gawk uses the first nonoption command-line argument as the text of the program source code. If the environment variable POSIXLY_CORRECT exists, then gawk behaves in strict POSIX mode, exactly as if you had supplied --posix. Many GNU programs look for this en‐ vironment variable to suppress extensions that conflict with POSIX, but gawk behaves differently: it suppresses all extensions, even those that do not conflict with POSIX, and behaves in strict POSIX mode. If --lint is supplied on the command line and gawk turns on POSIX mode because of POSIXLY_CORRECT, then it issues a warning message indicating that POSIX mode is in effect. You would typically set this variable in your 28 | Chapter 2: Running awk and gawk 2. Not recommended. shell’s startup file. For a Bourne-compatible shell (such as Bash), you would add these lines to the .profile file in your home directory: POSIXLY_CORRECT=true export POSIXLY_CORRECT For a C shell-compatible shell,2 you would add this line to the .login file in your home directory: setenv POSIXLY_CORRECT true Having POSIXLY_CORRECT set is not recommended for daily use, but it is good for testing the portability of your programs to other environments. Other Command-Line Arguments Any additional arguments on the command line are normally treated as input files to be processed in the order specified. However, an argument that has the form var=value, assigns the value value to the variable var—it does not specify a file at all. (See “Assigning variables on the command line” on page 116.) In the following example, count=1 is a variable assignment, not a filename: awk -f program.awk file1 count=1 file2 All the command-line arguments are made available to your awk program in the ARGV array (see “Predefined Variables” on page 160). Command-line options and the program text (if present) are omitted from ARGV. All other arguments, including variable assign‐ ments, are included. As each element of ARGV is processed, gawk sets ARGIND to the index in ARGV of the current element. Changing ARGC and ARGV in your awk program lets you control how awk processes the input files; this is described in more detail in “Using ARGC and ARGV” on page 170. The distinction between filename arguments and variable-assignment arguments is made when awk is about to open the next input file. At that point in execution, it checks the filename to see whether it is really a variable assignment; if so, awk sets the variable instead of reading a file. Therefore, the variables actually receive the given values after all previously specified files have been read. In particular, the values of variables assigned in this fashion are not available inside a BEGIN rule (see “The BEGIN and END Special Patterns” on page 145), because such rules are run before awk begins scanning the argument list. The variable values given on the command line are processed for escape sequences (see “Escape Sequences” on page 40). (d.c.) Other Command-Line Arguments | 29 In some very early implementations of awk, when a variable assignment occurred before any filenames, the assignment would happen before the BEGIN rule was executed. awk’s behavior was thus inconsistent; some command-line assignments were available inside the BEGIN rule, while others were not. Unfortunately, some applications came to depend upon this “feature.” When awk was changed to be more consistent, the -v option was added to accommodate applications that depended upon the old behavior. The variable assignment feature is most useful for assigning to variables such as RS, OFS, and ORS, which control input and output formats, before scanning the datafiles. It is also useful for controlling state if multiple passes are needed over a datafile. For example: awk 'pass == 1 { pass 1 stuff } pass == 2 { pass 2 stuff }' pass=1 mydata pass=2 mydata Given the variable assignment feature, the -F option for setting the value of FS is not strictly necessary. It remains for historical compatibility. Naming Standard Input Often, you may wish to read standard input together with other files. For example, you may wish to read one file, read standard input coming from a pipe, and then read another file. The way to name the standard input, with all versions of awk, is to use a single, standalone minus sign or dash, ‘-’. For example: some_command | awk -f myprog.awk file1 - file2 Here, awk first reads file1, then it reads the output of some_command, and finally it reads file2. You may also use "-" to name standard input when reading files with getline (see “Using getline from a File” on page 80). In addition, gawk allows you to specify the special filename /dev/stdin, both on the command line and with getline. Some other versions of awk also support this, but it is not standard. (Some operating systems provide a /dev/stdin file in the filesystem; however, gawk always processes this filename itself.) The Environment Variables gawk Uses A number of environment variables influence how gawk behaves. 30 | Chapter 2: Running awk and gawk 3. Semicolons on MS-Windows and MS-DOS. The AWKPATH Environment Variable In most awk implementations, you must supply a precise pathname for each program file, unless the file is in the current directory. But with gawk, if the filename supplied to the -f or -i options does not contain a directory separator ‘/’, then gawk searches a list of directories (called the search path) one by one, looking for a file with the specified name. The search path is a string consisting of directory names separated by colons.3 gawk gets its search path from the AWKPATH environment variable. If that variable does not exist, or if it has an empty value, gawk uses a default path (described shortly). The search path feature is particularly helpful for building libraries of useful awk func‐ tions. The library files can be placed in a standard directory in the default path and then specified on the command line with a short filename. Otherwise, you would have to type the full filename for each file. By using the -i or -f options, your command-line awk programs can use facilities in awk library files (see Chapter 10). Path searching is not done if gawk is in compatibility mode. This is true for both --traditional and --posix. See “Command-Line Op‐ tions” on page 22. If the source code file is not found after the initial search, the path is searched again after adding the suffix ‘.awk’ to the filename. gawk’s path search mechanism is similar to the shell’s. (See The Bourne-Again SHell manual.) It treats a null entry in the path as indicating the current directory. (A null entry is indicated by starting or ending the path with a colon or by placing two colons next to each other [‘::’].) To include the current directory in the path, either place . as an entry in the path or write a null entry in the path. Different past versions of gawk would also look explicitly in the current directory, either before or after the path search. As of ver‐ sion 4.1.2, this no longer happens; if you wish to look in the cur‐ rent directory, you must include . either as a separate entry or as a null entry in the search path. The Environment Variables gawk Uses | 31 4. Your version of gawk may use a different directory; it will depend upon how gawk was built and installed. The actual directory is the value of $(datadir) generated when gawk was configured. You probably don’t need to worry about this, though. The default value for AWKPATH is ‘.:/usr/local/share/awk’.4 Since . is included at the beginning, gawk searches first in the current directory and then in /usr/local/share/ awk. In practice, this means that you will rarely need to change the value of AWKPATH. gawk places the value of the search path that it used into ENVIRON["AWKPATH"]. This provides access to the actual search path value from within an awk program. Although you can change ENVIRON["AWKPATH"] within your awk program, this has no effect on the running program’s behavior. This makes sense: the AWKPATH environment variable is used to find the program source files. Once your program is running, all the files have been found, and gawk no longer needs to use AWKPATH. The AWKLIBPATH Environment Variable The AWKLIBPATH environment variable is similar to the AWKPATH variable, but it is used to search for loadable extensions (stored as system shared libraries) specified with the -l option rather than for source files. If the extension is not found, the path is searched again after adding the appropriate shared library suffix for the platform. For example, on GNU/Linux systems, the suffix ‘.so’ is used. The search path specified is also used for extensions loaded via the @load keyword (see “Loading Dynamic Extensions into Your Program” on page 36). If AWKLIBPATH does not exist in the environment, or if it has an empty value, gawk uses a default path; this is typically ‘/usr/local/lib/gawk’, although it can vary depending upon how gawk was built. gawk places the value of the search path that it used into ENVIRON["AWKLIBPATH"]. This provides access to the actual search path value from within an awk program. Other Environment Variables A number of other environment variables affect gawk’s behavior, but they are more specialized. Those in the following list are meant to be used by regular users: GAWK_MSEC_SLEEP Specifies the interval between connection retries, in milliseconds. On systems that do not support the usleep() system call, the value is rounded up to an integral number of seconds. 32 | Chapter 2: Running awk and gawk GAWK_READ_TIMEOUT Specifies the time, in milliseconds, for gawk to wait for input before returning with an error. See “Reading Input with a Timeout” on page 85. GAWK_SOCK_RETRIES Controls the number of times gawk attempts to retry a two-way TCP/IP (socket) connection before giving up. See “Using gawk for Network Programming” on page 337. POSIXLY_CORRECT Causes gawk to switch to POSIX-compatibility mode, disabling all traditional and GNU extensions. See “Command-Line Options” on page 22. The environment variables in the following list are meant for use by the gawk developers for testing and tuning. They are subject to change. The variables are: AWKBUFSIZE This variable only affects gawk on POSIX-compliant systems. With a value of ‘exact’, gawk uses the size of each input file as the size of the memory buffer to allocate for I/O. Otherwise, the value should be a number, and gawk uses that num‐ ber as the size of the buffer to allocate. (When this variable is not set, gawk uses the smaller of the file’s size and the “default” blocksize, which is usually the filesystem’s I/O blocksize.) AWK_HASH If this variable exists with a value of ‘gst’, gawk switches to using the hash function from GNU Smalltalk for managing arrays. This function may be marginally faster than the standard function. AWKREADFUNC If this variable exists, gawk switches to reading source files one line at a time, instead of reading in blocks. This exists for debugging problems on filesystems on non- POSIX operating systems where I/O is performed in records, not in blocks. GAWK_MSG_SRC If this variable exists, gawk includes the filename and line number within the gawk source code from which warning and/or fatal messages are generated. Its purpose is to help isolate the source of a message, as there are multiple places that produce the same warning or error message. GAWK_NO_DFA If this variable exists, gawk does not use the DFA regexp matcher for “does it match” kinds of tests. This can cause gawk to be slower. Its purpose is to help isolate dif‐ ferences between the two regexp matchers that gawk uses internally. (There aren’t The Environment Variables gawk Uses | 33 supposed to be differences, but occasionally theory and practice don’t coordinate with each other.) GAWK_NO_PP_RUN When gawk is invoked with the --pretty-print option, it will not run the program if this environment variable exists. This variable will not survive into the next major release. GAWK_STACKSIZE This specifies the amount by which gawk should grow its internal evaluation stack, when needed. INT_CHAIN_MAX This specifies the intended maximum number of items gawk will maintain on a hash chain for managing arrays indexed by integers. STR_CHAIN_MAX This specifies the intended maximum number of items gawk will maintain on a hash chain for managing arrays indexed by strings. TIDYMEM If this variable exists, gawk uses the mtrace() library calls from the GNU C Library to help track down possible memory leaks. gawk’s Exit Status If the exit statement is used with a value (see “The exit Statement” on page 159), then gawk exits with the numeric value given to it. Otherwise, if there were no problems during execution, gawk exits with the value of the C constant EXIT_SUCCESS. This is usually zero. If an error occurs, gawk exits with the value of the C constant EXIT_FAILURE. This is usually one. If gawk exits because of a fatal error, the exit status is two. On non-POSIX systems, this value may be mapped to EXIT_FAILURE. 34 | Chapter 2: Running awk and gawk Including Other Files into Your Program This section describes a feature that is specific to gawk. The @include keyword can be used to read external awk source files. This gives you the ability to split large awk source files into smaller, more manageable pieces, and also lets you reuse common awk code from various awk scripts. In other words, you can group together awk functions used to carry out specific tasks into external files. These files can be used just like function libraries, using the @include keyword in conjunction with the AWKPATH environment variable. Note that source files may also be included using the -i option. Let’s see an example. We’ll start with two (trivial) awk scripts, namely test1 and test2. Here is the test1 script: BEGIN { print "This is script test1." } and here is test2: @include "test1" BEGIN { print "This is script test2." } Running gawk with test2 produces the following result: $ gawk -f test2 This is script test1. This is script test2. gawk runs the test2 script, which includes test1 using the @include keyword. So, to include external awk source files, you just use @include followed by the name of the file to be included, enclosed in double quotes. Keep in mind that this is a language construct and the filename can‐ not be a string variable, but rather just a literal string constant in double quotes. The files to be included may be nested; e.g., given a third script, namely test3: @include "test2" BEGIN { print "This is script test3." } Including Other Files into Your Program | 35 Running gawk with the test3 script produces the following results: $ gawk -f test3 This is script test1. This is script test2. This is script test3. The filename can, of course, be a pathname. For example: @include "../io_funcs" and: @include "/usr/awklib/network" are both valid. The AWKPATH environment variable can be of great value when using @include. The same rules for the use of the AWKPATH variable in command-line file searches (see “The AWKPATH Environment Variable” on page 31) apply to @include also. This is very helpful in constructing gawk function libraries. If you have a large script with useful, general-purpose awk functions, you can break it down into library files and put those files in a special directory. You can then include those “libraries,” either by using the full pathnames of the files, or by setting the AWKPATH environment variable accordingly and then using @include with just the file part of the full pathname. Of course, you can keep library files in more than one directory; the more complex the working environment is, the more directories you may need to organize the files to be included. Given the ability to specify multiple -f options, the @include mechanism is not strictly necessary. However, the @include keyword can help you in constructing self-contained gawk programs, thus reducing the need for writing complex and tedious command lines. In particular, @include is very useful for writing CGI scripts to be run from web pages. As mentioned in “The AWKPATH Environment Variable” on page 31, the current di‐ rectory is always searched first for source files, before searching in AWKPATH; this also applies to files named with @include. Loading Dynamic Extensions into Your Program This section describes a feature that is specific to gawk. The @load keyword can be used to read external awk extensions (stored as system shared libraries). This allows you to link in compiled code that may offer superior performance and/or give you access to extended capabilities not supported by the awk language. The AWKLIBPATH variable is used to search for the extension. Using @load is completely equivalent to using the -l command-line option. 36 | Chapter 2: Running awk and gawk If the extension is not initially found in AWKLIBPATH, another search is conducted after appending the platform’s default shared library suffix to the filename. For example, on GNU/Linux systems, the suffix ‘.so’ is used: $ gawk '@load "ordchr"; BEGIN {print chr(65)}' A This is equivalent to the following example: $ gawk -lordchr 'BEGIN {print chr(65)}' A For command-line usage, the -l option is more convenient, but @load is useful for embedding inside an awk source file that requires access to an extension. Chapter 16 describes how to write extensions (in C or C++) that can be loaded with either @load or the -l option. It also describes the ordchr extension. Obsolete Options and/or Features This section describes features and/or command-line options from previous releases of gawk that either are not available in the current version or are still supported but dep‐ recated (meaning that they will not be in the next release). The process-related special files /dev/pid, /dev/ppid, /dev/pgrpid, and /dev/user were deprecated in gawk 3.1, but still worked. As of version 4.0, they are no longer interpreted specially by gawk. (Use PROCINFO instead; see “Built-in Variables That Con‐ vey Information” on page 163.) Undocumented Options and Features Use the Source, Luke! —Obi-Wan This section intentionally left blank. Summary • Use either ‘awk 'program' files’ or ‘awk -f program-file files’ to run awk. • The three standard options for all versions of awk are -f, -F, and -v. gawk supplies these and many others, as well as corresponding GNU-style long options. • Nonoption command-line arguments are usually treated as filenames, unless they have the form ‘var=value’, in which case they are taken as variable assignments to be performed at that point in processing the input. Obsolete Options and/or Features | 37 • All nonoption command-line arguments, excluding the program text, are placed in the ARGV array. Adjusting ARGC and ARGV affects how awk processes input. • You can use a single minus sign (‘-’) to refer to standard input on the command line. gawk also lets you use the special filename /dev/stdin. • gawk pays attention to a number of environment variables. AWKPATH, AWKLIBPATH, and POSIXLY_CORRECT are the most important ones. • gawk’s exit status conveys information to the program that invoked it. Use the exit statement from within an awk program to set the exit status. • gawk allows you to include other awk source files into your program using the @include statement and/or the -i and -f command-line options. • gawk allows you to load additional functions written in C or C++ using the @load statement and/or the -l option. (This advanced feature is described later, in Chap‐ ter 16.) 38 | Chapter 2: Running awk and gawk CHAPTER 3 Regular Expressions A regular expression, or regexp, is a way of describing a set of strings. Because regular expressions are such a fundamental part of awk programming, their format and use deserve a separate chapter. A regular expression enclosed in slashes (‘/’) is an awk pattern that matches every input record whose text belongs to that set. The simplest regular expression is a sequence of letters, numbers, or both. Such a regexp matches any string that contains that sequence. Thus, the regexp ‘foo’ matches any string containing ‘foo.’ Thus, the pattern /foo/ matches any input record containing the three adjacent characters ‘foo’ anywhere in the record. Other kinds of regexps let you specify more complicated classes of strings. Initially, the examples in this chapter are simple. As we explain more about how regular expressions work, we present more complicated instances. How to Use Regular Expressions A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is tested against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, the following prints the second field of each record where the string ‘li’ appears anywhere in the record: $ awk '/li/ { print $2 }' mail-list 555-5553 555-0542 555-6699 555-3430 Regular expressions can also be used in matching expressions. These expressions allow you to specify the string to match against; it need not be the entire current input record. The two operators ‘~’ and ‘!~’ perform regular expression comparisons. Expressions using these operators can be used as patterns, or in if, while, for, and do statements. 39 (See “Control Statements in Actions” on page 150.) For example, the following is true if the expression exp (taken as a string) matches regexp: exp ~ /regexp/ This example matches, or selects, all input records with the uppercase letter ‘J’ some‐ where in the first field: $ awk '$1 ~ /J/' inventory-shipped Jan 13 25 15 115 Jun 31 42 75 492 Jul 24 34 67 436 Jan 21 36 64 620 So does this: awk '{ if ($1 ~ /J/) print }' inventory-shipped This next example is true if the expression exp (taken as a character string) does not match regexp: exp !~ /regexp/ The following example matches, or selects, all input records whose first field does not contain the uppercase letter ‘J’: $ awk '$1 !~ /J/' inventory-shipped Feb 15 32 24 226 Mar 15 24 34 228 Apr 31 52 63 420 May 16 34 29 208 … When a regexp is enclosed in slashes, such as /foo/, we call it a regexp constant, much like 5.27 is a numeric constant and "foo" is a string constant. Escape Sequences Some characters cannot be included literally in string constants ("foo") or regexp con‐ stants (/foo/). Instead, they should be represented with escape sequences, which are character sequences beginning with a backslash (‘\’). One use of an escape sequence is to include a double-quote character in a string constant. Because a plain double quote ends the string, you must use ‘\"’ to represent an actual double-quote character as a part of the string. For example: $ awk 'BEGIN { print "He said \"hi!\" to her." }' He said "hi!" to her. The backslash character itself is another character that cannot be included normally; you must write ‘\\’ to put one backslash in the string or regexp. Thus, the string whose contents are the two characters ‘"’ and ‘\’ must be written "\"\\". 40 | Chapter 3: Regular Expressions Other escape sequences represent unprintable characters such as TAB or newline. There is nothing to stop you from entering most unprintable characters directly in a string constant or regexp constant, but they may look ugly. The following list presents all the escape sequences used in awk and what they represent. Unless noted otherwise, all these escape sequences apply to both string constants and regexp constants: \\ A literal backslash, ‘\’. \a The “alert” character, Ctrl-g, ASCII code 7 (BEL). (This often makes some sort of audible noise.) \b Backspace, Ctrl-h, ASCII code 8 (BS). \f Formfeed, Ctrl-l, ASCII code 12 (FF). \n Newline, Ctrl-j, ASCII code 10 (LF). \r Carriage return, Ctrl-m, ASCII code 13 (CR). \t Horizontal TAB, Ctrl-i, ASCII code 9 (HT). \v Vertical TAB, Ctrl-k, ASCII code 11 (VT). \nnn The octal value nnn, where nnn stands for 1 to 3 digits between ‘0’ and ‘7’. For example, the code for the ASCII ESC (escape) character is ‘\033’. \xhh… The hexadecimal value hh, where hh stands for a sequence of hexadecimal digits (‘0’–‘9’, and either ‘A’–‘F’ or ‘a’–‘f’). Like the same construct in ISO C, the escape sequence continues until the first nonhexadecimal digit is seen. (c.e.) However, using more than two hexadecimal digits produces undefined results. (The ‘\x’ es‐ cape sequence is not allowed in POSIX awk.) Escape Sequences | 41 The next major release of gawk will change, such that a maxi‐ mum of two hexadecimal digits following the ‘\x’ will be used. \/ A literal slash (necessary for regexp constants only). This sequence is used when you want to write a regexp constant that contains a slash (such as /.*:\/home\/ [[:alnum:]]+:.*/; the ‘[[:alnum:]]’ notation is discussed in “Using Bracket Ex‐ pressions” on page 46). Because the regexp is delimited by slashes, you need to escape any slash that is part of the pattern, in order to tell awk to keep processing the rest of the regexp. \" A literal double quote (necessary for string constants only). This sequence is used when you want to write a string constant that contains a double quote (such as "He said \"hi!\" to her."). Because the string is delimited by double quotes, you need to escape any quote that is part of the string in order to tell awk to keep pro‐ cessing the rest of the string. In gawk, a number of additional two-character sequences that begin with a backslash have special meaning in regexps. See “gawk-Specific Regexp Operators” on page 50. In a regexp, a backslash before any character that is not in the previous list and not listed in “gawk-Specific Regexp Operators” on page 50 means that the next character should be taken literally, even if it would normally be a regexp operator. For example, /a\+b/ matches the three characters ‘a+b’. For complete portability, do not use a backslash before any character not shown in the previous list or that is not an operator. Backslash Before Regular Characters If you place a backslash in a string constant before something that is not one of the characters previously listed, POSIX awk purposely leaves what happens as undefined. There are two choices: Strip the backslash out This is what BWK awk and gawk both do. For example, "a\qc" is the same as "aqc". (Because this is such an easy bug both to introduce and to miss, gawk warns you about it.) Consider ‘FS = "[ \t]+\|[ \t]+"’ to use vertical bars surrounded by whitespace as the field separator. There should be two backslashes in the string: ‘FS = "[ \t]+\\|[ \t]+"’.) 42 | Chapter 3: Regular Expressions Leave the backslash alone Some other awk implementations do this. In such implementations, typing "a\qc" is the same as typing "a\\qc". To summarize: • The escape sequences in the preceding list are always processed first, for both string constants and regexp constants. This happens very early, as soon as awk reads your program. • gawk processes both regexp constants and dynamic regexps (see “Using Dynamic Regexps” on page 49), for the special operators listed in “gawk-Specific Regexp Operators” on page 50. • A backslash before any other character means to treat that character literally. Escape Sequences for Metacharacters Suppose you use an octal or hexadecimal escape to represent a regexp metacharacter. (See “Regular Expression Operators” on page 43.) Does awk treat the character as a literal character or as a regexp operator? Historically, such characters were taken literally. (d.c.) However, the POSIX standard indicates that they should be treated as real metacharacters, which is what gawk does. In compatibility mode (see “Command-Line Options” on page 22), gawk treats the characters represented by octal and hexadecimal escape sequences literally when used in regexp constants. Thus, /a\52b/ is equivalent to /a\*b/. Regular Expression Operators You can combine regular expressions with special characters, called regular expression operators or metacharacters, to increase the power and versatility of regular expressions. The escape sequences described earlier in “Escape Sequences” on page 40 are valid inside a regexp. They are introduced by a ‘\’ and are recognized and converted into corre‐ sponding real characters as the very first step in processing regexps. Here is a list of metacharacters. All characters that are not escape sequences and that are not listed here stand for themselves: \ This suppresses the special meaning of a character when matching. For example, ‘\$’ matches the character ‘$’. Regular Expression Operators | 43 1. In other literature, you may see a bracket expression referred to as either a character set, a character class, or a character list. ^ This matches the beginning of a string. ‘^@chapter’ matches ‘@chapter’ at the be‐ ginning of a string, for example, and can be used to identify chapter beginnings in Texinfo source files. The ‘^’ is known as an anchor, because it anchors the pattern to match only at the beginning of the string. It is important to realize that ‘^’ does not match the beginning of a line (the point right after a ‘\n’ newline character) embedded in a string. The condition is not true in the following example: if ("line1\nLINE 2" ~ /^L/) … $ This is similar to ‘^’, but it matches only at the end of a string. For example, ‘p$’ matches a record that ends with a ‘p’. The ‘$’ is an anchor and does not match the end of a line (the point right before a ‘\n’ newline character) embedded in a string. The condition in the following example is not true: if ("line1\nLINE 2" ~ /1$/) … . (period) This matches any single character, including the newline character. For example, ‘.P’ matches any single character followed by a ‘P’ in a string. Using concatenation, we can make a regular expression such as ‘U.A’, which matches any three-character sequence that begins with ‘U’ and ends with ‘A’. In strict POSIX mode (see “Command-Line Options” on page 22), ‘.’ does not match the NUL character, which is a character with all bits equal to zero. Otherwise, NUL is just another character. Other versions of awk may not be able to match the NUL character. [...] This is called a bracket expression.1 It matches any one of the characters that are enclosed in the square brackets. For example, ‘[MVX]’ matches any one of the char‐ acters ‘M’, ‘V’, or ‘X’ in a string. A full discussion of what can be inside the square brackets of a bracket expression is given in “Using Bracket Expressions” on page 46. [^...] This is a complemented bracket expression. The first character after the ‘[’ must be a ‘^’. It matches any characters except those in the square brackets. For example, ‘[^awk]’ matches any character that is not an ‘a’, ‘w’, or ‘k’. 44 | Chapter 3: Regular Expressions | This is the alternation operator and it is used to specify alternatives. The ‘|’ has the lowest precedence of all the regular expression operators. For example, ‘^P| [aeiouy]’ matches any string that matches either ‘^P’ or ‘[aeiouy]’. This means it matches any string that starts with ‘P’ or contains (anywhere within it) a lowercase English vowel. The alternation applies to the largest possible regexps on either side. (...) Parentheses are used for grouping in regular expressions, as in arithmetic. They can be used to concatenate regular expressions containing the alternation operator, ‘|’. For example, ‘@(samp|code)\{[^}]+\}’ matches both ‘@code{foo}’ and ‘@samp{bar}’. (These are Texinfo formatting control sequences. The ‘+’ is explained further on in this list.) * This symbol means that the preceding regular expression should be repeated as many times as necessary to find a match. For example, ‘ph*’ applies the ‘*’ symbol to the preceding ‘h’ and looks for matches of one ‘p’ followed by any number of ‘h’s. This also matches just ‘p’ if no ‘h’s are present. There are two subtle points to understand about how ‘*’ works. First, the ‘*’ applies only to the single preceding regular expression component (e.g., in ‘ph*’, it applies just to the ‘h’). To cause ‘*’ to apply to a larger subexpression, use parentheses: ‘(ph)*’ matches ‘ph’, ‘phph’, ‘phphph’, and so on. Second, ‘*’ finds as many repetitions as possible. If the text to be matched is ‘phhhhhhhhhhhhhhooey’, ‘ph*’ matches all of the ‘h’s. + This symbol is similar to ‘*’, except that the preceding expression must be matched at least once. This means that ‘wh+y’ would match ‘why’ and ‘whhy’, but not ‘wy’, whereas ‘wh*y’ would match all three. ? This symbol is similar to ‘*’, except that the preceding expression can be matched either once or not at all. For example, ‘fe?d’ matches ‘fed’ and ‘fd’, but nothing else. {n} {n,} {n,m} One or two numbers inside braces denote an interval expression. If there is one number in the braces, the preceding regexp is repeated n times. If there are two numbers separated by a comma, the preceding regexp is repeated n to m times. If Regular Expression Operators | 45 2. Use two backslashes if you’re using a string constant with a regexp operator or function. there is one number followed by a comma, then the preceding regexp is repeated at least n times: wh{3}y Matches ‘whhhy’, but not ‘why’ or ‘whhhhy’. wh{3,5}y Matches ‘whhhy’, ‘whhhhy’, or ‘whhhhhy’ only. wh{2,}y Matches ‘whhy’, ‘whhhy’, and so on. Interval expressions were not traditionally available in awk. They were added as part of the POSIX standard to make awk and egrep consistent with each other. Initially, because old programs may use ‘{’ and ‘}’ in regexp constants, gawk did not match interval expressions in regexps. However, beginning with version 4.0, gawk does match interval expressions by de‐ fault. This is because compatibility with POSIX has become more important to most gawk users than compatibility with old programs. For programs that use ‘{’ and ‘}’ in regexp constants, it is good practice to always escape them with a backslash. Then the regexp constants are valid and work the way you want them to, using any version of awk.2 Finally, when ‘{’ and ‘}’ appear in regexp constants in a way that cannot be inter‐ preted as an interval expression (such as /q{a}/), then they stand for themselves. In regular expressions, the ‘*’, ‘+’, and ‘?’ operators, as well as the braces ‘{’ and ‘}’, have the highest precedence, followed by concatenation, and finally by ‘|’. As in arithmetic, parentheses can change how operators are grouped. In POSIX awk and gawk, the ‘*’, ‘+’, and ‘?’ operators stand for themselves when there is nothing in the regexp that precedes them. For example, /+/ matches a literal plus sign. However, many other versions of awk treat such a usage as a syntax error. If gawk is in compatibility mode (see “Command-Line Options” on page 22), interval expressions are not available in regular expressions. Using Bracket Expressions As mentioned earlier, a bracket expression matches any character among those listed between the opening and closing square brackets. 46 | Chapter 3: Regular Expressions Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, based upon the system’s native character set. For example, ‘[0-9]’ is equivalent to ‘[0123456789]’. (See “Regexp Ranges and Locales: A Long Sad Story” on page 464 for an explanation of how the POSIX standard and gawk have changed over time. This is mainly of historical interest.) To include one of the characters ‘\’, ‘]’, ‘-’, or ‘^’ in a bracket expression, put a ‘\’ in front of it. For example: [d\]] matches either ‘d’ or ‘]’. Additionally, if you place ‘]’ right after the opening ‘[’, the closing bracket is treated as one of the characters to be matched. The treatment of ‘\’ in bracket expressions is compatible with other awk implementa‐ tions and is also mandated by POSIX. The regular expressions in awk are a superset of the POSIX specification for Extended Regular Expressions (EREs). POSIX EREs are based on the regular expressions accepted by the traditional egrep utility. Character classes are a feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but the actual characters can vary from country to country and/or from character set to char‐ acter set. For example, the notion of what is an alphabetic character differs between the United States and France. A character class is only valid in a regexp inside the brackets of a bracket expression. Character classes consist of ‘[:’, a keyword denoting the class, and ‘:]’. Table 3-1 lists the character classes defined by the POSIX standard. For example, before the POSIX standard, you had to write /[A-Za-z0-9]/ to match alphanumeric characters. If your character set had other alphabetic characters in it, this would not match them. With the POSIX character classes, you can write /[[:alnum:]]/ to match the alphabetic and numeric characters in your character set. Table 3-1. POSIX character classes Class Meaning [:alnum:] Alphanumeric characters [:alpha:] Alphabetic characters [:blank:] Space and TAB characters [:cntrl:] Control characters [:digit:] Numeric characters [:graph:] Characters that are both printable and visible (a space is printable but not visible, whereas an ‘a’ is both) [:lower:] Lowercase alphabetic characters Using Bracket Expressions | 47 Class Meaning [:print:] Printable characters (characters that are not control characters) [:punct:] Punctuation characters (characters that are not letters, digits, control characters, or space characters) [:space:] Space characters (such as space, TAB, and formfeed, to name a few) [:upper:] Uppercase alphabetic characters [:xdigit:] Characters that are hexadecimal digits Some utilities that match regular expressions provide a nonstandard ‘[:ascii:]’ char‐ acter class; awk does not. However, you can simulate such a construct using ‘[\x00-\x7F]’. This matches all values numerically between zero and 127, which is the defined range of the ASCII character set. Use a complemented character list (‘[^\x00- \x7F]’) to match any single-byte characters that are not in the ASCII range. Two additional special sequences can appear in bracket expressions. These apply to non- ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character. They can also have several characters that are equivalent for collating, or sorting, purposes. (For example, in French, a plain “e” and a grave-accented “è” are equivalent.) These sequences are: Collating symbols Multicharacter collating elements enclosed between ‘[.’ and ‘.]’. For example, if ‘ch’ is a collating element, then ‘[[.ch.]]’ is a regexp that matches this collating element, whereas ‘[ch]’ is a regexp that matches either ‘c’ or ‘h’. Equivalence classes Locale-specific names for a list of characters that are equal. The name is enclosed between ‘[=’ and ‘=]’. For example, the name ‘e’ might be used to represent all of “ê,” “e,” “è,” and “é.” In this case, ‘[[=e=]]’ is a regexp that matches any of ‘e’, ‘ê’, ‘é’, or ‘è’. These features are very valuable in non-English-speaking locales. The library functions that gawk uses for regular expression match‐ ing currently recognize only POSIX character classes; they do not recognize collating symbols or equivalence classes. How Much Text Matches? Consider the following: echo aaaabcd | awk '{ sub(/a+/, ""); print }' 48 | Chapter 3: Regular Expressions This example uses the sub() function to make a change to the input record. (sub() replaces the first instance of any text matched by the first argument with the string provided as the second argument; see “String-Manipulation Functions” on page 194.) Here, the regexp /a+/ indicates “one or more ‘a’ characters,” and the replacement text is ‘’. The input contains four ‘a’ characters. awk (and POSIX) regular expressions always match the leftmost, longest sequence of input characters that can match. Thus, all four ‘a’ characters are replaced with ‘’ in this example: $ echo aaaabcd | awk '{ sub(/a+/, ""); print }' bcd For simple match/no-match tests, this is not so important. But when doing text matching and substitutions with the match(), sub(), gsub(), and gensub() functions, it is very important. Understanding this principle is also important for regexp-based record and field splitting (see “How Input Is Split into Records” on page 55, and also see “Specifying How Fields Are Separated” on page 65). Using Dynamic Regexps The righthand side of a ‘~’ or ‘!~’ operator need not be a regexp constant (i.e., a string of characters between slashes). It may be any expression. The expression is evaluated and converted to a string if necessary; the contents of the string are then used as the regexp. A regexp computed in this way is called a dynamic regexp or a computed regexp: BEGIN { digits_regexp = "[[:digit:]]+" } $0 ~ digits_regexp { print } This sets digits_regexp to a regexp that describes one or more digits, and tests whether the input record matches this regexp. When using the ‘~’ and ‘!~’ operators, be aware that there is a differ‐ ence between a regexp constant enclosed in slashes and a string con‐ stant enclosed in double quotes. If you are going to use a string con‐ stant, you have to understand that the string is, in essence, scanned twice: the first time when awk reads your program, and the second time when it goes to match the string on the lefthand side of the operator with the pattern on the right. This is true of any string- valued expression (such as digits_regexp, shown in the previous example), not just string constants. What difference does it make if the string is scanned twice? The answer has to do with escape sequences, and particularly with backslashes. To get a backslash into a regular expression inside a string, you have to type two backslashes. Using Dynamic Regexps | 49 For example, /\*/ is a regexp constant for a literal ‘*’. Only one backslash is needed. To do the same thing with a string, you have to type "\\*". The first backslash escapes the second one so that the string actually contains the two characters ‘\’ and ‘*’. Given that you can use both regexp and string constants to describe regular expressions, which should you use? The answer is “regexp constants,” for several reasons: • String constants are more complicated to write and more difficult to read. Using regexp constants makes your programs less error-prone. Not understanding the difference between the two kinds of constants is a common source of errors. • It is more efficient to use regexp constants. awk can note that you have supplied a regexp and store it internally in a form that makes pattern matching more efficient. When using a string constant, awk must first convert the string into this internal form and then perform the pattern matching. • Using regexp constants is better form; it shows clearly that you intend a regexp match. Using \n in Bracket Expressions of Dynamic Regexps Some older versions of awk do not allow the newline character to be used inside a bracket expression for a dynamic regexp: $ awk '$0 ~ "[ \t\n]"' error→ awk: newline in character class [ error→ ]... error→ source line number 1 error→ context is error→ $0 ~ "[ >>> \t\n]" <<< But a newline in a regexp constant works with no problem: $ awk '$0 ~ /[ \t\n]/' here is a sample line here is a sample line Ctrl-d gawk does not have this problem, and it isn’t likely to occur often in practice, but it’s worth noting for future reference. gawk-Specific Regexp Operators GNU software that deals with regular expressions provides a number of additional regexp operators. These operators are described in this section and are specific to gawk; they are not available in other awk implementations. Most of the additional oper‐ 50 | Chapter 3: Regular Expressions ators deal with word matching. For our purposes, a word is a sequence of one or more letters, digits, or underscores (‘_’): \s Matches any whitespace character. Think of it as shorthand )for ‘[[:space:]]’. \S Matches any character that) is not whitespace. Think of it as shorthand for ‘[^[:space:]]’. \w Matches any word-constituent character—that is, it matches any letter, digit, or underscore. Think of it as shorthand for ‘[[:alnum:]_]’. \W Matches any character that is not word-constituent. Think of it as shorthand for ‘[^[:alnum:]_]’. \< Matches the empty string at the beginning of a word. For example, /\ Matches the empty string at the end of a word. For example, /stow\>/ matches ‘stow’ but not ‘stowaway’. \y Matches the empty string at either the beginning or the end of a word (i.e., the word boundary). For example, ‘\yballs?\y’ matches either ‘ball’ or ‘balls’, as a separate word. \B Matches the empty string that occurs between two word-constituent characters. For example, /\Brat\B/ matches ‘crate’, but it does not match ‘dirty rat’. ‘\B’ is essentially the opposite of ‘\y’. There are two other operators that work on buffers. In Emacs, a buffer is, naturally, an Emacs buffer. Other GNU programs, including gawk, consider the entire string to match as the buffer. The operators are: \` Matches the empty string at the beginning of a buffer (string) \' Matches the empty string at the end of a buffer (string) gawk-Specific Regexp Operators | 51 Because ‘^’ and ‘$’ always work in terms of the beginning and end of strings, these operators don’t add any new capabilities for awk. They are provided for compatibility with other GNU software. In other GNU software, the word-boundary operator is ‘\b’. However, that conflicts with the awk language’s definition of ‘\b’ as backspace, so gawk uses a different letter. An alternative method would have been to require two backslashes in the GNU operators, but this was deemed too confusing. The current method of using ‘\y’ for the GNU ‘\b’ appears to be the lesser of two evils. The various command-line options (see “Command-Line Options” on page 22) control how gawk interprets characters in regexps: No options In the default case, gawk provides all the facilities of POSIX regexps and the previ‐ ously described GNU regexp operators. --posix Match only POSIX regexps; the GNU operators are not special (e.g., ‘\w’ matches a literal ‘w’). Interval expressions are allowed. --traditional Match traditional Unix awk regexps. The GNU operators are not special, and in‐ terval expressions are not available. Because BWK awk supports them, the POSIX character classes (‘[[:alnum:]]’, etc.) are available. Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regexp metacharacters. --re-interval Allow interval expressions in regexps, if --traditional has been provided. Other‐ wise, interval expressions are available by default. Case Sensitivity in Matching Case is normally significant in regular expressions, both when matching ordinary char‐ acters (i.e., not metacharacters) and inside bracket expressions. Thus, a ‘w’ in a regular expression matches only a lowercase ‘w’ and not an uppercase ‘W’. The simplest way to do a case-independent match is to use a bracket expression—for example, ‘[Ww]’. However, this can be cumbersome if you need to use it often, and it can make the regular expressions harder to read. There are two alternatives that you might prefer. One way to perform a case-insensitive match at a particular point in the program is to convert the data to a single case, using the tolower() or toupper() built-in string 52 | Chapter 3: Regular Expressions 3. Experienced C and C++ programmers will note that it is possible, using something like ‘IGNORECASE = 1 && /foObAr/ { … }’ and ‘IGNORECASE = 0 || /foobar/ { … }’. However, this is somewhat obscure and we don’t recommend it. 4. If you don’t understand this, don’t worry about it; it just means that gawk does the right thing. functions (which we haven’t discussed yet; see “String-Manipulation Functions” on page 194). For example: tolower($1) ~ /foo/ { … } converts the first field to lowercase before matching against it. This works in any POSIX- compliant awk. Another method, specific to gawk, is to set the variable IGNORECASE to a nonzero value (see “Predefined Variables” on page 160). When IGNORECASE is not zero, all regexp and string operations ignore case. Changing the value of IGNORECASE dynamically controls the case sensitivity of the pro‐ gram as it runs. Case is significant by default because IGNORECASE (like most variables) is initialized to zero: x = "aB" if (x ~ /ab/) … # this test will fail IGNORECASE = 1 if (x ~ /ab/) … # now it will succeed In general, you cannot use IGNORECASE to make certain rules case insensitive and other rules case sensitive, as there is no straightforward way to set IGNORECASE just for the pattern of a particular rule.3 To do this, use either bracket expressions or tolower(). However, one thing you can do with IGNORECASE only is dynamically turn case sensitivity on or off for all the rules at once. IGNORECASE can be set on the command line or in a BEGIN rule (see “Other Command- Line Arguments” on page 29; also see “Startup and cleanup actions” on page 145). Setting IGNORECASE from the command line is a way to make a program case insensitive without having to edit it. In multibyte locales, the equivalences between upper- and lowercase characters are tes‐ ted based on the wide-character values of the locale’s character set. Otherwise, the char‐ acters are tested based on the ISO-8859-1 (ISO Latin-1) character set. This character set is a superset of the traditional 128 ASCII characters, which also provides a number of characters suitable for use with European languages.4 The value of IGNORECASE has no effect if gawk is in compatibility mode (see “Command- Line Options” on page 22). Case is always significant in compatibility mode. Case Sensitivity in Matching | 53 Summary • Regular expressions describe sets of strings to be matched. In awk, regular expres‐ sion constants are written enclosed between slashes: /.../. • Regexp constants may be used standalone in patterns and in conditional expres‐ sions, or as part of matching expressions using the ‘~’ and ‘!~’ operators. • Escape sequences let you represent nonprintable characters and also let you rep‐ resent regexp metacharacters as literal characters to be matched. • Regexp operators provide grouping, alternation, and repetition. • Bracket expressions give you a shorthand for specifying sets of characters that can match at a particular point in a regexp. Within bracket expressions, POSIX character classes let you specify certain groups of characters in a locale-independent fashion. • Regular expressions match the leftmost longest text in the string being matched. This matters for cases where you need to know the extent of the match, such as for text substitution and when the record separator is a regexp. • Matching expressions may use dynamic regexps (i.e., string values treated as regular expressions). • gawk’s IGNORECASE variable lets you control the case sensitivity of regexp matching. In other awk versions, use tolower() or toupper(). 54 | Chapter 3: Regular Expressions CHAPTER 4 Reading Input Files In the typical awk program, awk reads all input either from the standard input (by default, this is the keyboard, but often it is a pipe from another command) or from files whose names you specify on the awk command line. If you specify input files, awk reads them in order, processing all the data from one before going on to the next. The name of the current input file can be found in the predefined variable FILENAME (see “Predefined Variables” on page 160). The input is read in units called records, and is processed by the rules of your program one record at a time. By default, each record is one line. Each record is automatically split into chunks called fields. This makes it more convenient for programs to work on the parts of a record. On rare occasions, you may need to use the getline command. The getline command is valuable both because it can do explicit input from any number of files, and because the files used with it do not have to be named on the awk command line (see “Explicit Input with getline” on page 77). How Input Is Split into Records awk divides the input for your program into records and fields. It keeps track of the number of records that have been read so far from the current input file. This value is stored in a predefined variable called FNR, which is reset to zero every time a new file is started. Another predefined variable, NR, records the total number of input records read so far from all datafiles. It starts at zero, but is never automatically reset to zero. Record Splitting with Standard awk Records are separated by a character called the record separator. By default, the record separator is the newline character. This is why records are, by default, single lines. To 55 use a different character for the record separator, simply assign that character to the predefined variable RS. Like any other variable, the value of RS can be changed in the awk program with the assignment operator, ‘=’ (see “Assignment Expressions” on page 122). The new record- separator character should be enclosed in quotation marks, which indicate a string constant. Often, the right time to do this is at the beginning of execution, before any input is processed, so that the very first record is read with the proper separator. To do this, use the special BEGIN pattern (see “The BEGIN and END Special Patterns” on page 145). For example: awk 'BEGIN { RS = "u" } { print $0 }' mail-list changes the value of RS to ‘u’, before reading any input. The new value is a string whose first character is the letter “u”; as a result, records are separated by the letter “u”. Then the input file is read, and the second rule in the awk program (the action with no pattern) prints each record. Because each print statement adds a newline at the end of its output, this awk program copies the input with each ‘u’ changed to a newline. Here are the results of running the program on mail-list: $ awk 'BEGIN { RS = "u" } > { print $0 }' mail-list Amelia 555-5553 amelia.zodiac sq e@gmail.com F Anthony 555-3412 anthony.assert ro@hotmail.com A Becky 555-7685 becky.algebrar m@gmail.com A Bill 555-1675 bill.drowning@hotmail.com A Broderick 555-0542 broderick.aliq otiens@yahoo.com R Camilla 555-2912 camilla.inf sar m@skynet.be R Fabi s 555-1234 fabi s. ndevicesim s@ cb.ed F J lie 555-6699 j lie.perscr tabor@skeeve.com F Martin 555-6480 martin.codicib s@hotmail.com A Sam 56 | Chapter 4: Reading Input Files el 555-3430 sam el.lanceolis@sh .ed A Jean-Pa l 555-2127 jeanpa l.campanor m@ny .ed R Note that the entry for the name ‘Bill’ is not split. In the original datafile (see “Datafiles for the Examples” on page 11), the line looks like this: Bill 555-1675 bill.drowning@hotmail.com A It contains no ‘u’, so there is no reason to split the record, unlike the others, which each have one or more occurrences of the ‘u’. In fact, this record is treated as part of the previous record; the newline separating them in the output is the original newline in the datafile, not the one added by awk when it printed the record! Another way to change the record separator is on the command line, using the variable- assignment feature (see “Other Command-Line Arguments” on page 29): awk '{ print $0 }' RS="u" mail-list This sets RS to ‘u’ before processing mail-list. Using an alphabetic character such as ‘u’ for the record separator is highly likely to produce strange results. Using an unusual character such as ‘/’ is more likely to produce correct behavior in the majority of cases, but there are no guarantees. The moral is: Know Your Data. When using regular characters as the record separator, there is one unusual case that occurs when gawk is being fully POSIX-compliant (see “Command-Line Options” on page 22). Then, the following (extreme) pipeline prints a surprising ‘1’: $ echo | gawk --posix 'BEGIN { RS = "a" } ; { print NF }' 1 There is one field, consisting of a newline. The value of the built-in variable NF is the number of fields in the current record. (In the normal case, gawk treats the newline as whitespace, printing ‘0’ as the result. Most other versions of awk also act this way.) Reaching the end of an input file terminates the current input record, even if the last character in the file is not the character in RS. (d.c.) The empty string "" (a string without any characters) has a special meaning as the value of RS. It means that records are separated by one or more blank lines and nothing else. See “Multiple-Line Records” on page 75 for more details. How Input Is Split into Records | 57 If you change the value of RS in the middle of an awk run, the new value is used to delimit subsequent records, but the record currently being processed, as well as records already processed, are not affected. After the end of the record has been determined, gawk sets the variable RT to the text in the input that matched RS. Record Splitting with gawk When using gawk, the value of RS is not limited to a one-character string. It can be any regular expression (see Chapter 3). (c.e.) In general, each record ends at the next string that matches the regular expression; the next record starts at the end of the matching string. This general rule is actually at work in the usual case, where RS contains just a newline: a record ends at the beginning of the next matching string (the next newline in the input), and the following record starts just after the end of this string (at the first character of the following line). The newline, because it matches RS, is not part of either record. When RS is a single character, RT contains the same single character. However, when RS is a regular expression, RT contains the actual input text that matched the regular expression. If the input file ends without any text matching RS, gawk sets RT to the null string. The following example illustrates both of these features. It sets RS equal to a regular expression that matches either a newline or a series of one or more uppercase letters with optional leading and/or trailing whitespace: $ echo record 1 AAAA record 2 BBBB record 3 | > gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" } > { print "Record =", $0,"and RT = [" RT "]" }' Record = record 1 and RT = [ AAAA ] Record = record 2 and RT = [ BBBB ] Record = record 3 and RT = [ ] The square brackets delineate the contents of RT, letting you see the leading and trailing whitespace. The final value of RT is a newline. See “A Simple Stream Editor” on page 312 for a more useful example of RS as a regexp and RT. If you set RS to a regular expression that allows optional trailing text, such as ‘RS = "abc(XYZ)?"’, it is possible, due to implementation constraints, that gawk may match the leading part of the regular expression, but not the trailing part, particularly if the input text that could match the trailing part is fairly long. gawk attempts to avoid this problem, but currently, there’s no guarantee that this will never happen. 58 | Chapter 4: Reading Input Files 1. At least that we know about. Remember that in awk, the ‘^’ and ‘$’ anchor metacharacters match the beginning and end of a string, and not the beginning and end of a line. As a result, something like ‘RS = "^[[:upper:]]"’ can only match at the beginning of a file. This is because gawk views the in‐ put file as one long string that happens to contain newline charac‐ ters. It is thus best to avoid anchor metacharacters in the value of RS. The use of RS as a regular expression and the RT variable are gawk extensions; they are not available in compatibility mode (see “Command-Line Options” on page 22). In compatibility mode, only the first character of the value of RS determines the end of the record. RS = “\0” Is Not Portable There are times when you might want to treat an entire datafile as a single record. The only way to make this happen is to give RS a value that you know doesn’t occur in the input file. This is hard to do in a general way, such that a program always works for arbitrary input files. You might think that for text files, the NUL character, which consists of a character with all bits equal to zero, is a good value to use for RS in this case: BEGIN { RS = "\0" } # whole file becomes one record? gawk in fact accepts this, and uses the NUL character for the record separator. This works for certain special files, such as /proc/environ on GNU/Linux systems, where the NUL character is in fact the record separator. However, this usage is not portable to most other awk implementations. Almost all other awk implementations1 store strings internally as C-style strings. C strings use the NUL character as the string terminator. In effect, this means that ‘RS = "\0"’ is the same as ‘RS = ""’. (d.c.) It happens that recent versions of mawk can use the NUL character as a record separator. However, this is a special case: mawk does not allow embedded NUL characters in strings. (This may change in a future version of mawk.) See “Reading a Whole File at Once” on page 252 for an interesting way to read whole files. If you are using gawk, see “Reading an Entire File” on page 450 for another option. How Input Is Split into Records | 59 2. In POSIX awk, newlines are not considered whitespace for separating fields. Examining Fields When awk reads an input record, the record is automatically parsed or separated by the awk utility into chunks called fields. By default, fields are separated by whitespace, like words in a line. Whitespace in awk means any string of one or more spaces, TABs, or newlines;2 other characters that are considered whitespace by other languages (such as formfeed, vertical tab, etc.) are not considered whitespace by awk. The purpose of fields is to make it more convenient for you to refer to these pieces of the record. You don’t have to use them—you can operate on the whole record if you want—but fields are what make simple awk programs so powerful. You use a dollar sign (‘$’) to refer to a field in an awk program, followed by the number of the field you want. Thus, $1 refers to the first field, $2 to the second, and so on. (Unlike in the Unix shells, the field numbers are not limited to single digits. $127 is the 127th field in the record.) For example, suppose the following is a line of input: This seems like a pretty nice example. Here the first field, or $1, is ‘This’, the second field, or $2, is ‘seems’, and so on. Note that the last field, $7, is ‘example.’. Because there is no space between the ‘e’ and the ‘.’, the period is considered part of the seventh field. NF is a predefined variable whose value is the number of fields in the current record. awk automatically updates the value of NF each time it reads a record. No matter how many fields there are, the last field in a record can be represented by $NF. So, $NF is the same as $7, which is ‘example.’. If you try to reference a field beyond the last one (such as $8 when the record has only seven fields), you get the empty string. (If used in a numeric operation, you get zero.) The use of $0, which looks like a reference to the “zeroth” field, is a special case: it represents the whole input record. Use it when you are not interested in specific fields. Here are some more examples: $ awk '$1 ~ /li/ { print $0 }' mail-list Amelia 555-5553 amelia.zodiacusque@gmail.com F Julie 555-6699 julie.perscrutabor@skeeve.com F This example prints each record in the file mail-list whose first field contains the string ‘li’. By contrast, the following example looks for ‘li’ in the entire record and prints the first and last fields for each matching input record: 60 | Chapter 4: Reading Input Files 3. A binary operator, such as ‘*’ for multiplication, is one that takes two operands. The distinction is required because awk also has unary (one-operand) and ternary (three-operand) operators. $ awk '/li/ { print $1, $NF }' mail-list Amelia F Broderick R Julie F Samuel A Nonconstant Field Numbers A field number need not be a constant. Any expression in the awk language can be used after a ‘$’ to refer to a field. The value of the expression specifies the field number. If the value is a string, rather than a number, it is converted to a number. Consider this example: awk '{ print $NR }' Recall that NR is the number of records read so far: one in the first record, two in the second, and so on. So this example prints the first field of the first record, the second field of the second record, and so on. For the twentieth record, field number 20 is printed; most likely, the record has fewer than 20 fields, so this prints a blank line. Here is another example of using expressions as field numbers: awk '{ print $(2*2) }' mail-list awk evaluates the expression ‘(2*2)’ and uses its value as the number of the field to print. The ‘*’ represents multiplication, so the expression ‘2*2’ evaluates to four. The parentheses are used so that the multiplication is done before the ‘$’ operation; they are necessary whenever there is a binary operator3 in the field-number expression. This example, then, prints the type of relationship (the fourth field) for every line of the file mail-list. (All of the awk operators are listed, in order of decreasing precedence, in “Operator Precedence (How Operators Nest)” on page 136.) If the field number you compute is zero, you get the entire record. Thus, ‘$(2-2)’ has the same value as $0. Negative field numbers are not allowed; trying to reference one usually terminates the program. (The POSIX standard does not define what happens when you reference a negative field number. gawk notices this and terminates your program. Other awk implementations may behave differently.) As mentioned in “Examining Fields” on page 60, awk stores the current record’s number of fields in the built-in variable NF (also see “Predefined Variables” on page 160). Thus, the expression $NF is not a special feature—it is the direct consequence of evaluating NF and using its value as a field number. Nonconstant Field Numbers | 61 Changing the Contents of a Field The contents of a field, as seen by awk, can be changed within an awk program; this changes what awk perceives as the current input record. (The actual input is untouched; awk never modifies the input file.) Consider the following example and its output: $ awk '{ nboxes = $3 ; $3 = $3 - 10 > print nboxes, $3 }' inventory-shipped 25 15 32 22 24 14 … The program first saves the original value of field three in the variable nboxes. The ‘-’ sign represents subtraction, so this program reassigns field three, $3, as the original value of field three minus ten: ‘$3 - 10’. (See “Arithmetic Operators” on page 119.) Then it prints the original and new values for field three. (Someone in the warehouse made a consistent mistake while inventorying the red boxes.) For this to work, the text in $3 must make sense as a number; the string of characters must be converted to a number for the computer to do arithmetic on it. The number resulting from the subtraction is converted back to a string of characters that then be‐ comes field three. See “Conversion of Strings and Numbers” on page 117. When the value of a field is changed (as perceived by awk), the text of the input record is recalculated to contain the new field where the old one was. In other words, $0 changes to reflect the altered field. Thus, this program prints a copy of the input file, with 10 subtracted from the second field of each line: $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped Jan 3 25 15 115 Feb 5 32 24 226 Mar 5 24 34 228 … It is also possible to assign contents to fields that are out of range. For example: $ awk '{ $6 = ($5 + $4 + $3 + $2) > print $6 }' inventory-shipped 166 297 301 … We’ve just created $6, whose value is the sum of fields $2, $3, $4, and $5. The ‘+’ sign represents addition. For the file inventory-shipped, $6 represents the total number of parcels shipped for a particular month. 62 | Chapter 4: Reading Input Files Creating a new field changes awk’s internal copy of the current input record, which is the value of $0. Thus, if you do ‘print $0’ after adding a field, the record printed includes the new field, with the appropriate number of field separators between it and the pre‐ viously existing fields. This recomputation affects and is affected by NF (the number of fields; see “Examining Fields” on page 60). For example, the value of NF is set to the number of the highest field you create. The exact format of $0 is also affected by a feature that has not been discussed yet: the output field separator, OFS, used to separate the fields (see “Output Separa‐ tors” on page 91). Note, however, that merely referencing an out-of-range field does not change the value of either $0 or NF. Referencing an out-of-range field only produces an empty string. For example: if ($(NF+1) != "") print "can't happen" else print "everything is normal" should print ‘everything is normal’, because NF+1 is certain to be out of range. (See “The if-else Statement” on page 150 for more information about awk’s if-else statements. See “Variable Typing and Comparison Expressions” on page 128 for more information about the ‘!=’ operator.) It is important to note that making an assignment to an existing field changes the value of $0 but does not change the value of NF, even when you assign the empty string to a field. For example: $ echo a b c d | awk '{ OFS = ":"; $2 = "" > print $0; print NF }' a::c:d 4 The field is still there; it just has an empty value, delimited by the two colons between ‘a’ and ‘c’. This example shows what happens if you create a new field: $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new" > print $0; print NF }' a::c:d::new 6 The intervening field, $5, is created with an empty value (indicated by the second pair of adjacent colons), and NF is updated with the value six. Changing the Contents of a Field | 63 Decrementing NF throws away the values of the fields after the new value of NF and recomputes $0. (d.c.) Here is an example: $ echo a b c d e f | awk '{ print "NF =", NF; > NF = 3; print $0 }' NF = 6 a b c Some versions of awk don’t rebuild $0 when NF is decremented. Finally, there are times when it is convenient to force awk to rebuild the entire record, using the current values of the fields and OFS. To do this, use the seemingly innocuous assignment: $1 = $1 # force record to be reconstituted print $0 # or whatever else with $0 This forces awk to rebuild the record. It does help to add a comment, as we’ve shown here. There is a flip side to the relationship between $0 and the fields. Any assignment to $0 causes the record to be reparsed into fields using the current value of FS. This also applies to any built-in function that updates $0, such as sub() and gsub() (see “String- Manipulation Functions” on page 194). Understanding $0 It is important to remember that $0 is the full record, exactly as it was read from the input. This includes any leading or trailing whitespace, and the exact whitespace (or other characters) that separates the fields. It is a common error to try to change the field separators in a record simply by setting FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record. But this does not work, because nothing was done to change the record itself. Instead, you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as described earlier. 64 | Chapter 4: Reading Input Files Specifying How Fields Are Separated The field separator, which is either a single character or a regular expression, controls the way awk splits an input record into fields. awk scans the input record for character sequences that match the separator; the fields themselves are the text between the matches. In the examples that follow, we use the bullet symbol (•) to represent spaces in the output. If the field separator is ‘oo’, then the following line: moo goo gai pan is split into three fields: ‘m’, ‘•g’, and ‘•gai•pan’. Note the leading spaces in the values of the second and third fields. The field separator is represented by the predefined variable FS. Shell programmers take note: awk does not use the name IFS that is used by the POSIX-compliant shells (such as the Unix Bourne shell, sh, or Bash). The value of FS can be changed in the awk program with the assignment operator, ‘=’ (see “Assignment Expressions” on page 122). Often, the right time to do this is at the beginning of execution before any input has been processed, so that the very first record is read with the proper separator. To do this, use the special BEGIN pattern (see “The BEGIN and END Special Patterns” on page 145). For example, here we set the value of FS to the string ",": awk 'BEGIN { FS = "," } ; { print $2 }' Given the input line: John Q. Smith, 29 Oak St., Walamazoo, MI 42139 this awk program extracts and prints the string ‘•29•Oak•St.’. Sometimes the input data contains separator characters that don’t separate fields the way you thought they would. For instance, the person’s name in the example we just used might have a title or suffix attached, such as: John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139 The same program would extract ‘•LXIX’ instead of ‘•29•Oak•St.’. If you were expecting the program to print the address, you would be surprised. The moral is to choose your data layout and separator characters carefully to prevent such problems. (If the data is not in a form that is easy to process, perhaps you can massage it first with a separate awk program.) Specifying How Fields Are Separated | 65 Whitespace Normally Separates Fields Fields are normally separated by whitespace sequences (spaces, TABs, and newlines), not by single spaces. Two spaces in a row do not delimit an empty field. The default value of the field separator FS is a string containing a single space, " ". If awk interpreted this value in the usual way, each space character would separate fields, so two spaces in a row would make an empty field between them. The reason this does not happen is that a single space as the value of FS is a special case—it is taken to specify the default manner of delimiting fields. If FS is any other single character, such as ",", then each occurrence of that character separates two fields. Two consecutive occurrences delimit an empty field. If the character occurs at the beginning or the end of the line, that too delimits an empty field. The space character is the only single character that does not follow these rules. Using Regular Expressions to Separate Fields The previous subsection discussed the use of single characters or simple strings as the value of FS. More generally, the value of FS may be a string containing any regular expression. In this case, each match in the record for the regular expression separates fields. For example, the assignment: FS = ", \t" makes every area of an input line that consists of a comma followed by a space and a TAB into a field separator. For a less trivial example of a regular expression, try using single spaces to separate fields the way single commas are used. FS can be set to "[ ]" (left bracket, space, right bracket). This regular expression matches a single space and nothing else (see Chapter 3). There is an important difference between the two cases of ‘FS = " "’ (a single space) and ‘FS = "[ \t\n]+"’ (a regular expression matching one or more spaces, TABs, or newlines). For both values of FS, fields are separated by runs (multiple adjacent occur‐ rences) of spaces, TABs, and/or newlines. However, when the value of FS is " ", awk first strips leading and trailing whitespace from the record and then decides where the fields are. For example, the following pipeline prints ‘b’: $ echo ' a b c d ' | awk '{ print $2 }' b However, this pipeline prints ‘a’ (note the extra spaces around each letter): $ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t\n]+" } > { print $2 }' a In this case, the first field is null, or empty. 66 | Chapter 4: Reading Input Files The stripping of leading and trailing whitespace also comes into play whenever $0 is recomputed. For instance, study this pipeline: $ echo ' a b c d' | awk '{ print; $2 = $2; print }' a b c d a b c d The first print statement prints the record as it was read, with leading whitespace intact. The assignment to $2 rebuilds $0 by concatenating $1 through $NF together, separated by the value of OFS (which is a space by default). Because the leading whitespace was ignored when finding $1, it is not part of the new $0. Finally, the last print statement prints the new $0. There is an additional subtlety to be aware of when using regular expressions for field splitting. It is not well specified in the POSIX standard, or anywhere else, what ‘^’ means when splitting fields. Does the ‘^’ match only at the beginning of the entire record? Or is each field separator a new string? It turns out that different awk versions answer this question differently, and you should not rely on any specific behavior in your pro‐ grams. (d.c.) As a point of information, BWK awk allows ‘^’ to match only at the beginning of the record. gawk also works this way. For example: $ echo 'xxAA xxBxx C' | > gawk -F '(^x+)|( +)' '{ for (i = 1; i <= NF; i++) > printf "-->%s<--\n", $i }' --><-- -->AA<-- -->xxBxx<-- -->C<-- Making Each Character a Separate Field There are times when you may want to examine each character of a record separately. This can be done in gawk by simply assigning the null string ("") to FS. (c.e.) In this case, each individual character in the record becomes a separate field. For example: $ echo a b | gawk 'BEGIN { FS = "" } > { > for (i = 1; i <= NF; i = i + 1) > print "Field", i, "is", $i > }' Field 1 is a Field 2 is Field 3 is b Traditionally, the behavior of FS equal to "" was not defined. In this case, most versions of Unix awk simply treat the entire record as only having one field. (d.c.) In compatibility Specifying How Fields Are Separated | 67 mode (see “Command-Line Options” on page 22), if FS is the null string, then gawk also behaves this way. Setting FS from the Command Line FS can be set on the command line. Use the -F option to do so. For example: awk -F, 'program' input-files sets FS to the ‘,’ character. Notice that the option uses an uppercase ‘F’ instead of a lowercase ‘f’. The latter option (-f) specifies a file containing an awk program. The value used for the argument to -F is processed in exactly the same way as assign‐ ments to the predefined variable FS. Any special characters in the field separator must be escaped appropriately. For example, to use a ‘\’ as the field separator on the command line, you would have to type: # same as FS = "\\" awk -F\\\\ '…' files … Because ‘\’ is used for quoting in the shell, awk sees ‘-F\\’. Then awk processes the ‘\\’ for escape characters (see “Escape Sequences” on page 40), finally yielding a single ‘\’ to use for the field separator. As a special case, in compatibility mode (see “Command-Line Options” on page 22), if the argument to -F is ‘t’, then FS is set to the TAB character. If you type ‘-F\t’ at the shell, without any quotes, the ‘\’ gets deleted, so awk figures that you really want your fields to be separated with TABs and not ‘t’s. Use ‘-v FS="t"’ or ‘-F"[t]"’ on the com‐ mand line if you really do want to separate your fields with ‘t’s. Use ‘-F '\t'’ when not in compatibility mode to specify that TABs separate fields. As an example, let’s use an awk program file called edu.awk that contains the pat‐ tern /edu/ and the action ‘print $1’: /edu/ { print $1 } Let’s also set FS to be the ‘-’ character and run the program on the file mail-list. The following a university, and the first three digits of their phone numbers: $ awk -F- -f edu.awk mail-list Fabius 555 Samuel 555 Jean Note the third line of output. The third line in the original file looked like this: Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R 68 | Chapter 4: Reading Input Files 4. Thanks to Andrew Schorr for this tip. The ‘-’ as part of the person’s name was used as the field separator, instead of the ‘-’ in the phone number that was originally intended. This demonstrates why you have to be careful in choosing your field and record separators. Perhaps the most common use of a single character as the field separator occurs when processing the Unix system password file. On many Unix systems, each user has a sep‐ arate entry in the system password file, with one line per user. The information in these lines is separated by colons. The first field is the user’s login name and the second is the user’s encrypted or shadow password. (A shadow password is indicated by the presence of a single ‘x’ in the second field.) A password file entry might look like this: arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash The following program searches the system password file and prints the entries for users whose full name is not indicated: awk -F: '$5 == ""' /etc/passwd Making the Full Line Be a Single Field Occasionally, it’s useful to treat the whole input line as a single field. This can be done easily and portably simply by setting FS to "\n" (a newline):4 awk -F'\n' 'program' files … When you do this, $1 is the same as $0. Changing FS Does Not Affect the Fields According to the POSIX standard, awk is supposed to behave as if each record is split into fields at the time it is read. In particular, this means that if you change the value of FS after a record is read, the values of the fields (i.e., how they were split) should reflect the old value of FS, not the new one. However, many older implementations of awk do not work this way. Instead, they defer splitting the fields until a field is actually referenced. The fields are split using the current value of FS! (d.c.) This behavior can be difficult to diagnose. The following example illustrates the difference between the two methods: sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }' which usually prints: root Specifying How Fields Are Separated | 69 5. The sed utility is a “stream editor.” Its behavior is also defined by the POSIX standard. on an incorrect implementation of awk, while gawk prints the full first line of the file, something like: root:x:0:0:Root:/: (The sed5 command prints just the first line of /etc/passwd.) Field-Splitting Summary It is important to remember that when you assign a string constant as the value of FS, it undergoes normal awk string processing. For example, with Unix awk and gawk, the assignment ‘FS = "\.."’ assigns the character string ".." to FS (the backslash is strip‐ ped). This creates a regexp meaning “fields are separated by occurrences of any two characters.” If instead you want fields to be separated by a literal period followed by any single character, use ‘FS = "\\.."’. The following list summarizes how fields are split, based on the value of FS (‘==’ means “is equal to”): FS == " " Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored. This is the default. FS == any other single character Fields are separated by each occurrence of the character. Multiple successive oc‐ currences delimit empty fields, as do leading and trailing occurrences. The char‐ acter can even be a regexp metacharacter; it does not need to be escaped. FS == regexp Fields are separated by occurrences of characters that match regexp. Leading and trailing matches of regexp delimit empty fields. FS == "" Each individual character in the record becomes a separate field. (This is a common extension; it is not specified by the POSIX standard.) 70 | Chapter 4: Reading Input Files FS and IGNORECASE The IGNORECASE variable (see “Built-in Variables That Control awk” on page 160) affects field splitting only when the value of FS is a regexp. It has no effect when FS is a single character, even if that character is a letter. Thus, in the following code: FS = "c" IGNORECASE = 1 $0 = "aCa" print $1 The output is ‘aCa’. If you really want to split fields on an alphabetic character while ignoring case, use a regexp that will do it for you (e.g., ‘FS = "[c]"’). In this case, IGNORECASE will take effect. Reading Fixed-Width Data This section discusses an advanced feature of gawk. If you are a novice awk user, you might want to skip it on the first reading. gawk provides a facility for dealing with fixed-width fields with no distinctive field separator. For example, data of this nature arises in the input for old Fortran programs where numbers are run together, or in the output of programs that did not anticipate the use of their output as input for other programs. An example of the latter is a table where all the columns are lined up by the use of a variable number of spaces and empty fields are just spaces. Clearly, awk’s normal field splitting based on FS does not work well in this case. Although a portable awk program can use a series of substr() calls on $0 (see “String-Manipulation Functions” on page 194), this is awkward and inefficient for a large number of fields. The splitting of an input record into fixed-width fields is specified by assigning a string containing space-separated numbers to the built-in variable FIELDWIDTHS. Each number specifies the width of the field, including columns between fields. If you want to ignore the columns between fields, you can specify the width as a separate field that is subsequently ignored. It is a fatal error to supply a field width that is not a positive number. The following data is the output of the Unix w utility. It is useful to illustrate the use of FIELDWIDTHS: 10:06pm up 21 days, 14:04, 23 users User tty login idle JCPU PCPU what hzuo ttyV0 8:58pm 9 5 vi p24.tex hzang ttyV3 6:37pm 50 -csh eklye ttyV5 9:53pm 7 1 em thes.tex Reading Fixed-Width Data | 71 dportein ttyV6 8:17pm 1:47 -csh gierd ttyD3 10:00pm 1 elm dave ttyD4 9:47pm 4 4 w brent ttyp0 26Jun91 4:46 26:46 4:41 bash dave ttyq4 26Jun9115days 46 46 wnewmail The following program takes this input, converts the idle time to number of seconds, and prints out the first two fields and the calculated idle time: BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" } NR > 2 { idle = $4 sub(/^ +/, "", idle) # strip leading spaces if (idle == "") idle = 0 if (idle ~ /:/) { split(idle, t, ":") idle = t[1] * 60 + t[2] } if (idle ~ /days/) idle *= 24 * 60 * 60 print $1, $2, idle } The preceding program uses a number of awk features that haven’t been introduced yet. Running the program on the data produces the following results: hzuo ttyV0 0 hzang ttyV3 50 eklye ttyV5 0 dportein ttyV6 107 gierd ttyD3 1 dave ttyD4 0 brent ttyp0 286 dave ttyq4 1296000 Another (possibly more practical) example of fixed-width input data is the input from a deck of balloting cards. In some parts of the United States, voters mark their choices by punching holes in computer cards. These cards are then processed to count the votes for any particular candidate or on any particular issue. Because a voter may choose not to vote on some issue, any column on the card may be empty. An awk program for processing such data could use the FIELDWIDTHS feature to simplify reading the data. (Of course, getting gawk to run on a system with card readers is another story!) 72 | Chapter 4: Reading Input Files 6. The CSV format lacked a formal standard definition for many years. RFC 4180 standardizes the most common practices. Assigning a value to FS causes gawk to use FS for field splitting again. Use ‘FS = FS’ to make this happen, without having to know the current value of FS. In order to tell which kind of field splitting is in effect, use PROCINFO["FS"] (see “Built-in Variables That Convey Information” on page 163). The value is "FS" if regular field splitting is being used, or is "FIELDWIDTHS" if fixed-width field splitting is being used: if (PROCINFO["FS"] == "FS") regular field splitting … else if (PROCINFO["FS"] == "FIELDWIDTHS") fixed-width field splitting … else content-based field splitting … (see next section) This information is useful when writing a function that needs to temporarily change FS or FIELDWIDTHS, read some records, and then restore the original settings (see “Reading the User Database” on page 264 for an example of such a function). Defining Fields by Content This section discusses an advanced feature of gawk. If you are a novice awk user, you might want to skip it on the first reading. Normally, when using FS, gawk defines the fields as the parts of the record that occur in between each field separator. In other words, FS defines what a field is not, instead of what a field is. However, there are times when you really want to define the fields by what they are, and not by what they are not. The most notorious such case is so-called comma-separated values (CSV) data. Many spreadsheet programs, for example, can export their data into text files, where each record is terminated with a newline, and fields are separated by commas. If commas only separated the data, there wouldn’t be an issue. The problem comes when one of the fields contains an embedded comma. In such cases, most programs embed the field in double quotes.6 So, we might have data like this: Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA The FPAT variable offers a solution for cases like this. The value of FPAT should be a string that provides a regular expression. This regular expression describes the contents of each field. In the case of CSV data as presented here, each field is either “anything that is not a comma,” or “a double quote, anything that is not a double quote, and a closing double quote.” If written as a regular expression constant (see Chapter 3), we would have /([^,] Defining Fields by Content | 73 +)|("[^"]+")/. Writing this as a string requires us to escape the double quotes, leading to: FPAT = "([^,]+)|(\"[^\"]+\")" Putting this to use, here is a simple program to parse the data: BEGIN { FPAT = "([^,]+)|(\"[^\"]+\")" } { print "NF = ", NF for (i = 1; i <= NF; i++) { printf("$%d = <%s>\n", i, $i) } } When run, we get the following: $ gawk -f simple-csv.awk addresses.csv NF = 7 $1 = $2 = $3 = <"1234 A Pretty Street, NE"> $4 = $5 = $6 = <12345-6789> $7 = Note the embedded comma in the value of $3. A straightforward improvement when processing CSV data of this sort would be to remove the quotes when they occur, with something like this: if (substr($i, 1, 1) == "\"") { len = length($i) $i = substr($i, 2, len - 2) # Get text within the two quotes } As with FS, the IGNORECASE variable (see “Built-in Variables That Control awk” on page 160) affects field splitting with FPAT. Assigning a value to FPAT overrides field splitting with FS and with FIELDWIDTHS. Similar to FIELDWIDTHS, the value of PROCINFO["FS"] will be "FPAT" if content-based field splitting is being used. 74 | Chapter 4: Reading Input Files Some programs export CSV data that contains embedded newlines between the double quotes. gawk provides no way to deal with this. Even though a formal specification for CSV data exists, there isn’t much more to be done; the FPAT mechanism provides an elegant solution for the majority of cases, and the gawk developers are satis‐ fied with that. As written, the regexp used for FPAT requires that each field contain at least one char‐ acter. A straightforward modification (changing the first ‘+’ to ‘*’) allows fields to be empty: FPAT = "([^,]*)|(\"[^\"]+\")" Finally, the patsplit() function makes the same functionality available for splitting regular strings (see “String-Manipulation Functions” on page 194). To recap, gawk provides three independent methods to split input records into fields. The mechanism used is based on which of the three variables—FS, FIELDWIDTHS, or FPAT—was last assigned to. Multiple-Line Records In some databases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multiline records. The first step in doing this is to choose your data format. One technique is to use an unusual character or string to separate records. For example, you could use the formfeed character (written ‘\f’ in awk, as in C) to separate them, making each record a page of the file. To do this, just set the variable RS to "\f" (a string containing the formfeed character). Any other character could equally well be used, as long as it won’t be part of the data in a record. Another technique is to have blank lines separate records. By a special dispensation, an empty string as the value of RS indicates that records are separated by one or more blank lines. When RS is set to the empty string, each record always ends at the first blank line encountered. The next record doesn’t start until the first nonblank line that follows. No matter how many blank lines appear in a row, they all act as one record separator. (Blank lines must be completely empty; lines that contain only whitespace do not count.) You can achieve the same effect as ‘RS = ""’ by assigning the string "\n\n+" to RS. This regexp matches the newline at the end of the record and one or more blank lines after the record. In addition, a regular expression always matches the longest possible se‐ quence when there is a choice (see “How Much Text Matches?” on page 48). So, the next record doesn’t start until the first nonblank line that follows—no matter how many blank lines appear in a row, they are considered one record separator. Multiple-Line Records | 75 7. When FS is the null string ("") or a regexp, this special feature of RS does not apply. It does apply to the default field separator of a single space: ‘FS = " "’. However, there is an important difference between ‘RS = ""’ and ‘RS = "\n\n+"’. In the first case, leading newlines in the input datafile are ignored, and if a file ends without extra blank lines after the last record, the final newline is removed from the record. In the second case, this special processing is not done. (d.c.) Now that the input is separated into records, the second step is to separate the fields in the records. One way to do this is to divide each of the lines into fields in the normal manner. This happens by default as the result of a special feature. When RS is set to the empty string and FS is set to a single character, the newline character always acts as a field separator. This is in addition to whatever field separations result from FS.7 The original motivation for this special exception was probably to provide useful be‐ havior in the default case (i.e., FS is equal to " "). This feature can be a problem if you really don’t want the newline character to separate fields, because there is no way to prevent it. However, you can work around this by using the split() function to break up the record manually (see “String-Manipulation Functions” on page 194). If you have a single-character field separator, you can work around the special feature in a different way, by making FS into a regexp for that single character. For example, if the field sep‐ arator is a percent character, instead of ‘FS = "%"’, use ‘FS = "[%]"’. Another way to separate fields is to put each field on a separate line: to do this, just set the variable FS to the string "\n". (This single-character separator matches a single newline.) A practical example of a datafile organized this way might be a mailing list, where blank lines separate the entries. Consider a mailing list in a file named address es, which looks like this: Jane Doe 123 Main Street Anywhere, SE 12345-6789 John Smith 456 Tree-lined Avenue Smallville, MW 98765-4321 … A simple program to process this file is as follows: # addrs.awk --- simple mailing list program # Records are separated by blank lines. # Each line is one field. BEGIN { RS = "" ; FS = "\n" } { 76 | Chapter 4: Reading Input Files print "Name is:", $1 print "Address is:", $2 print "City and State are:", $3 print "" } Running the program produces the following output: $ awk -f addrs.awk addresses Name is: Jane Doe Address is: 123 Main Street City and State are: Anywhere, SE 12345-6789 Name is: John Smith Address is: 456 Tree-lined Avenue City and State are: Smallville, MW 98765-4321 … See “Printing Mailing Labels” on page 304 for a more realistic program dealing with ad‐ dress lists. The following list summarizes how records are split, based on the value of RS: RS == "\n" Records are separated by the newline character (‘\n’). In effect, every line in the datafile is a separate record, including blank lines. This is the default. RS == any single character Records are separated by each occurrence of the character. Multiple successive oc‐ currences delimit empty records. RS == "" Records are separated by runs of blank lines. When FS is a single character, then the newline character always serves as a field separator, in addition to whatever value FS may have. Leading and trailing newlines in a file are ignored. RS == regexp Records are separated by occurrences of characters that match regexp. Leading and trailing matches of regexp delimit empty records. (This is a gawk extension; it is not specified by the POSIX standard.) If not in compatibility mode (see “Command-Line Options” on page 22), gawk sets RT to the input text that matched the value specified by RS. But if the input file ended without any text that matches RS, then gawk sets RT to the null string. Explicit Input with getline So far we have been getting our input data from awk’s main input stream—either the standard input (usually your keyboard, sometimes the output from another program) Explicit Input with getline | 77 or the files specified on the command line. The awk language has a special built-in command called getline that can be used to read input under your explicit control. The getline command is used in several different ways and should not be used by beginners. The examples that follow the explanation of the getline command include material that has not been covered yet. Therefore, come back and study the getline command after you have reviewed the rest of Parts I and II and have a good knowledge of how awk works. The getline command returns 1 if it finds a record and 0 if it encounters the end of the file. If there is some error in getting a record, such as a file that cannot be opened, then getline returns −1. In this case, gawk sets the variable ERRNO to a string describing the error that occurred. In the following examples, command stands for a string value that represents a shell command. When --sandbox is specified (see “Command-Line Options” on page 22), reading lines from files, pipes, and coprocesses is disabled. Using getline with No Arguments The getline command can be used without arguments to read input from the current input file. All it does in this case is read the next input record and split it up into fields. This is useful if you’ve finished processing the current record, but want to do some special processing on the next record right now. For example: # Remove text between /* and */, inclusive { if ((i = index($0, "/*")) != 0) { out = substr($0, 1, i - 1) # leading part of the string rest = substr($0, i + 2) # ... */ ... j = index(rest, "*/") # is */ in trailing part? if (j > 0) { rest = substr(rest, j + 2) # remove comment } else { while (j == 0) { # get more text if (getline <= 0) { print("unexpected EOF or error:", ERRNO) > "/dev/stderr" exit } # build up the line using string concatenation rest = rest $0 j = index(rest, "*/") # is */ in trailing part? 78 | Chapter 4: Reading Input Files if (j != 0) { rest = substr(rest, j + 2) break } } } # build up the output line using string concatenation $0 = out rest } print $0 } This awk program deletes C-style comments (‘/* … */’) from the input. It uses a number of features we haven’t covered yet, including string concatenation (see “String Concat‐ enation” on page 121) and the index() and substr() built-in functions (see “String- Manipulation Functions” on page 194). By replacing the ‘print $0’ with other statements, you could perform more complicated processing on the decommented in‐ put, such as searching for matches of a regular expression. (This program has a subtle problem—it does not work if one comment ends and another begins on the same line.) This form of the getline command sets NF, NR, FNR, RT, and the value of $0. The new value of $0 is used to test the patterns of any subsequent rules. The original value of $0 that triggered the rule that executed getline is lost. By contrast, the next statement reads a new record but immediately begins processing it normally, starting with the first rule in the program. See “The next Statement” on page 157. Using getline into a Variable You can use ‘getline var’ to read the next record from awk’s input into the variable var. No other processing is done. For example, suppose the next line is a comment or a special string, and you want to read it without triggering any rules. This form of getline allows you to read that line and store it in a variable so that the main read-a- line-and-check-each-rule loop of awk never sees it. The following example swaps every two lines of input: { if ((getline tmp) > 0) { print tmp print $0 } else print $0 } It takes the following list: Explicit Input with getline | 79 wan tew free phore and produces these results: tew wan phore free The getline command used in this way sets only the variables NR, FNR, and RT (and, of course, var). The record is not split into fields, so the values of the fields (including $0) and the value of NF do not change. Using getline from a File Use ‘getline < file’ to read the next record from file. Here, file is a string-valued expression that specifies the filename. ‘< file’ is called a redirection because it directs input to come from a different place. For example, the following program reads its input record from the file secondary.input when it encounters a first field with a value equal to 10 in the current input file: { if ($1 == 10) { getline < "secondary.input" print } else print } Because the main input stream is not used, the values of NR and FNR are not changed. However, the record it reads is split into fields in the normal manner, so the values of $0 and the other fields are changed, resulting in a new value of NF. RT is also set. According to POSIX, ‘getline < expression’ is ambiguous if expression contains unparenthesized operators other than ‘$’; for example, ‘getline < dir "/" file’ is ambiguous because the concatenation operator (not discussed yet; see “String Concat‐ enation” on page 121) is not parenthesized. You should write it as ‘getline < (dir "/" file)’ if you want your program to be portable to all awk implementations. Using getline into a Variable from a File Use ‘getline var < file’ to read input from the file file, and put it in the variable var. As earlier, file is a string-valued expression that specifies the file from which to read. 80 | Chapter 4: Reading Input Files 8. This is not quite true. RT could be changed if RS is a regular expression. In this version of getline, none of the predefined variables are changed and the record is not split into fields. The only variable changed is var.8 For example, the following program copies all the input files to the output, except for records that say ‘@include filename’. Such a record is replaced by the contents of the file filename: { if (NF == 2 && $1 == "@include") { while ((getline line < $2) > 0) print line close($2) } else print } Note here how the name of the extra input file is not built into the program; it is taken directly from the data, specifically from the second field on the @include line. The close() function is called to ensure that if two identical @include lines appear in the input, the entire specified file is included twice. See “Closing Input and Output Redirections” on page 105. One deficiency of this program is that it does not process nested @include statements (i.e., @include statements in included files) the way a true macro preprocessor would. See “An Easy Way to Use Library Functions” on page 314 for a program that does handle nested @include statements. Using getline from a Pipe Omniscience has much to recommend it. Failing that, attention to details would be useful. —Brian Kernighan The output of a command can also be piped into getline, using ‘command | getline’. In this case, the string command is run as a shell command and its output is piped into awk to be used as input. This form of getline reads one record at a time from the pipe. For example, the following program copies its input to its output, except for lines that begin with ‘@execute’, which are replaced by the output produced by running the rest of the line as a shell command: { if ($1 == "@execute") { tmp = substr($0, 10) # Remove "@execute" while ((tmp | getline) > 0) print close(tmp) } else Explicit Input with getline | 81 print } The close() function is called to ensure that if two identical ‘@execute’ lines appear in the input, the command is run for each one. Given the input: foo bar baz @execute who bletch the program might produce: foo bar baz arnold ttyv0 Jul 13 14:22 miriam ttyp0 Jul 13 14:23 (murphy:0) bill ttyp1 Jul 13 14:23 (murphy:0) bletch Notice that this program ran the command who and printed the result. (If you try this program yourself, you will of course get different results, depending upon who is logged in on your system.) This variation of getline splits the record into fields, sets the value of NF, and recom‐ putes the value of $0. The values of NR and FNR are not changed. RT is set. According to POSIX, ‘expression | getline’ is ambiguous if expression contains unparenthesized operators other than ‘$’—for example, ‘"echo " "date" | getline’ is ambiguous because the concatenation operator is not parenthesized. You should write it as ‘("echo " "date") | getline’ if you want your program to be portable to all awk implementations. Unfortunately, gawk has not been consistent in its treatment of a construct like ‘"echo " "date" | getline’. Most versions, includ‐ ing the current version, treat it at as ‘("echo " "date") | get line’. (This is also how BWK awk behaves.) Some versions instead treat it as ‘"echo " ("date" | getline)’. (This is how mawk be‐ haves.) In short, always use explicit parentheses, and then you won’t have to worry. Using getline into a Variable from a Pipe When you use ‘command | getline var’, the output of command is sent through a pipe to getline and into the variable var. For example, the following program reads the 82 | Chapter 4: Reading Input Files current date and time into the variable current_time, using the date utility, and then prints it: BEGIN { "date" | getline current_time close("date") print "Report printed on " current_time } In this version of getline, none of the predefined variables are changed and the record is not split into fields. However, RT is set. Using getline from a Coprocess Reading input into getline from a pipe is a one-way operation. The command that is started with ‘command | getline’ only sends data to your awk program. On occasion, you might want to send data to another program for processing and then read the results back. gawk allows you to start a coprocess, with which two-way com‐ munications are possible. This is done with the ‘|&’ operator. Typically, you write data to the coprocess first and then read the results back, as shown in the following: print "some query" |& "db_server" "db_server" |& getline which sends a query to db_server and then reads the results. The values of NR and FNR are not changed, because the main input stream is not used. However, the record is split into fields in the normal manner, thus changing the values of $0, of the other fields, and of NF and RT. Coprocesses are an advanced feature. They are discussed here only because this is the section on getline. See “Two-Way Communications with Another Process” on page 334, where coprocesses are discussed in more detail. Using getline into a Variable from a Coprocess When you use ‘command |& getline var’, the output from the coprocess command is sent through a two-way pipe to getline and into the variable var. In this version of getline, none of the predefined variables are changed and the record is not split into fields. The only variable changed is var. However, RT is set. Points to Remember About getline Here are some miscellaneous points about getline that you should bear in mind: Explicit Input with getline | 83 • When getline changes the value of $0 and NF, awk does not automatically jump to the start of the program and start testing the new record against every pattern. However, the new record is tested against any subsequent rules. • Some very old awk implementations limit the number of pipelines that an awk program may have open to just one. In gawk, there is no such limit. You can open as many pipelines (and coprocesses) as the underlying operating system permits. • An interesting side effect occurs if you use getline without a redirection inside a BEGIN rule. Because an unredirected getline reads from the command-line datafiles, the first getline command causes awk to set the value of FILENAME. Nor‐ mally, FILENAME does not have a value inside BEGIN rules, because you have not yet started to process the command-line datafiles. (d.c.) (See “The BEGIN and END Special Patterns” on page 145; also see “Built-in Variables That Convey Informa‐ tion” on page 163.) • Using FILENAME with getline (‘getline < FILENAME’) is likely to be a source of confusion. awk opens a separate input stream from the current input file. However, by not using a variable, $0 and NF are still updated. If you’re doing this, it’s probably by accident, and you should reconsider what it is you’re trying to accomplish. • The next section presents a table summarizing the getline variants and which variables they can affect. It is worth noting that those variants that do not use re‐ direction can cause FILENAME to be updated if they cause awk to start reading a new input file. • If the variable being assigned is an expression with side effects, different versions of awk behave differently upon encountering end-of-file. Some versions don’t eval‐ uate the expression; many versions (including gawk) do. Here is an example, cour‐ tesy of Duncan Moore: BEGIN { system("echo 1 > f") while ((getline a[++c] < "f") > 0) { } print c } Here, the side effect is the ‘++c’. Is c incremented if end-of-file is encountered before the element in a is assigned? gawk treats getline like a function call, and evaluates the expression ‘a[++c]’ before attempting to read from f. However, some versions of awk only evaluate the ex‐ pression once they know that there is a string value to be assigned. 84 | Chapter 4: Reading Input Files 9. This assumes that standard input is the keyboard. Summary of getline Variants Table 4-1 summarizes the eight variants of getline, listing which predefined variables are set by each one, and whether the variant is standard or a gawk extension. Note: for each variant, gawk sets the RT predefined variable. Table 4-1. getline variants and what they set Variant Effect awk / gawk getline Sets $0, NF, FNR, NR, and RT awk getline var Sets var, FNR, NR, and RT awk getline < file Sets $0, NF, and RT awk getline var < file Sets var and RT awk command | getline Sets $0, NF, and RT awk command | getline var Sets var and RT awk command |& getline Sets $0, NF, and RT gawk command |& getline var Sets var and RT gawk Reading Input with a Timeout This section describes a feature that is specific to gawk. You may specify a timeout in milliseconds for reading input from the keyboard, a pipe, or two-way communication, including TCP/IP sockets. This can be done on a per-input, per-command, or per-connection basis, by setting a special element in the PROCINFO array (see “Built-in Variables That Convey Information” on page 163): PROCINFO["input_name", "READ_TIMEOUT"] = timeout in milliseconds When set, this causes gawk to time out and return failure if no data is available to read within the specified timeout period. For example, a TCP client can decide to give up on receiving any response from the server after a certain amount of time: Service = "/inet/tcp/0/localhost/daytime" PROCINFO[Service, "READ_TIMEOUT"] = 100 if ((Service |& getline) > 0) print $0 else if (ERRNO != "") print ERRNO Here is how to read interactively from the user9 without waiting for more than five seconds: Reading Input with a Timeout | 85 PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000 while ((getline < "/dev/stdin") > 0) print $0 gawk terminates the read operation if input does not arrive after waiting for the timeout period, returns failure, and sets ERRNO to an appropriate string value. A negative or zero value for the timeout is the same as specifying no timeout at all. A timeout can also be set for reading from the keyboard in the implicit loop that reads input records and matches them against patterns, like so: $ gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 } > { print "You entered: " $0 }' gawk You entered: gawk In this case, failure to respond within five seconds results in the following error message: error→ gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input file `-': Connection timed out The timeout can be set or changed at any time, and will take effect on the next attempt to read from the input device. In the following example, we start with a timeout value of one second, and progressively reduce it by one-tenth of a second until we wait in‐ definitely for the input to arrive: PROCINFO[Service, "READ_TIMEOUT"] = 1000 while ((Service |& getline) > 0) { print $0 PROCINFO[Service, "READ_TIMEOUT"] -= 100 } You should not assume that the read operation will block exactly after the tenth record has been printed. It is possible that gawk will read and buffer more than one record’s worth of data the first time. Be‐ cause of this, changing the value of timeout like the preceding exam‐ ple is not very useful. If the PROCINFO element is not present and the GAWK_READ_TIMEOUT environment vari‐ able exists, gawk uses its value to initialize the timeout value. The exclusive use of the environment variable to specify timeout has the disadvantage of not being able to control it on a per-command or per-connection basis. gawk considers a timeout event to be an error even though the attempt to read from the underlying device may succeed in a later attempt. This is a limitation, and it also means that you cannot use this to multiplex input from two or more sources. Assigning a timeout value prevents read operations from blocking indefinitely. But bear in mind that there are other ways gawk can stall waiting for an input device to be ready. 86 | Chapter 4: Reading Input Files A network client can sometimes take a long time to establish a connection before it can start reading any data, or the attempt to open a FIFO special file for reading can block indefinitely until some other process opens it for writing. Directories on the Command Line According to the POSIX standard, files named on the awk command line must be text files; it is a fatal error if they are not. Most versions of awk treat a directory on the command line as a fatal error. By default, gawk produces a warning for a directory on the command line, but otherwise ignores it. This makes it easier to use shell wildcards with your awk program: $ gawk -f whizprog.awk * Directories could kill this program If either of the --posix or --traditional options is given, then gawk reverts to treating a directory on the command line as a fatal error. See “Reading Directories” on page 448 for a way to treat directories as usable data from an awk program. Summary • Input is split into records based on the value of RS. The possibilities are as follows: Value of RS Records are split on… awk / gawk Any single character That character awk The empty string ("") Runs of two or more newlines awk A regexp Text that matches the regexp gawk • FNR indicates how many records have been read from the current input file; NR indicates how many records have been read in total. • gawk sets RT to the text matched by RS. • After splitting the input into records, awk further splits the records into individual fields, named $1, $2, and so on. $0 is the whole record, and NF indicates how many fields there are. The default way to split fields is between whitespace characters. • Fields may be referenced using a variable, as in $NF. Fields may also be assigned values, which causes the value of $0 to be recomputed when it is later referenced. Assigning to a field with a number greater than NF creates the field and rebuilds the record, using OFS to separate the fields. Incrementing NF does the same thing. Dec‐ rementing NF throws away fields and rebuilds the record. • Field splitting is more complicated than record splitting: Directories on the Command Line | 87 Field separator value Fields are split … awk / gawk FS == " " On runs of whitespace awk FS == any single character On that character awk FS == regexp On text matching the regexp awk FS == "" Such that each individual character is a separate field gawk FIELDWIDTHS == list of columns Based on character position gawk FPAT == regexp On the text surrounding text matching the regexp gawk • Using ‘FS = "\n"’ causes the entire record to be a single field (assuming that new‐ lines separate records). • FS may be set from the command line using the -F option. This can also be done using command-line variable assignment. • Use PROCINFO["FS"] to see how fields are being split. • Use getline in its various forms to read additional records, from the default input stream, from a file, or from a pipe or coprocess. • Use PROCINFO[file, "READ_TIMEOUT"] to cause reads to time out for file. • Directories on the command line are fatal for standard awk; gawk ignores them if not in POSIX mode. 88 | Chapter 4: Reading Input Files CHAPTER 5 Printing Output One of the most common programming actions is to print, or output, some or all of the input. Use the print statement for simple output, and the printf statement for fancier formatting. The print statement is not limited when computing which values to print. However, with two exceptions, you cannot specify how to print them—how many col‐ umns, whether to use exponential notation or not, and so on. (For the exceptions, see “Output Separators” on page 91 and “Controlling Numeric Output with print” on page 92.) For printing with specifications, you need the printf statement (see “Using printf Statements for Fancier Printing” on page 93). Besides basic and formatted printing, this chapter also covers I/O redirections to files and pipes, introduces the special filenames that gawk processes internally, and discusses the close() built-in function. The print Statement Use the print statement to produce output with simple, standardized formatting. You specify only the strings or numbers to print, in a list separated by commas. They are output, separated by single spaces, followed by a newline. The statement looks like this: print item1, item2, … The entire list of items may be optionally enclosed in parentheses. The parentheses are necessary if any of the item expressions uses the ‘>’ relational operator; otherwise it could be confused with an output redirection (see “Redirecting Output of print and printf” on page 100). The items to print can be constant strings or numbers, fields of the current record (such as $1), variables, or any awk expression. Numeric values are converted to strings and then printed. 89 The simple statement ‘print’ with no items is equivalent to ‘print $0’: it prints the entire current record. To print a blank line, use ‘print ""’. To print a fixed piece of text, use a string constant, such as "Don't Panic", as one item. If you forget to use the double- quote characters, your text is taken as an awk expression, and you will probably get an error. Keep in mind that a space is printed between any two items. Note that the print statement is a statement and not an expression—you can’t use it in the pattern part of a pattern–action statement, for example. print Statement Examples Each print statement makes at least one line of output. However, it isn’t limited to only one line. If an item value is a string containing a newline, the newline is output along with the rest of the string. A single print statement can make any number of lines this way. The following is an example of printing a string that contains embedded newlines: $ awk 'BEGIN { print "line one\nline two\nline three" }' line one line two line three The next example, which is run on the inventory-shipped file, prints the first two fields of each input record, with a space between them: $ awk '{ print $1, $2 }' inventory-shipped Jan 13 Feb 15 Mar 15 … A common mistake in using the print statement is to omit the comma between two items. This often has the effect of making the items run together in the output, with no space. The reason for this is that juxtaposing two string expressions in awk means to concatenate them. Here is the same program, without the comma: $ awk '{ print $1 $2 }' inventory-shipped Jan13 Feb15 Mar15 … To someone unfamiliar with the inventory-shipped file, neither example’s output makes much sense. A heading line at the beginning would make it clearer. Let’s add some headings to our table of months ($1) and green crates shipped ($2). We do this using a BEGIN rule (see “The BEGIN and END Special Patterns” on page 145) so that the headings are only printed once: 90 | Chapter 5: Printing Output awk 'BEGIN { print "Month Crates" print "----- ------" } { print $1, $2 }' inventory-shipped When run, the program prints the following: Month Crates ----- ------ Jan 13 Feb 15 Mar 15 … The only problem, however, is that the headings and the table data don’t line up! We can fix this by printing some spaces between the two fields: awk 'BEGIN { print "Month Crates" print "----- ------" } { print $1, " ", $2 }' inventory-shipped Lining up columns this way can get pretty complicated when there are many columns to fix. Counting spaces for two or three columns is simple, but any more than this can take up a lot of time. This is why the printf statement was created (see “Using printf Statements for Fancier Printing” on page 93); one of its specialties is lining up columns of data. You can continue either a print or printf statement simply by putting a newline after any comma (see “awk Statements Versus Lines” on page 16). Output Separators As mentioned previously, a print statement contains a list of items separated by com‐ mas. In the output, the items are normally separated by single spaces. However, this doesn’t need to be the case; a single space is simply the default. Any string of characters may be used as the output field separator by setting the predefined variable OFS. The initial value of this variable is the string " " (i.e., a single space). The output from an entire print statement is called an output record. Each print statement outputs one output record, and then outputs a string called the output record separator (or ORS). The initial value of ORS is the string "\n" (i.e., a newline character). Thus, each print statement normally makes a separate line. In order to change how output fields and records are separated, assign new values to the variables OFS and ORS. The usual place to do this is in the BEGIN rule (see “The BEGIN and END Special Patterns” on page 145), so that it happens before any input is processed. Output Separators | 91 It can also be done with assignments on the command line, before the names of the input files, or using the -v command-line option (see “Command-Line Options” on page 22). The following example prints the first and second fields of each input record, separated by a semicolon, with a blank line added after each newline: $ awk 'BEGIN { OFS = ";"; ORS = "\n\n" } > { print $1, $2 }' mail-list Amelia;555-5553 Anthony;555-3412 Becky;555-7685 Bill;555-1675 Broderick;555-0542 Camilla;555-2912 Fabius;555-1234 Julie;555-6699 Martin;555-6480 Samuel;555-3430 Jean-Paul;555-2127 If the value of ORS does not contain a newline, the program’s output runs together on a single line. Controlling Numeric Output with print When printing numeric values with the print statement, awk internally converts each number to a string of characters and prints that string. awk uses the sprintf() func‐ tion to do this conversion (see “String-Manipulation Functions” on page 194). For now, it suffices to say that the sprintf() function accepts a format specification that tells it how to format numbers (or strings), and that there are a number of different ways in which numbers can be formatted. The different format specifications are discussed more fully in “Format-Control Letters” on page 94. The predefined variable OFMT contains the format specification that print uses with sprintf() when it wants to convert a number to a string for printing. The default value of OFMT is "%.6g". The way print prints numbers can be changed by supplying a different format specification for the value of OFMT, as shown in the following example: 92 | Chapter 5: Printing Output $ awk 'BEGIN { > OFMT = "%.0f" # print numbers as integers (rounds) > print 17.23, 17.54 }' 17 18 According to the POSIX standard, awk’s behavior is undefined if OFMT contains anything but a floating-point conversion specification. (d.c.) Using printf Statements for Fancier Printing For more precise control over the output format than what is provided by print, use printf. With printf you can specify the width to use for each item, as well as various formatting choices for numbers (such as what output base to use, whether to print an exponent, whether to print a sign, and how many digits to print after the decimal point). Introduction to the printf Statement A simple printf statement looks like this: printf format, item1, item2, … As for print, the entire list of arguments may optionally be enclosed in parentheses. Here too, the parentheses are necessary if any of the item expressions uses the ‘>’ rela‐ tional operator; otherwise, it can be confused with an output redirection (see “Redi‐ recting Output of print and printf” on page 100). The difference between printf and print is the format argument. This is an expression whose value is taken as a string; it specifies how to output each of the other argu‐ ments. It is called the format string. The format string is very similar to that in the ISO C library function printf(). Most of format is text to output verbatim. Scattered among this text are format specifiers— one per item. Each format specifier says to output the next item in the argument list at that place in the format. The printf statement does not automatically append a newline to its output. It outputs only what the format string specifies. So if a newline is needed, you must include one in the format string. The output separator variables OFS and ORS have no effect on printf statements. For example: $ awk 'BEGIN { > ORS = "\nOUCH!\n"; OFS = "+" > msg = "Don\47t Panic!" > printf "%s\n", msg > }' Don't Panic! Here, neither the ‘+’ nor the ‘OUCH!’ appears in the output message. Using printf Statements for Fancier Printing | 93 Format-Control Letters A format specifier starts with the character ‘%’ and ends with a format-control letter—it tells the printf statement how to output one item. The format-control letter specifies what kind of value to print. The rest of the format specifier is made up of optional modifiers that control how to print the value, such as the field width. Here is a list of the format-control letters: %c Print a number as a character; thus, ‘printf "%c", 65’ outputs the letter ‘A’. The output for a string value is the first character of the string. The POSIX standard says the first character of a string is print‐ ed. In locales with multibyte characters, gawk attempts to con‐ vert the leading bytes of the string into a valid wide character and then to print the multibyte encoding of that character. Similar‐ ly, when printing a numeric value, gawk allows the value to be within the numeric range of values that can be held in a wide character. If the conversion to multibyte encoding fails, gawk uses the low eight bits of the value as the character to print. Other awk versions generally restrict themselves to printing the first byte of a string or to numeric values within the range of a single byte (0–255). %d, %i Print a decimal integer. The two control letters are equivalent. (The ‘%i’ specification is for compatibility with ISO C.) %e, %E Print a number in scientific (exponential) notation. For example: printf "%4.3e\n", 1950 prints ‘1.950e+03’, with a total of four significant figures, three of which follow the decimal point. (The ‘4.3’ represents two modifiers, discussed in the next subsection.) ‘%E’ uses ‘E’ instead of ‘e’ in the output. %f Print a number in floating-point notation. For example: printf "%4.3f", 1950 prints ‘1950.000’, with a total of four significant figures, three of which follow the decimal point. (The ‘4.3’ represents two modifiers, discussed in the next sub‐ section.) 94 | Chapter 5: Printing Output On systems supporting IEEE 754 floating-point format, values representing nega‐ tive infinity are formatted as ‘-inf’ or ‘-infinity’, and positive infinity as ‘inf’ or ‘infinity’. The special “not a number” value formats as ‘-nan’ or ‘nan’ (see “Other Stuff to Know” on page 379). %F Like ‘%f’, but the infinity and “not a number” values are spelled using uppercase letters. The ‘%F’ format is a POSIX extension to ISO C; not all systems support it. On those that don’t, gawk uses ‘%f’ instead. %g, %G Print a number in either scientific notation or in floating-point notation, whichever uses fewer characters; if the result is printed in scientific notation, ‘%G’ uses ‘E’ in‐ stead of ‘e’. %o Print an unsigned octal integer (see “Octal and hexadecimal numbers” on page 112). %s Print a string. %u Print an unsigned decimal integer. (This format is of marginal use, because all numbers in awk are floating point; it is provided primarily for compatibility with C.) %x, %X Print an unsigned hexadecimal integer; ‘%X’ uses the letters ‘A’ through ‘F’ instead of ‘a’ through ‘f’ (see “Octal and hexadecimal numbers” on page 112). %% Print a single ‘%’. This does not consume an argument and it ignores any modifiers. When using the integer format-control letters for values that are out‐ side the range of the widest C integer type, gawk switches to the ‘%g’ format specifier. If --lint is provided on the command line (see “Command-Line Options” on page 22), gawk warns about this. Oth‐ er versions of awk may print invalid values or do something else entirely. (d.c.) Using printf Statements for Fancier Printing | 95 Modifiers for printf Formats A format specification can also include modifiers that can control how much of the item’s value is printed, as well as how much space it gets. The modifiers come between the ‘%’ and the format-control letter. We use the bullet symbol “•” in the following examples to represent spaces in the output. Here are the possible modifiers, in the order in which they may appear: N$ An integer constant followed by a ‘$’ is a positional specifier. Normally, format specifications are applied to arguments in the order given in the format string. With a positional specifier, the format specification is applied to a specific argument, instead of what would be the next argument in the list. Positional specifiers begin counting with one. Thus: printf "%s %s\n", "don't", "panic" printf "%2$s %1$s\n", "panic", "don't" prints the famous friendly message twice. At first glance, this feature doesn’t seem to be of much use. It is in fact a gawk extension, intended for use in translating messages at runtime. See “Rearranging printf Arguments” on page 351, which describes how and why to use positional speci‐ fiers. For now, we ignore them. - (Minus) The minus sign, used before the width modifier (see later on in this list), says to left-justify the argument within its specified width. Normally, the argument is printed right-justified in the specified width. Thus: printf "%-4s", "foo" prints ‘foo•’. space For numeric conversions, prefix positive values with a space and negative values with a minus sign. + The plus sign, used before the width modifier (see later on in this list), says to always supply a sign for numeric conversions, even if the data to format is positive. The ‘+’ overrides the space modifier. # Use an “alternative form” for certain control letters. For ‘%o’, supply a leading zero. For ‘%x’ and ‘%X’, supply a leading ‘0x’ or ‘0X’ for a nonzero result. For ‘%e’, ‘%E’, ‘%f’, and ‘%F’, the result always contains a decimal point. For ‘%g’ and ‘%G’, trailing zeros are not removed from the result. 96 | Chapter 5: Printing Output 0 A leading ‘0’ (zero) acts as a flag indicating that output should be padded with zeros instead of spaces. This applies only to the numeric output formats. This flag only has an effect when the field width is wider than the value to print. ' A single quote or apostrophe character is a POSIX extension to ISO C. It indicates that the integer part of a floating-point value, or the entire part of an integer decimal value, should have a thousands-separator character in it. This only works in locales that support such characters. For example: $ cat thousands.awk Show source program BEGIN { printf "%'d\n", 1234567 } $ LC_ALL=C gawk -f thousands.awk 1234567 Results in "C" locale $ LC_ALL=en_US.UTF-8 gawk -f thousands.awk 1,234,567 Results in US English UTF locale For more information about locales and internationalization issues, see “Where You Are Makes a Difference” on page 138. The ‘'’ flag is a nice feature, but its use complicates things: it becomes difficult to use it in command-line programs. For in‐ formation on appropriate quoting tricks, see “Shell Quoting Is‐ sues” on page 8. width This is a number specifying the desired minimum width of a field. Inserting any number between the ‘%’ sign and the format-control character forces the field to expand to this width. The default way to do this is to pad with spaces on the left. For example: printf "%4s", "foo" prints ‘•foo’. The value of width is a minimum width, not a maximum. If the item value requires more than width characters, it can be as wide as necessary. Thus, the following: printf "%4s", "foobar" prints ‘foobar’. Preceding the width with a minus sign causes the output to be padded with spaces on the right, instead of on the left. Using printf Statements for Fancier Printing | 97 .prec A period followed by an integer constant specifies the precision to use when print‐ ing. The meaning of the precision varies by control letter: %d, %i, %o, %u, %x, %X Minimum number of digits to print. %e, %E, %f, %F Number of digits to the right of the decimal point. %g, %G Maximum number of significant digits. %s Maximum number of characters from the string that should print. Thus, the following: printf "%.4s", "foobar" prints ‘foob’. The C library printf’s dynamic width and prec capability (e.g., "%*.*s") is support‐ ed. Instead of supplying explicit width and/or prec values in the format string, they are passed in the argument list. For example: w = 5 p = 3 s = "abcdefg" printf "%*.*s\n", w, p, s is exactly equivalent to: s = "abcdefg" printf "%5.3s\n", s Both programs output ‘••abc’. Earlier versions of awk did not support this capability. If you must use such a version, you may simulate this feature by using concatenation to build up the format string, like so: w = 5 p = 3 s = "abcdefg" printf "%" w "." p "s\n", s This is not particularly easy to read, but it does work. C programmers may be used to supplying additional modifiers (‘h’, ‘j’, ‘l’, ‘L’, ‘t’, and ‘z’) in printf format strings. These are not valid in awk. Most awk implementations silently ignore them. If --lint is provided on the command line (see “Command-Line Op‐ tions” on page 22), gawk warns about their use. If --posix is supplied, their use is a fatal error. 98 | Chapter 5: Printing Output Examples Using printf The following simple example shows how to use printf to make an aligned table: awk '{ printf "%-10s %s\n", $1, $2 }' mail-list This command prints the names of the people ($1) in the file mail-list as a string of 10 characters that are left-justified. It also prints the phone numbers ($2) next on the line. This produces an aligned two-column table of names and phone numbers, as shown here: $ awk '{ printf "%-10s %s\n", $1, $2 }' mail-list Amelia 555-5553 Anthony 555-3412 Becky 555-7685 Bill 555-1675 Broderick 555-0542 Camilla 555-2912 Fabius 555-1234 Julie 555-6699 Martin 555-6480 Samuel 555-3430 Jean-Paul 555-2127 In this case, the phone numbers had to be printed as strings because the numbers are separated by dashes. Printing the phone numbers as numbers would have produced just the first three digits: ‘555’. This would have been pretty confusing. It wasn’t necessary to specify a width for the phone numbers because they are last on their lines. They don’t need to have spaces after them. The table could be made to look even nicer by adding headings to the tops of the col‐ umns. This is done using a BEGIN rule (see “The BEGIN and END Special Patterns” on page 145) so that the headers are only printed once, at the beginning of the awk program: awk 'BEGIN { print "Name Number" print "---- ------" } { printf "%-10s %s\n", $1, $2 }' mail-list The preceding example mixes print and printf statements in the same program. Using just printf statements can produce the same results: awk 'BEGIN { printf "%-10s %s\n", "Name", "Number" printf "%-10s %s\n", "----", "------" } { printf "%-10s %s\n", $1, $2 }' mail-list Printing each column heading with the same format specification used for the column elements ensures that the headings are aligned just like the columns. The fact that the same format specification is used three times can be emphasized by storing it in a variable, like this: Using printf Statements for Fancier Printing | 99 awk 'BEGIN { format = "%-10s %s\n" printf format, "Name", "Number" printf format, "----", "------" } { printf format, $1, $2 }' mail-list Redirecting Output of print and printf So far, the output from print and printf has gone to the standard output, usually the screen. Both print and printf can also send their output to other places. This is called redirection. When --sandbox is specified (see “Command-Line Options” on page 22), redirecting output to files, pipes, and coprocesses is disabled. A redirection appears after the print or printf statement. Redirections in awk are written just like redirections in shell commands, except that they are written inside the awk program. There are four forms of output redirection: output to a file, output appended to a file, output through a pipe to another command, and output to a coprocess. We show them all for the print statement, but they work identically for printf: print items > output-file This redirection prints the items into the output file named output-file. The file‐ name output-file can be any expression. Its value is changed to a string and then used as a filename (see Chapter 6). When this type of redirection is used, the output-file is erased before the first output is written to it. Subsequent writes to the same output-file do not erase output-file, but append to it. (This is different from how you use redirections in shell scripts.) If output-file does not exist, it is created. For example, here is how an awk program can write a list of peoples’ names to one file named name-list, and a list of phone numbers to another file named phone-list: $ awk '{ print $2 > "phone-list" > print $1 > "name-list" }' mail-list $ cat phone-list 555-5553 555-3412 … $ cat name-list Amelia Anthony … 100 | Chapter 5: Printing Output Each output file contains one name or number per line. print items >> output-file This redirection prints the items into the preexisting output file named output- file. The difference between this and the single-‘>’ redirection is that the old con‐ tents (if any) of output-file are not erased. Instead, the awk output is appended to the file. If output-file does not exist, then it is created. print items | command It is possible to send output to another program through a pipe instead of into a file. This redirection opens a pipe to command, and writes the values of items through this pipe to another process created to execute command. The redirection argument command is actually an awk expression. Its value is con‐ verted to a string whose contents give the shell command to be run. For example, the following produces two files, one unsorted list of peoples’ names, and one list sorted in reverse alphabetical order: awk '{ print $1 > "names.unsorted" command = "sort -r > names.sorted" print $1 | command }' mail-list The unsorted list is written with an ordinary redirection, while the sorted list is written by piping through the sort utility. The next example uses redirection to mail a message to the mailing list bug- system. This might be useful when trouble is encountered in an awk script run periodically for system maintenance: report = "mail bug-system" print("Awk script failed:", $0) | report print("at record number", FNR, "of", FILENAME) | report close(report) The close() function is called here because it’s a good idea to close the pipe as soon as all the intended output has been sent to it. See “Closing Input and Output Re‐ directions” on page 105 for more information. This example also illustrates the use of a variable to represent a file or command— it is not necessary to always use a string constant. Using a variable is generally a good idea, because (if you mean to refer to that same file or command) awk requires that the string value be written identically every time. print items |& command This redirection prints the items to the input of command. The difference between this and the single-‘|’ redirection is that the output from command can be read with getline. Thus, command is a coprocess, which works together with but is subsidiary to the awk program. Redirecting Output of print and printf | 101 This feature is a gawk extension, and is not available in POSIX awk. See “Using getline from a Coprocess” on page 83 for a brief discussion and “Two-Way Com‐ munications with Another Process” on page 334 for a more complete discussion. Redirecting output using ‘>’, ‘>>’, ‘|’, or ‘|&’ asks the system to open a file, pipe, or cop‐ rocess only if the particular file or command you specify has not already been written to by your program or if it has been closed since it was last written to. It is a common error to use ‘>’ redirection for the first print to a file, and then to use ‘>>’ for subsequent output: # clear the file print "Don't panic" > "guide.txt" … # append print "Avoid improbability generators" >> "guide.txt" This is indeed how redirections must be used from the shell. But in awk, it isn’t necessary. In this kind of case, a program should use ‘>’ for all the print statements, because the output file is only opened once. (It happens that if you mix ‘>’ and ‘>>’ the output is produced in the expected order. However, mixing the operators for the same file is definitely poor style, and is confusing to readers of your program.) As mentioned earlier (see “Points to Remember About getline” on page 83), many older awk implementations limit the number of pipelines that an awk program may have open to just one! In gawk, there is no such limit. gawk allows a program to open as many pipelines as the underlying operating system permits. Piping into sh A particularly powerful way to use redirection is to build command lines and pipe them into the shell, sh. For example, suppose you have a list of files brought over from a system where all the filenames are stored in uppercase, and you wish to rename them to have names in all lowercase. The following program is both simple and efficient: { printf("mv %s %s\n", $0, tolower($0)) | "sh" } END { close("sh") } The tolower() function returns its argument string with all uppercase characters con‐ verted to lowercase (see “String-Manipulation Functions” on page 194). The program builds up a list of command lines, using the mv utility to rename the files. It then sends the list to the shell for execution. See “Quoting Strings to Pass to the Shell” on page 253 for a function that can help in generating command lines to be fed to the shell. 102 | Chapter 5: Printing Output 1. The “tty” in /dev/tty stands for “Teletype,” a serial terminal. Special Files for Standard Preopened Data Streams Running programs conventionally have three input and output streams already avail‐ able to them for reading and writing. These are known as the standard input, standard output, and standard error output. These open streams (and any other open files or pipes) are often referred to by the technical term file descriptors. These streams are, by default, connected to your keyboard and screen, but they are often redirected with the shell, via the ‘<’, ‘<<’, ‘>’, ‘>>’, ‘>&’, and ‘|’ operators. Standard error is typically used for writing error messages; the reason there are two separate streams, standard output and standard error, is so that they can be redirected separately. In traditional implementations of awk, the only way to write an error message to stan‐ dard error in an awk program is as follows: print "Serious error detected!" | "cat 1>&2" This works by opening a pipeline to a shell command that can access the standard error stream that it inherits from the awk process. This is far from elegant, and it also requires a separate process. So people writing awk programs often don’t do this. Instead, they send the error messages to the screen, like this: print "Serious error detected!" > "/dev/tty" (/dev/tty is a special file supplied by the operating system that is connected to your keyboard and screen. It represents the “terminal,”1 which on modern systems is a key‐ board and screen, not a serial console.) This generally has the same effect, but not always: although the standard error stream is usually the screen, it can be redirected; when that happens, writing to the screen is not correct. In fact, if awk is run from a background job, it may not have a terminal at all. Then opening /dev/tty fails. gawk, BWK awk, and mawk provide special filenames for accessing the three standard streams. If the filename matches one of these special names when gawk (or one of the others) redirects input or output, then it directly uses the descriptor that the filename stands for. These special filenames work for all operating systems that gawk has been ported to, not just those that are POSIX-compliant: /dev/stdin The standard input (file descriptor 0). /dev/stdout The standard output (file descriptor 1). Special Files for Standard Preopened Data Streams | 103 /dev/stderr The standard error output (file descriptor 2). With these facilities, the proper way to write an error message then becomes: print "Serious error detected!" > "/dev/stderr" Note the use of quotes around the filename. Like with any other redirection, the value must be a string. It is a common error to omit the quotes, which leads to confusing results. gawk does not treat these filenames as special when in POSIX-compatibility mode. However, because BWK awk supports them, gawk does support them even when invoked with the --traditional option (see “Command-Line Options” on page 22). Special Filenames in gawk Besides access to standard input, standard output, and standard error, gawk provides access to any open file descriptor. Additionally, there are special filenames reserved for TCP/IP networking. Accessing Other Open Files with gawk Besides the /dev/stdin, /dev/stdout, and /dev/stderr special filenames mentioned earlier, gawk provides syntax for accessing any other inherited open file: /dev/fd/N The file associated with file descriptor N. Such a file must be opened by the program initiating the awk execution (typically the shell). Unless special pains are taken in the shell from which gawk is invoked, only descriptors 0, 1, and 2 are available. The filenames /dev/stdin, /dev/stdout, and /dev/stderr are essentially aliases for /dev/fd/0, /dev/fd/1, and /dev/fd/2, respectively. However, those names are more self-explanatory. Note that using close() on a filename of the form "/dev/fd/N ", for file descriptor numbers above two, does actually close the given file descriptor. Special Files for Network Communications gawk programs can open a two-way TCP/IP connection, acting as either a client or a server. This is done using a special filename of the form: /net-type/protocol/local-port/remote-host/remote-port The net-type is one of ‘inet’, ‘inet4’, or ‘inet6’. The protocol is one of ‘tcp’ or ‘udp’, and the other fields represent the other essential pieces of information for making a 104 | Chapter 5: Printing Output networking connection. These filenames are used with the ‘|&’ operator for communi‐ cating with a coprocess (see “Two-Way Communications with Another Process” on page 334). This is an advanced feature, mentioned here only for completeness. Full dis‐ cussion is delayed until “Using gawk for Network Programming” on page 337. Special Filename Caveats Here are some things to bear in mind when using the special filenames that gawk provides: • Recognition of the filenames for the three standard preopened files is disabled only in POSIX mode. • Recognition of the other special filenames is disabled if gawk is in compatibility mode (either --traditional or --posix; see “Command-Line Options” on page 22). • gawk always interprets these special filenames. For example, using ‘/dev/fd/4’ for output actually writes on file descriptor 4, and not on a new file descriptor that is dup()ed from file descriptor 4. Most of the time this does not matter; however, it is important to not close any of the files related to file descriptors 0, 1, and 2. Doing so results in unpredictable behavior. Closing Input and Output Redirections If the same filename or the same shell command is used with getline more than once during the execution of an awk program (see “Explicit Input with getline” on page 77), the file is opened (or the command is executed) the first time only. At that time, the first record of input is read from that file or command. The next time the same file or com‐ mand is used with getline, another record is read from it, and so on. Similarly, when a file or pipe is opened for output, awk remembers the filename or command associated with it, and subsequent writes to the same file or command are appended to the previous writes. The file or pipe stays open until awk exits. This implies that special steps are necessary in order to read the same file again from the beginning, or to rerun a shell command (rather than reading more output from the same command). The close() function makes these things possible: close(filename) or: close(command) Closing Input and Output Redirections | 105 The argument filename or command can be any expression. Its value must exactly match the string that was used to open the file or start the command (spaces and other “irrel‐ evant” characters included). For example, if you open a pipe with this: "sort -r names" | getline foo then you must close it with this: close("sort -r names") Once this function call is executed, the next getline from that file or command, or the next print or printf to that file or command, reopens the file or reruns the command. Because the expression that you use to close a file or pipeline must exactly match the expression used to open the file or run the command, it is good practice to use a variable to store the filename or command. The previous example becomes the following: sortcom = "sort -r names" sortcom | getline foo … close(sortcom) This helps avoid hard-to-find typographical errors in your awk programs. Here are some of the reasons for closing an output file: • To write a file and read it back later on in the same awk program. Close the file after writing it, then begin reading it with getline. • To write numerous files, successively, in the same awk program. If the files aren’t closed, eventually awk may exceed a system limit on the number of open files in one process. It is best to close each one when the program has finished writing it. • To make a command finish. When output is redirected through a pipe, the com‐ mand reading the pipe normally continues to try to read input as long as the pipe is open. Often this means the command cannot really do its work until the pipe is closed. For example, if output is redirected to the mail program, the message is not actually sent until the pipe is closed. • To run the same program a second time, with the same arguments. This is not the same thing as giving more input to the first run! For example, suppose a program pipes output to the mail program. If it outputs several lines redirected to this pipe without closing it, they make a single message of several lines. By contrast, if the program closes the pipe after each line of output, then each line makes a separate message. If you use more files than the system allows you to have open, gawk attempts to multiplex the available open files among your datafiles. gawk’s ability to do this depends upon the facilities of your operating system, so it may not always work. It is therefore both good practice and good portability advice to always use close() on your files when you are 106 | Chapter 5: Printing Output 2. The technical terminology is rather morbid. The finished child is called a “zombie,” and cleaning up after it is referred to as “reaping.” done with them. In fact, if you are using a lot of pipes, it is essential that you close commands when done. For example, consider something like this: { … command = ("grep " $1 " /some/file | my_prog -q " $3) while ((command | getline) > 0) { process output of command } # need close(command) here } This example creates a new pipeline based on data in each record. Without the call to close() indicated in the comment, awk creates child processes to run the commands, until it eventually runs out of file descriptors for more pipelines. Even though each command has finished (as indicated by the end-of-file return status from getline), the child process is not terminated;2 more importantly, the file descriptor for the pipe is not closed and released until close() is called or awk exits. close() silently does nothing if given an argument that does not represent a file, pipe, or coprocess that was opened with a redirection. In such a case, it returns a negative value, indicating an error. In addition, gawk sets ERRNO to a string indicating the error. Note also that ‘close(FILENAME)’ has no “magic” effects on the implicit loop that reads through the files named on the command line. It is, more likely, a close of a file that was never opened with a redirection, so awk silently does nothing, except return a negative value. When using the ‘|&’ operator to communicate with a coprocess, it is occasionally useful to be able to close one end of the two-way pipe without closing the other. This is done by supplying a second argument to close(). As in any other call to close(), the first argument is the name of the command or special file used to start the coprocess. The second argument should be a string, with either of the values "to" or "from". Case does not matter. As this is an advanced feature, discussion is delayed until “Two-Way Com‐ munications with Another Process” on page 334, which describes it in more detail and gives an example. Closing Input and Output Redirections | 107 3. This is a full 16-bit value as returned by the wait() system call. See the system manual pages for information on how to decode this value. Using close()’s Return Value In many older versions of Unix awk, the close() function is actually a statement. (d.c.) It is a syntax error to try and use the return value from close(): command = "…" command | getline info retval = close(command) # syntax error in many Unix awks gawk treats close() as a function. The return value is −1 if the argument names some‐ thing that was never opened with a redirection, or if there is a system problem closing the file or process. In these cases, gawk sets the predefined variable ERRNO to a string describing the problem. In gawk, when closing a pipe or coprocess (input or output), the return value is the exit status of the command.3 Otherwise, it is the return value from the system’s close() or fclose() C functions when closing input or output files, respectively. This value is zero if the close succeeds, or −1 if it fails. The POSIX standard is very vague; it says that close() returns zero on success and a nonzero value otherwise. In general, different implementations vary in what they report when closing pipes; thus, the return value cannot be used portably. (d.c.) In POSIX mode (see “Command-Line Options” on page 22), gawk just returns zero when closing a pipe. Summary • The print statement prints comma-separated expressions. Each expression is sep‐ arated by the value of OFS and terminated by the value of ORS. OFMT provides the conversion format for numeric values for the print statement. • The printf statement provides finer-grained control over output, with format- control letters for different data types and various flags that modify the behavior of the format-control letters. • Output from both print and printf may be redirected to files, pipes, and coprocesses. 108 | Chapter 5: Printing Output • gawk provides special filenames for access to standard input, output, and error, and for network communications. • Use close() to close open file, pipe, and coprocess redirections. For coprocesses, it is possible to close only one direction of the communications. Summary | 109 CHAPTER 6 Expressions Expressions are the basic building blocks of awk patterns and actions. An expression evaluates to a value that you can print, test, or pass to a function. Additionally, an ex‐ pression can assign a new value to a variable or a field by using an assignment operator. An expression can serve as a pattern or action statement on its own. Most other kinds of statements contain one or more expressions that specify the data on which to operate. As in other languages, expressions in awk can include variables, array references, con‐ stants, and function calls, as well as combinations of these with various operators. Constants, Variables, and Conversions Expressions are built up from values and the operations performed upon them. This section describes the elementary objects that provide the values used in expressions. Constant Expressions The simplest type of expression is the constant, which always has the same value. There are three types of constants: numeric, string, and regular expression. Each is used in the appropriate context when you need a data value that isn’t going to change. Numeric constants can have different forms, but are internally stored in an identical manner. 111 1. The internal representation of all numbers, including integers, uses double-precision floating-point num‐ bers. On most modern systems, these are in IEEE 754 standard format. See Chapter 15 for much more information. Numeric and string constants A numeric constant stands for a number. This number can be an integer, a decimal fraction, or a number in scientific (exponential) notation.1 Here are some examples of numeric constants that all have the same value: 105 1.05e+2 1050e-1 A string constant consists of a sequence of characters enclosed in double quotation marks. For example: "parrot" represents the string whose contents are ‘parrot’. Strings in gawk can be of any length, and they can contain any of the possible eight-bit ASCII characters, including ASCII NUL (character code zero). Other awk implementations may have difficulty with some character codes. Octal and hexadecimal numbers In awk, all numbers are in decimal (i.e., base 10). Many other programming languages allow you to specify numbers in other bases, often octal (base 8) and hexadecimal (base 16). In octal, the numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, and so on. Just as ‘11’ in decimal is 1 times 10 plus 1, so ‘11’ in octal is 1 times 8 plus 1. This equals 9 in decimal. In hexadecimal, there are 16 digits. Because the everyday decimal number system only has ten digits (‘0’–‘9’), the letters ‘a’ through ‘f’ are used to represent the rest. (Case in the letters is usually irrelevant; hexadecimal ‘a’ and ‘A’ have the same value.) Thus, ‘11’ in hexadecimal is 1 times 16 plus 1, which equals 17 in decimal. Just by looking at plain ‘11’, you can’t tell what base it’s in. So, in C, C++, and other languages derived from C, there is a special notation to signify the base. Octal numbers start with a leading ‘0’, and hexadecimal numbers start with a leading ‘0x’ or ‘0X’: 11 Decimal value 11 011 Octal 11, decimal value 9 0x11 Hexadecimal 11, decimal value 17 112 | Chapter 6: Expressions This example shows the difference: $ gawk 'BEGIN { printf "%d, %d, %d\n", 011, 11, 0x11 }' 9, 11, 17 Being able to use octal and hexadecimal constants in your programs is most useful when working with data that cannot be represented conveniently as characters or as regular numbers, such as binary data of various sorts. gawk allows the use of octal and hexadecimal constants in your program text. However, such numbers in the input data are not treated differently; doing so by default would break old programs. (If you really need to do this, use the --non-decimal-data command-line option; see “Allowing Nondecimal Input Data” on page 327.) If you have octal or hexadecimal data, you can use the strtonum() function (see “String- Manipulation Functions” on page 194) to convert the data into a number. Most of the time, you will want to use octal or hexadecimal constants when working with the built- in bit-manipulation functions; see “Bit-Manipulation Functions” on page 217 for more information. Unlike in some early C implementations, ‘8’ and ‘9’ are not valid in octal constants. For example, gawk treats ‘018’ as decimal 18: $ gawk 'BEGIN { print "021 is", 021 ; print 018 }' 021 is 17 18 Octal and hexadecimal source code constants are a gawk extension. If gawk is in com‐ patibility mode (see “Command-Line Options” on page 22), they are not available. A Constant’s Base Does Not Affect Its Value Once a numeric constant has been converted internally into a number, gawk no longer remembers what the original form of the constant was; the internal value is always used. This has particular consequences for conversion of numbers to strings: $ gawk 'BEGIN { printf "0x11 is <%s>\n", 0x11 }' 0x11 is <17> Regular expression constants A regexp constant is a regular expression description enclosed in slashes, such as /^beginning and end$/. Most regexps used in awk programs are constant, but the ‘~’ and ‘!~’ matching operators can also match computed or dynamic regexps (which are typically just ordinary strings or variables that contain a regexp, but could be more complex expressions). Constants, Variables, and Conversions | 113 Using Regular Expression Constants When used on the righthand side of the ‘~’ or ‘!~’ operators, a regexp constant merely stands for the regexp that is to be matched. However, regexp constants (such as /foo/) may be used like simple expressions. When a regexp constant appears by itself, it has the same meaning as if it appeared in a pattern (i.e., ‘($0 ~ /foo/)’). (d.c.) See “Ex‐ pressions as Patterns” on page 142. This means that the following two code segments: if ($0 ~ /barfly/ || $0 ~ /camelot/) print "found" and: if (/barfly/ || /camelot/) print "found" are exactly equivalent. One rather bizarre consequence of this rule is that the following Boolean expression is valid, but does not do what its author probably intended: # Note that /foo/ is on the left of the ~ if (/foo/ ~ $1) print "found foo" This code is “obviously” testing $1 for a match against the regexp /foo/. But in fact, the expression ‘/foo/ ~ $1’ really means ‘($0 ~ /foo/) ~ $1’. In other words, first match the input record against the regexp /foo/. The result is either zero or one, depending upon the success or failure of the match. That result is then matched against the first field in the record. Because it is unlikely that you would ever really want to make this kind of test, gawk issues a warning when it sees this construct in a program. Another consequence of this rule is that the assignment statement: matches = /foo/ assigns either 0 or 1 to the variable matches, depending upon the contents of the current input record. Constant regular expressions are also used as the first argument for the gensub(), sub(), and gsub() functions, as the second argument of the match() function, and as the third argument of the split() and patsplit() functions (see “String-Manipulation Functions” on page 194). Modern implementations of awk, including gawk, allow the third argument of split() to be a regexp constant, but some older implementations do not. (d.c.) Because some built-in functions accept regexp constants as arguments, con‐ fusion can arise when attempting to use regexp constants as arguments to user-defined functions (see “User-Defined Functions” on page 220). For example: function mysub(pat, repl, str, global) { if (global) gsub(pat, repl, str) else sub(pat, repl, str) 114 | Chapter 6: Expressions return str } { … text = "hi! hi yourself!" mysub(/hi/, "howdy", text, 1) … } In this example, the programmer wants to pass a regexp constant to the user-defined function mysub(), which in turn passes it on to either sub() or gsub(). However, what really happens is that the pat parameter is assigned a value of either one or zero, de‐ pending upon whether or not $0 matches /hi/. gawk issues a warning when it sees a regexp constant used as a parameter to a user-defined function, because passing a truth value in this way is probably not what was intended. Variables Variables are ways of storing values at one point in your program for use later in another part of your program. They can be manipulated entirely within the program text, and they can also be assigned values on the awk command line. Using variables in a program Variables let you give names to values and refer to them later. Variables have already been used in many of the examples. The name of a variable must be a sequence of letters, digits, or underscores, and it may not begin with a digit. Here, a letter is any one of the 52 upper- and lowercase English letters. Other characters that may be defined as letters in non-English locales are not valid in variable names. Case is significant in variable names; a and A are distinct variables. A variable name is a valid expression by itself; it represents the variable’s current value. Variables are given new values with assignment operators, increment operators, and decrement operators (see “Assignment Expressions” on page 122). In addition, the sub() and gsub() functions can change a variable’s value, and the match(), split(), and patsplit() functions can change the contents of their array parameters (see “String- Manipulation Functions” on page 194). A few variables have special built-in meanings, such as FS (the field separator) and NF (the number of fields in the current input record). See “Predefined Variables” on page 160 for a list of the predefined variables. These predefined variables can be used and assigned just like all other variables, but their values are also used or changed automat‐ ically by awk. All predefined variables’ names are entirely uppercase. Constants, Variables, and Conversions | 115 Variables in awk can be assigned either numeric or string values. The kind of value a variable holds can change over the life of a program. By default, variables are initialized to the empty string, which is zero if converted to a number. There is no need to explicitly initialize a variable in awk, which is what you would do in C and in most other traditional languages. Assigning variables on the command line Any awk variable can be set by including a variable assignment among the arguments on the command line when awk is invoked (see “Other Command-Line Arguments” on page 29). Such an assignment has the following form: variable=text With it, a variable is set either at the beginning of the awk run or in between input files. When the assignment is preceded with the -v option, as in the following: -v variable=text the variable is set at the very beginning, even before the BEGIN rules execute. The -v option and its assignment must precede all the filename arguments, as well as the pro‐ gram text. (See “Command-Line Options” on page 22 for more information about the -v option.) Otherwise, the variable assignment is performed at a time determined by its position among the input file arguments—after the processing of the preceding input file argument. For example: awk '{ print $n }' n=4 inventory-shipped n=2 mail-list prints the value of field number n for all input records. Before the first file is read, the command line sets the variable n equal to four. This causes the fourth field to be printed in lines from inventory-shipped. After the first file has finished, but before the second file is started, n is set to two, so that the second field is printed in lines from mail-list: $ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list 15 24 … 555-5553 555-3412 … Command-line arguments are made available for explicit examination by the awk pro‐ gram in the ARGV array (see “Using ARGC and ARGV” on page 170). awk processes the values of command-line assignments for escape sequences (see “Escape Sequences” on page 40). (d.c.) 116 | Chapter 6: Expressions 2. Pathological cases can require up to 752 digits (!), but we doubt that you need to worry about this. Conversion of Strings and Numbers Number-to-string and string-to-number conversion are generally straightforward. There can be subtleties to be aware of; this section discusses this important facet of awk. How awk converts between strings and numbers Strings are converted to numbers and numbers are converted to strings, if the context of the awk program demands it. For example, if the value of either foo or bar in the expression ‘foo + bar’ happens to be a string, it is converted to a number before the addition is performed. If numeric values appear in string concatenation, they are con‐ verted to strings. Consider the following: two = 2; three = 3 print (two three) + 4 This prints the (numeric) value 27. The numeric values of the variables two and three are converted to strings and concatenated together. The resulting string is converted back to the number 23, to which 4 is then added. If, for some reason, you need to force a number to be converted to a string, concatenate that number with the empty string, "". To force a string to be converted to a number, add zero to that string. A string is converted to a number by interpreting any numeric prefix of the string as numerals: "2.5" converts to 2.5, "1e3" converts to 1,000, and "25fix" has a numeric value of 25. Strings that can’t be interpreted as valid numbers convert to zero. The exact manner in which numbers are converted into strings is controlled by the awk predefined variable CONVFMT (see “Predefined Variables” on page 160). Numbers are con‐ verted using the sprintf() function with CONVFMT as the format specifier (see “String- Manipulation Functions” on page 194). CONVFMT’s default value is "%.6g", which creates a value with at most six significant digits. For some applications, you might want to change it to specify more precision. On most modern machines, 17 digits is usually enough to capture a floating-point number’s value exactly.2 Strange results can occur if you set CONVFMT to a string that doesn’t tell sprintf() how to format floating-point numbers in a useful way. For example, if you forget the ‘%’ in the format, awk converts all numbers to the same constant string. As a special case, if a number is an integer, then the result of converting it to a string is always an integer, no matter what the value of CONVFMT may be. Given the following code fragment: Constants, Variables, and Conversions | 117 CONVFMT = "%2.2f" a = 12 b = a "" b has the value "12", not "12.00". (d.c.) Pre-POSIX awk Used OFMT for String Conversion Prior to the POSIX standard, awk used the value of OFMT for converting numbers to strings. OFMT specifies the output format to use when printing numbers with print. CONVFMT was introduced in order to separate the semantics of conversion from the se‐ mantics of printing. Both CONVFMT and OFMT have the same default value: "%.6g". In the vast majority of cases, old awk programs do not change their behavior. See “The print Statement” on page 89 for more information on the print statement. Locales can influence conversion Where you are can matter when it comes to converting between numbers and strings. The local character set and language—the locale—can affect numeric formats. In par‐ ticular, for awk programs, it affects the decimal point character and the thousands- separator character. The "C" locale, and most English-language locales, use the period character (‘.’) as the decimal point and don’t have a thousands separator. However, many (if not most) European and non-English locales use the comma (‘,’) as the decimal point character. European locales often use either a space or a period as the thousands sepa‐ rator, if they have one. The POSIX standard says that awk always uses the period as the decimal point when reading the awk program source code, and for command-line variable assignments (see “Other Command-Line Arguments” on page 29). However, when interpreting input data, for print and printf output, and for number-to-string conversion, the local dec‐ imal point character is used. (d.c.) In all cases, numbers in source code and in input data cannot have a thousands separator. Here are some examples indicating the difference in behavior, on a GNU/Linux system: $ export POSIXLY_CORRECT=1 Force POSIX behavior $ gawk 'BEGIN { printf "%g\n", 3.1415927 }' 3.14159 $ LC_ALL=en_DK.utf-8 gawk 'BEGIN { printf "%g\n", 3.1415927 }' 3,14159 $ echo 4,321 | gawk '{ print $1 + 1 }' 5 $ echo 4,321 | LC_ALL=en_DK.utf-8 gawk '{ print $1 + 1 }' 5,321 118 | Chapter 6: Expressions The en_DK.utf-8 locale is for English in Denmark, where the comma acts as the decimal point separator. In the normal "C" locale, gawk treats ‘4,321’ as 4, while in the Danish locale, it’s treated as the full number including the fractional part, 4.321. Some earlier versions of gawk fully complied with this aspect of the standard. However, many users in non-English locales complained about this behavior, because their data used a period as the decimal point, so the default behavior was restored to use a period as the decimal point character. You can use the --use-lc-numeric option (see “Command-Line Options” on page 22) to force gawk to use the locale’s decimal point character. (gawk also uses the locale’s decimal point character when in POSIX mode, either via --posix or the POSIXLY_CORRECT environment variable, as shown previously.) Table 6-1 describes the cases in which the locale’s decimal point character is used and when a period is used. Some of these features have not been described yet. Table 6-1. Locale decimal point versus a period Feature Default --posix or --use-lc-numeric %'g Use locale Use locale %g Use period Use locale Input Use period Use locale strtonum() Use period Use locale Finally, modern-day formal standards and the IEEE standard floating-point represen‐ tation can have an unusual but important effect on the way gawk converts some special string values to numbers. The details are presented in “Standards Versus Existing Prac‐ tice” on page 389. Operators: Doing Something with Values This section introduces the operators that make use of the values provided by constants and variables. Arithmetic Operators The awk language uses the common arithmetic operators when evaluating expres‐ sions. All of these arithmetic operators follow normal precedence rules and work as you would expect them to. The following example uses a file named grades, which contains a list of student names as well as three test scores per student (it’s a small class): Pat 100 97 58 Sandy 84 72 93 Chris 72 92 89 Operators: Doing Something with Values | 119 This program takes the file grades and prints the average of the scores: $ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3 > print $1, avg }' grades Pat 85 Sandy 83 Chris 84.3333 The following list provides the arithmetic operators in awk, in order from the highest precedence to the lowest: x ^ y x ** y Exponentiation; x raised to the y power. ‘2 ^ 3’ has the value eight; the character sequence ‘**’ is equivalent to ‘^’. (c.e.) - x Negation. + x Unary plus; the expression is converted to a number. x * y Multiplication. x / y Division; because all numbers in awk are floating-point numbers, the result is not rounded to an integer—‘3 / 4’ has the value 0.75. (It is a common mistake, espe‐ cially for C programmers, to forget that all numbers in awk are floating point, and that division of integer-looking constants produces a real number, not an integer.) x % y Remainder; further discussion is provided in the text, just after this list. x + y Addition. x - y Subtraction. Unary plus and minus have the same precedence, the multiplication operators all have the same precedence, and addition and subtraction have the same precedence. When computing the remainder of ‘x % y’, the quotient is rounded toward zero to an integer and multiplied by y. This result is subtracted from x; this operation is sometimes known as “trunc-mod.” The following relation always holds: b * int(a / b) + (a % b) == a 120 | Chapter 6: Expressions 3. It happens that BWK awk, gawk, and mawk all “get it right,” but you should not rely on this. One possibly undesirable effect of this definition of remainder is that ‘x % y’ is negative if x is negative. Thus: -17 % 8 = -1 In other awk implementations, the signedness of the remainder may be machine- dependent. The POSIX standard only specifies the use of ‘^’ for exponentiation. For maximum portability, do not use the ‘**’ operator. String Concatenation It seemed like a good idea at the time. —Brian Kernighan There is only one string operation: concatenation. It does not have a specific operator to represent it. Instead, concatenation is performed by writing expressions next to one another, with no operator. For example: $ awk '{ print "Field number one: " $1 }' mail-list Field number one: Amelia Field number one: Anthony … Without the space in the string constant after the ‘:’, the line runs together. For example: $ awk '{ print "Field number one:" $1 }' mail-list Field number one:Amelia Field number one:Anthony … Because string concatenation does not have an explicit operator, it is often necessary to ensure that it happens at the right time by using parentheses to enclose the items to concatenate. For example, you might expect that the following code fragment concat‐ enates file and name: file = "file" name = "name" print "something meaningful" > file name This produces a syntax error with some versions of Unix awk.3 It is necessary to use the following: print "something meaningful" > (file name) Operators: Doing Something with Values | 121 Parentheses should be used around concatenation in all but the most common contexts, such as on the righthand side of ‘=’. Be careful about the kinds of expressions used in string concatenation. In particular, the order of evaluation of expressions used for con‐ catenation is undefined in the awk language. Consider this example: BEGIN { a = "don't" print (a " " (a = "panic")) } It is not defined whether the second assignment to a happens before or after the value of a is retrieved for producing the concatenated value. The result could be either ‘don't panic’, or ‘panic panic’. The precedence of concatenation, when mixed with other operators, is often counter- intuitive. Consider this example: $ awk 'BEGIN { print -12 " " -24 }' -12-24 This “obviously” is concatenating −12, a space, and −24. But where did the space dis‐ appear to? The answer lies in the combination of operator precedences and awk’s au‐ tomatic conversion rules. To get the desired result, write the program this way: $ awk 'BEGIN { print -12 " " (-24) }' -12 -24 This forces awk to treat the ‘-’ on the ‘-24’ as unary. Otherwise, it’s parsed as follows: −12 (" "− 24) ⇒ −12 (0 − 24) ⇒ −12 (−24) ⇒ −12−24 As mentioned earlier, when mixing concatenation with other operators, parenthesize. Otherwise, you’re never quite sure what you’ll get. Assignment Expressions An assignment is an expression that stores a (usually different) value into a variable. For example, let’s assign the value one to the variable z: z = 1 After this expression is executed, the variable z has the value one. Whatever old value z had before the assignment is forgotten. Assignments can also store string values. For example, the following stores the value "this food is good" in the variable message: 122 | Chapter 6: Expressions thing = "food" predicate = "good" message = "this " thing " is " predicate This also illustrates string concatenation. The ‘=’ sign is called an assignment operator. It is the simplest assignment operator because the value of the righthand operand is stored unchanged. Most operators (addition, concatenation, and so on) have no effect except to compute a value. If the value isn’t used, there’s no reason to use the operator. An assignment operator is different; it does produce a value, but even if you ignore it, the assignment still makes itself felt through the alteration of the variable. We call this a side effect. The lefthand operand of an assignment need not be a variable (see “Variables” on page 115); it can also be a field (see “Changing the Contents of a Field” on page 62) or an array element (see Chapter 8). These are all called lvalues, which means they can appear on the lefthand side of an assignment operator. The righthand operand may be any expression; it produces the new value that the assignment stores in the specified variable, field, or array element. (Such values are called rvalues.) It is important to note that variables do not have permanent types. A variable’s type is simply the type of whatever value was last assigned to it. In the following program fragment, the variable foo has a numeric value at first, and a string value later on: foo = 1 print foo foo = "bar" print foo When the second assignment gives foo a string value, the fact that it previously had a numeric value is forgotten. String values that do not begin with a digit have a numeric value of zero. After executing the following code, the value of foo is five: foo = "a string" foo = foo + 5 Using a variable as a number and then later as a string can be con‐ fusing and is poor programming style. The previous two examples illustrate how awk works, not how you should write your programs! An assignment is an expression, so it has a value—the same value that is assigned. Thus, ‘z = 1’ is an expression with the value one. One consequence of this is that you can write multiple assignments together, such as: x = y = z = 5 Operators: Doing Something with Values | 123 This example stores the value five in all three variables (x, y, and z). It does so because the value of ‘z = 5’, which is five, is stored into y and then the value of ‘y = z = 5’, which is five, is stored into x. Assignments may be used anywhere an expression is called for. For example, it is valid to write ‘x != (y = 1)’ to set y to one, and then test whether x equals one. But this style tends to make programs hard to read; such nesting of assignments should be avoided, except perhaps in a one-shot program. Aside from ‘=’, there are several other assignment operators that do arithmetic with the old value of the variable. For example, the operator ‘+=’ computes a new value by adding the righthand value to the old value of the variable. Thus, the following assignment adds five to the value of foo: foo += 5 This is equivalent to the following: foo = foo + 5 Use whichever makes the meaning of your program clearer. There are situations where using ‘+=’ (or any assignment operator) is not the same as simply repeating the lefthand operand in the righthand expression. For example: # Thanks to Pat Rankin for this example BEGIN { foo[rand()] += 5 for (x in foo) print x, foo[x] bar[rand()] = bar[rand()] + 5 for (x in bar) print x, bar[x] } The indices of bar are practically guaranteed to be different, because rand() returns different values each time it is called. (Arrays and the rand() function haven’t been covered yet. See Chapter 8 and “Numeric Functions” on page 192 for more information.) This example illustrates an important fact about assignment operators: the lefthand expression is only evaluated once. It is up to the implementation as to which expression is evaluated first, the lefthand or the righthand. Consider this example: i = 1 a[i += 2] = i + 1 The value of a[3] could be either two or four. 124 | Chapter 6: Expressions Table 6-2 lists the arithmetic assignment operators. In each case, the righthand operand is an expression whose value is converted to a number. Table 6-2. Arithmetic assignment operators Operator Effect lvalue += increment Add increment to the value of lvalue. lvalue -= decrement Subtract decrement from the value of lvalue. lvalue *= coefficient Multiply the value of lvalue by coefficient. lvalue /= divisor Divide the value of lvalue by divisor. lvalue %= modulus Set lvalue to its remainder by modulus. lvalue ^= power Raise lvalue to the power power. lvalue **= power Raise lvalue to the power power. (c.e.) Only the ‘^=’ operator is specified by POSIX. For maximum porta‐ bility, do not use the ‘**=’ operator. Syntactic Ambiguities Between ‘/=’ and Regular Expressions There is a syntactic ambiguity between the /= assignment operator and regexp constants whose first character is an ‘=’. (d.c.) This is most notable in some commercial awk versions. For example: $ awk /==/ /dev/null error→ awk: syntax error at source line 1 error→ context is error→ >>> /= <<< error→ awk: bailing out at source line 1 A workaround is: awk '/[=]=/' /dev/null gawk does not have this problem; BWK awk and mawk also do not. Increment and Decrement Operators Increment and decrement operators increase or decrease the value of a variable by one. An assignment operator can do the same thing, so the increment operators add no power to the awk language; however, they are convenient abbreviations for very common operations. Operators: Doing Something with Values | 125 The operator used for adding one is written ‘++’. It can be used to increment a variable either before or after taking its value. To pre-increment a variable v, write ‘++v’. This adds one to the value of v—that new value is also the value of the expression. (The assignment expression ‘v += 1’ is completely equivalent.) Writing the ‘++’ after the variable specifies post-increment. This increments the variable value just the same; the difference is that the value of the increment expression itself is the variable’s old value. Thus, if foo has the value four, then the expression ‘foo++’ has the value four, but it changes the value of foo to five. In other words, the operator returns the old value of the variable, but with the side effect of incrementing it. The post-increment ‘foo++’ is nearly the same as writing ‘(foo += 1) - 1’. It is not perfectly equivalent because all numbers in awk are floating point—in floating point, ‘foo + 1 - 1’ does not necessarily equal foo. But the difference is minute as long as you stick to numbers that are fairly small (less than 1012). Fields and array elements are incremented just like variables. (Use ‘$(i++)’ when you want to do a field reference and a variable increment at the same time. The parentheses are necessary because of the precedence of the field reference operator ‘$’.) The decrement operator ‘--’ works just like ‘++’, except that it subtracts one instead of adding it. As with ‘++’, it can be used before the lvalue to pre-decrement or after it to post-decrement. Following is a summary of increment and decrement expressions: ++lvalue Increment lvalue, returning the new value as the value of the expression. lvalue++ Increment lvalue, returning the old value of lvalue as the value of the expression. --lvalue Decrement lvalue, returning the new value as the value of the expression. (This expression is like ‘++lvalue’, but instead of adding, it subtracts.) lvalue-- Decrement lvalue, returning the old value of lvalue as the value of the expression. (This expression is like ‘lvalue++’, but instead of adding, it subtracts.) Operator Evaluation Order Doctor, it hurts when I do this! Then don’t do that! —Groucho Marx What happens for something like the following? b = 6 print b += b++ 126 | Chapter 6: Expressions Or something even stranger? b = 6 b += ++b + b++ print b In other words, when do the various side effects prescribed by the postfix operators (‘b++’) take effect? When side effects happen is implementation-defined. In other words, it is up to the particular version of awk. The result for the first example may be 12 or 13, and for the second, it may be 22 or 23. In short, doing things like this is not recommended and definitely not anything that you can rely upon for portability. You should avoid such things in your own programs. Truth Values and Conditions In certain contexts, expression values also serve as “truth values”; i.e., they determine what should happen next as the program runs. This section describes how awk defines “true” and “false” and how values are compared. True and False in awk Many programming languages have a special representation for the concepts of “true” and “false.” Such languages usually use the special constants true and false, or perhaps their uppercase equivalents. However, awk is different. It borrows a very simple concept of true and false from C. In awk, any nonzero numeric value or any nonempty string value is true. Any other value (zero or the null string, "") is false. The following program prints ‘A strange truth value’ three times: BEGIN { if (3.1415927) print "A strange truth value" if ("Four Score And Seven Years Ago") print "A strange truth value" if (j = 57) print "A strange truth value" } There is a surprising consequence of the “nonzero or non-null” rule: the string constant "0" is actually true, because it is non-null. (d.c.) Truth Values and Conditions | 127 Variable Typing and Comparison Expressions The Guide is definitive. Reality is frequently inaccurate. —Douglas Adams, The Hitchhiker’s Guide to the Galaxy Unlike in other programming languages, in awk variables do not have a fixed type. Instead, they can be either a number or a string, depending upon the value that is assigned to them. We look now at how variables are typed, and how awk compares variables. String type versus numeric type The POSIX standard introduced the concept of a numeric string, which is simply a string that looks like a number—for example, " +2". This concept is used for determining the type of a variable. The type of the variable is important because the types of two variables determine how they are compared. Variable typing follows these rules: • A numeric constant or the result of a numeric operation has the numeric attribute. • A string constant or the result of a string operation has the string attribute. • Fields, getline input, FILENAME, ARGV elements, ENVIRON elements, and the ele‐ ments of an array created by match(), split(), and patsplit() that are numeric strings have the strnum attribute. Otherwise, they have the string attribute. Unini‐ tialized variables also have the strnum attribute. • Attributes propagate across assignments but are not changed by any use. The last rule is particularly important. In the following program, a has numeric type, even though it is later used in a string operation: BEGIN { a = 12.345 b = a " is a cute number" print b } When two operands are compared, either string comparison or numeric comparison may be used. This depends upon the attributes of the operands, according to the fol‐ lowing symmetric matrix: STRING NUMERIC STRNUM STRING string string string NUMERIC string numeric numeric STRNUM string numeric numeric 128 | Chapter 6: Expressions The basic idea is that user input that looks numeric—and only user input—should be treated as numeric, even though it is actually made of characters and is therefore also a string. Thus, for example, the string constant " +3.14", when it appears in program source code, is a string—even though it looks numeric—and is never treated as a number for comparison purposes. In short, when one operand is a “pure” string, such as a string constant, then a string comparison is performed. Otherwise, a numeric comparison is performed. This point bears additional emphasis. All user input is made of characters, and so is first and foremost of string type; input strings that look numeric are additionally given the strnum attribute. Thus, the six-character input string ‘ +3.14’ receives the strnum at‐ tribute. In contrast, the eight characters " +3.14" appearing in program text comprise a string constant. The following examples print ‘1’ when the comparison between the two different constants is true, and ‘0’ otherwise: $ echo ' +3.14' | awk '{ print($0 == " +3.14") }' True 1 $ echo ' +3.14' | awk '{ print($0 == "+3.14") }' False 0 $ echo ' +3.14' | awk '{ print($0 == "3.14") }' False 0 $ echo ' +3.14' | awk '{ print($0 == 3.14) }' True 1 $ echo ' +3.14' | awk '{ print($1 == " +3.14") }' False 0 $ echo ' +3.14' | awk '{ print($1 == "+3.14") }' True 1 $ echo ' +3.14' | awk '{ print($1 == "3.14") }' False 0 $ echo ' +3.14' | awk '{ print($1 == 3.14) }' True 1 Comparison operators Comparison expressions compare strings or numbers for relationships such as equali‐ ty. They are written using relational operators, which are a superset of those in C. Table 6-3 describes them. Table 6-3. Relational operators Expression Result x < y True if x is less than y x <= y True if x is less than or equal to y x > y True if x is greater than y x >= y True if x is greater than or equal to y x == y True if x is equal to y Truth Values and Conditions | 129 Expression Result x != y True if x is not equal to y x ~ y True if the string x matches the regexp denoted by y x !~ y True if the string x does not match the regexp denoted by y subscript in array True if the array array has an element with the subscript subscript Comparison expressions have the value one if true and zero if false. When comparing operands of mixed types, numeric operands are converted to strings using the value of CONVFMT (see “Conversion of Strings and Numbers” on page 117). Strings are compared by comparing the first character of each, then the second character of each, and so on. Thus, "10" is less than "9". If there are two strings where one is a prefix of the other, the shorter string is less than the longer one. Thus, "abc" is less than "abcd". It is very easy to accidentally mistype the ‘==’ operator and leave off one of the ‘=’ char‐ acters. The result is still valid awk code, but the program does not do what is intended: if (a = b) # oops! should be a == b … else … Unless b happens to be zero or the null string, the if part of the test always succeeds. Because the operators are so similar, this kind of error is very difficult to spot when scanning the source code. The following list of expressions illustrates the kinds of comparisons awk performs, as well as what the result of each comparison is: 1.5 <= 2.0 Numeric comparison (true) "abc" >= "xyz" String comparison (false) 1.5 != " +2" String comparison (true) "1e2" < "3" String comparison (true) a = 2; b = "2" a == b String comparison (true) 130 | Chapter 6: Expressions 4. Technically, string comparison is supposed to behave the same way as if the strings were compared with the C strcoll() function. a = 2; b = " +2" a == b String comparison (false) In this example: $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }' false the result is ‘false’ because both $1 and $2 are user input. They are numeric strings— therefore both have the strnum attribute, dictating a numeric comparison. The purpose of the comparison rules and the use of numeric strings is to attempt to produce the behavior that is “least surprising,” while still “doing the right thing.” String comparisons and regular expression comparisons are very different. For example: x == "foo" has the value one, or is true if the variable x is precisely ‘foo’. By contrast: x ~ /foo/ has the value one if x contains ‘foo’, such as "Oh, what a fool am I!". The righthand operand of the ‘~’ and ‘!~’ operators may be either a regexp constant (/…/) or an ordinary expression. In the latter case, the value of the expression as a string is used as a dynamic regexp (see “How to Use Regular Expressions” on page 39; also see “Using Dynamic Regexps” on page 49). A constant regular expression in slashes by itself is also an expression. /regexp/ is an abbreviation for the following comparison expression: $0 ~ /regexp/ One special place where /foo/ is not an abbreviation for ‘$0 ~ /foo/’ is when it is the righthand operand of ‘~’ or ‘!~’. See “Using Regular Expression Constants” on page 114, where this is discussed in more detail. String comparison with POSIX rules The POSIX standard says that string comparison is performed based on the locale’s collating order. This is the order in which characters sort, as defined by the locale (for more discussion, see “Where You Are Makes a Difference” on page 138). This order is usually very different from the results obtained when doing straight character-by- character comparison.4 Truth Values and Conditions | 131 Because this behavior differs considerably from existing practice, gawk only implements it when in POSIX mode (see “Command-Line Options” on page 22). Here is an example to illustrate the difference, in an en_US.UTF-8 locale: $ gawk 'BEGIN { printf("ABC < abc = %s\n", > ("ABC" < "abc" ? "TRUE" : "FALSE")) }' ABC < abc = TRUE $ gawk --posix 'BEGIN { printf("ABC < abc = %s\n", > ("ABC" < "abc" ? "TRUE" : "FALSE")) }' ABC < abc = FALSE Boolean Expressions A Boolean expression is a combination of comparison expressions or matching expres‐ sions, using the Boolean operators “or” (‘||’), “and” (‘&&’), and “not” (‘!’), along with parentheses to control nesting. The truth value of the Boolean expression is computed by combining the truth values of the component expressions. Boolean expressions are also referred to as logical expressions. The terms are equivalent. Boolean expressions can be used wherever comparison and matching expressions can be used. They can be used in if, while, do, and for statements (see “Control Statements in Actions” on page 150). They have numeric values (one if true, zero if false) that come into play if the result of the Boolean expression is stored in a variable or used in arithmetic. In addition, every Boolean expression is also a valid pattern, so you can use one as a pattern to control the execution of rules. The Boolean operators are: boolean1 && boolean2 True if both boolean1 and boolean2 are true. For example, the following statement prints the current input record if it contains both ‘edu’ and ‘li’: if ($0 ~ /edu/ && $0 ~ /li/) print The subexpression boolean2 is evaluated only if boolean1 is true. This can make a difference when boolean2 contains expressions that have side effects. In the case of ‘$0 ~ /foo/ && ($2 == bar++)’, the variable bar is not incremented if there is no substring ‘foo’ in the record. boolean1 || boolean2 True if at least one of boolean1 or boolean2 is true. For example, the following statement prints all records in the input that contain either ‘edu’ or ‘li’: if ($0 ~ /edu/ || $0 ~ /li/) print The subexpression boolean2 is evaluated only if boolean1 is false. This can make a difference when boolean2 contains expressions that have side effects. (Thus, this 132 | Chapter 6: Expressions 5. This program has a bug; it prints lines starting with ‘END’. How would you fix it? test never really distinguishes records that contain both ‘edu’ and ‘li’—as soon as ‘edu’ is matched, the full test succeeds.) ! boolean True if boolean is false. For example, the following program prints ‘no home!’ in the unusual event that the HOME environment variable is not defined: BEGIN { if (! ("HOME" in ENVIRON)) print "no home!" } (The in operator is described in “Referring to an Array Element” on page 175.) The ‘&&’ and ‘||’ operators are called short-circuit operators because of the way they work. Evaluation of the full expression is “short-circuited” if the result can be deter‐ mined partway through its evaluation. Statements that end with ‘&&’ or ‘||’ can be continued simply by putting a newline after them. But you cannot put a newline in front of either of these operators without using backslash continuation (see “awk Statements Versus Lines” on page 16). The actual value of an expression using the ‘!’ operator is either one or zero, depending upon the truth value of the expression it is applied to. The ‘!’ operator is often useful for changing the sense of a flag variable from false to true and back again. For example, the following program is one way to print lines in between special bracketing lines: $1 == "START" { interested = ! interested; next } interested { print } $1 == "END" { interested = ! interested; next } The variable interested, as with all awk variables, starts out initialized to zero, which is also false. When a line is seen whose first field is ‘START’, the value of interested is toggled to true, using ‘!’. The next rule prints lines as long as interested is true. When a line is seen whose first field is ‘END’, interested is toggled back to false.5 Most commonly, the ‘!’ operator is used in the conditions of if and while statements, where it often makes more sense to phrase the logic in the negative: if (! some condition || some other condition) { … do whatever processing … } Truth Values and Conditions | 133 The next statement is discussed in “The next Statement” on page 157. next tells awk to skip the rest of the rules, get the next record, and start processing the rules over again at the top. The reason it’s there is to avoid printing the bracketing ‘START’ and ‘END’ lines. Conditional Expressions A conditional expression is a special kind of expression that has three operands. It allows you to use one expression’s value to select one of two other expressions. The conditional expression in awk is the same as in the C language, as shown here: selector ? if-true-exp : if-false-exp There are three subexpressions. The first, selector, is always computed first. If it is “true” (not zero or not null), then if-true-exp is computed next, and its value becomes the value of the whole expression. Otherwise, if-false-exp is computed next, and its value becomes the value of the whole expression. For example, the following expression produces the absolute value of x: x >= 0 ? x : -x Each time the conditional expression is computed, only one of if-true-exp and if- false-exp is used; the other is ignored. This is important when the expressions have side effects. For example, this conditional expression examines element i of either array a or array b, and increments i: x == y ? a[i++] : b[i++] This is guaranteed to increment i exactly once, because each time only one of the two increment expressions is executed and the other is not. See Chapter 8 for more infor‐ mation about arrays. As a minor gawk extension, a statement that uses ‘?:’ can be continued simply by putting a newline after either character. However, putting a newline in front of either character does not work without using backslash continuation (see “awk Statements Versus Lines” on page 16). If --posix is specified (see “Command-Line Options” on page 22), this extension is disabled. Function Calls A function is a name for a particular calculation. This enables you to ask for it by name at any point in the program. For example, the function sqrt() computes the square root of a number. A fixed set of functions are built in, which means they are available in every awk pro‐ gram. The sqrt() function is one of these. See “Built-in Functions” on page 191 for a list 134 | Chapter 6: Expressions of built-in functions and their descriptions. In addition, you can define functions for use in your program. See “User-Defined Functions” on page 220 for instructions on how to do this. Finally, gawk lets you write functions in C or C++ that may be called from your program (see Chapter 16). The way to use a function is with a function call expression, which consists of the func‐ tion name followed immediately by a list of arguments in parentheses. The arguments are expressions that provide the raw materials for the function’s calculations. When there is more than one argument, they are separated by commas. If there are no argu‐ ments, just write ‘()’ after the function name. The following examples show function calls with and without arguments: sqrt(x^2 + y^2) one argument atan2(y, x) two arguments rand() no arguments Do not put any space between the function name and the opening parenthesis! A user-defined function name looks just like the name of a variable—a space would make the expression look like concat‐ enation of a variable with an expression inside parentheses. With built-in functions, space before the parenthesis is harmless, but it is best not to get into the habit of using space to avoid mistakes with user-defined functions. Each function expects a particular number of arguments. For example, the sqrt() function must be called with a single argument, the number of which to take the square root: sqrt(argument) Some of the built-in functions have one or more optional arguments. If those arguments are not supplied, the functions use a reasonable default value. See “Built-in Func‐ tions” on page 191 for full details. If arguments are omitted in calls to user-defined func‐ tions, then those arguments are treated as local variables. Such local variables act like the empty string if referenced where a string value is required, and like zero if referenced where a numeric value is required (see “User-Defined Functions” on page 220). As an advanced feature, gawk provides indirect function calls, which is a way to choose the function to call at runtime, instead of when you write the source code to your pro‐ gram. We defer discussion of this feature until later; see “Indirect Function Calls” on page 231. Like every other expression, the function call has a value, often called the return value, which is computed by the function based on the arguments you give it. In this example, the return value of ‘sqrt(argument)’ is the square root of argument. The following program reads numbers, one number per line, and prints the square root of each one: Function Calls | 135 $ awk '{ print "The square root of", $1, "is", sqrt($1) }' 1 The square root of 1 is 1 3 The square root of 3 is 1.73205 5 The square root of 5 is 2.23607 Ctrl-d A function can also have side effects, such as assigning values to certain variables or doing I/O. This program shows how the match() function (see “String-Manipulation Functions” on page 194) changes the variables RSTART and RLENGTH: { if (match($1, $2)) print RSTART, RLENGTH else print "no match" } Here is a sample run: $ awk -f matchit.awk aaccdd c+ 3 2 foo bar no match abcdefg e 5 1 Operator Precedence (How Operators Nest) Operator precedence determines how operators are grouped when different operators appear close by in one expression. For example, ‘*’ has higher precedence than ‘+’; thus, ‘a + b * c’ means to multiply b and c, and then add a to the product (i.e., ‘a + (b * c)’). The normal precedence of the operators can be overruled by using parentheses. Think of the precedence rules as saying where the parentheses are assumed to be. In fact, it is wise to always use parentheses whenever there is an unusual combination of operators, because other people who read the program may not remember what the precedence is in this case. Even experienced programmers occasionally forget the exact rules, which leads to mistakes. Explicit parentheses help prevent any such mistakes. When operators of equal precedence are used together, the leftmost operator groups first, except for the assignment, conditional, and exponentiation operators, which group in the opposite order. Thus, ‘a - b + c’ groups as ‘(a - b) + c’ and ‘a = b = c’ groups as ‘a = (b = c)’. Normally the precedence of prefix unary operators does not matter, because there is only one way to interpret them: innermost first. Thus, ‘$++i’ means ‘$(++i)’ and ‘++ 136 | Chapter 6: Expressions $x’ means ‘++($x)’. However, when another operator follows the operand, then the precedence of the unary operators can matter. ‘$x^2’ means ‘($x)^2’, but ‘-x^2’ means ‘-(x^2)’, because ‘-’ has lower precedence than ‘^’, whereas ‘$’ has higher precedence. Also, operators cannot be combined in a way that violates the precedence rules; for example, ‘$$0++--’ is not a valid expression because the first ‘$’ has higher precedence than the ‘++’; to avoid the problem the expression can be rewritten as ‘$($0++)--’. This list presents awk’s operators, in order of highest to lowest precedence: (…) Grouping. $ Field reference. ++ -- Increment, decrement. ^ ** Exponentiation. These operators group right to left. + - ! Unary plus, minus, logical “not.” * / % Multiplication, division, remainder. + - Addition, subtraction. String concatenation There is no special symbol for concatenation. The operands are simply written side by side (see “String Concatenation” on page 121). < <= == != > >= >> | |& Relational and redirection. The relational operators and the redirections have the same precedence level. Characters such as ‘>’ serve both as relationals and as redi‐ rections; the context distinguishes between the two meanings. Note that the I/O redirection operators in print and printf statements belong to the statement level, not to expressions. The redirection does not produce an ex‐ pression that could be the operand of another operator. As a result, it does not make sense to use a redirection operator near another operator of lower precedence without parentheses. Such combinations (e.g., ‘print foo > a ? b : c’) result in syntax errors. The correct way to write this statement is ‘print foo > (a ? b : c)’. ~ !~ Matching, nonmatching. Operator Precedence (How Operators Nest) | 137 in Array membership. && Logical “and.” || Logical “or.” ?: Conditional. This operator groups right to left. = += -= *= /= %= ^= **= Assignment. These operators group right to left. The ‘|&’, ‘**’, and ‘**=’ operators are not specified by POSIX. For maximum portability, do not use them. Where You Are Makes a Difference Modern systems support the notion of locales: a way to tell the system about the local character set and language. The ISO C standard defines a default "C" locale, which is an environment that is typical of what many C programmers are used to. Once upon a time, the locale setting used to affect regexp matching, but this is no longer true (see “Regexp Ranges and Locales: A Long Sad Story” on page 464). Locales can affect record splitting. For the normal case of ‘RS = "\n"’, the locale is largely irrelevant. For other single-character record separators, setting ‘LC_ALL=C’ in the envi‐ ronment will give you much better performance when reading records. Otherwise, gawk has to make several function calls, per input character, to find the record terminator. Locales can affect how dates and times are formatted (see “Time Functions” on page 211). For example, a common way to abbreviate the date September 4, 2015, in the United States is “9/4/15.” In many countries in Europe, however, it is abbreviated “4.9.15.” Thus, the ‘%x’ specification in a "US" locale might produce ‘9/4/15’, while in a "EUROPE" locale, it might produce ‘4.9.15’. According to POSIX, string comparison is also affected by locales (similar to regular expressions). The details are presented in “String comparison with POSIX rules” on page 131. 138 | Chapter 6: Expressions Finally, the locale affects the value of the decimal point character used when gawk parses input data. This is discussed in detail in “Conversion of Strings and Numbers” on page 117. Summary • Expressions are the basic elements of computation in programs. They are built from constants, variables, function calls, and combinations of the various kinds of values with operators. • awk supplies three kinds of constants: numeric, string, and regexp. gawk lets you specify numeric constants in octal and hexadecimal (bases 8 and 16) as well as decimal (base 10). In certain contexts, a standalone regexp constant such as /foo/ has the same meaning as ‘$0 ~ /foo/’. • Variables hold values between uses in computations. A number of built-in variables provide information to your awk program, and a number of others let you control how awk behaves. • Numbers are automatically converted to strings, and strings to numbers, as needed by awk. Numeric values are converted as if they were formatted with sprintf() using the format in CONVFMT. Locales can influence the conversions. • awk provides the usual arithmetic operators (addition, subtraction, multiplication, division, modulus), and unary plus and minus. It also provides comparison oper‐ ators, Boolean operators, an array membership testing operator, and regexp match‐ ing operators. String concatenation is accomplished by placing two expressions next to each other; there is no explicit operator. The three-operand ‘?:’ operator provides an “if-else” test within expressions. • Assignment operators provide convenient shorthands for common arithmetic operations. • In awk, a value is considered to be true if it is nonzero or non-null. Otherwise, the value is false. • A variable’s type is set upon each assignment and may change over its lifetime. The type determines how it behaves in comparisons (string or numeric). • Function calls return a value that may be used as part of a larger expression. Ex‐ pressions used to pass parameter values are fully evaluated before the function is called. awk provides built-in and user-defined functions; this is described in Chapter 9. Summary | 139 • Operator precedence specifies the order in which operations are performed, unless explicitly overridden by parentheses. awk’s operator precedence is compatible with that of C. • Locales can affect the format of data as output by an awk program, and occasionally the format for data read as input. 140 | Chapter 6: Expressions CHAPTER 7 Patterns, Actions, and Variables As you have already seen, each awk statement consists of a pattern with an associated action. This chapter describes how you build patterns and actions, what kinds of things you can do within actions, and awk’s predefined variables. The pattern–action rules and the statements available for use within actions form the core of awk programming. In a sense, everything covered up to here has been the foun‐ dation that programs are built on top of. Now it’s time to start building something useful. Pattern Elements Patterns in awk control the execution of rules—a rule is executed when its pattern matches the current input record. The following is a summary of the types of awk patterns: /regular expression/ A regular expression. It matches when the text of the input record fits the regular expression. (See Chapter 3.) expression A single expression. It matches when its value is nonzero (if a number) or non-null (if a string). (See “Expressions as Patterns” on page 142.) begpat, endpat A pair of patterns separated by a comma, specifying a range of records. The range includes both the initial record that matches begpat and the final record that matches endpat. (See “Specifying Record Ranges with Patterns” on page 143.) 141 BEGIN END Special patterns for you to supply startup or cleanup actions for your awk program. (See “The BEGIN and END Special Patterns” on page 145.) BEGINFILE ENDFILE Special patterns for you to supply startup or cleanup actions to be done on a per- file basis. (See “The BEGINFILE and ENDFILE Special Patterns” on page 147.) empty The empty pattern matches every input record. (See “The Empty Pattern” on page 148.) Regular Expressions as Patterns Regular expressions are one of the first kinds of patterns presented in this book. This kind of pattern is simply a regexp constant in the pattern part of a rule. Its meaning is ‘$0 ~ /pattern/’. The pattern matches when the input record matches the regexp. For example: /foo|bar|baz/ { buzzwords++ } END { print buzzwords, "buzzwords seen" } Expressions as Patterns Any awk expression is valid as an awk pattern. The pattern matches if the expression’s value is nonzero (if a number) or non-null (if a string). The expression is reevaluated each time the rule is tested against a new input record. If the expression uses fields such as $1, the value depends directly on the new input record’s text; otherwise, it depends on only what has happened so far in the execution of the awk program. Comparison expressions, using the comparison operators described in “Variable Typing and Comparison Expressions” on page 128, are a very common kind of pattern. Regexp matching and nonmatching are also very common expressions. The left operand of the ‘~’ and ‘!~’ operators is a string. The right operand is either a constant regular expression enclosed in slashes (/regexp/), or any expression whose string value is used as a dy‐ namic regular expression (see “Using Dynamic Regexps” on page 49). The following example prints the second field of each input record whose first field is precisely ‘li’: $ awk '$1 == "li" { print $2 }' mail-list (There is no output, because there is no person with the exact name ‘li’.) Contrast this with the following regular expression match, which accepts any record with a first field that contains ‘li’: 142 | Chapter 7: Patterns, Actions, and Variables $ awk '$1 ~ /li/ { print $2 }' mail-list 555-5553 555-6699 A regexp constant as a pattern is also a special case of an expression pattern. The ex‐ pression /li/ has the value one if ‘li’ appears in the current input record. Thus, as a pattern, /li/ matches any record containing ‘li’. Boolean expressions are also commonly used as patterns. Whether the pattern matches an input record depends on whether its subexpressions match. For example, the fol‐ lowing command prints all the records in mail-list that contain both ‘edu’ and ‘li’: $ awk '/edu/ && /li/' mail-list Samuel 555-3430 samuel.lanceolis@shu.edu A The following command prints all records in mail-list that contain either ‘edu’ or ‘li’ (or both, of course): $ awk '/edu/ || /li/' mail-list Amelia 555-5553 amelia.zodiacusque@gmail.com F Broderick 555-0542 broderick.aliquotiens@yahoo.com R Fabius 555-1234 fabius.undevicesimus@ucb.edu F Julie 555-6699 julie.perscrutabor@skeeve.com F Samuel 555-3430 samuel.lanceolis@shu.edu A Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R The following command prints all records in mail-list that do not contain the string ‘li’: $ awk '! /li/' mail-list Anthony 555-3412 anthony.asserturo@hotmail.com A Becky 555-7685 becky.algebrarum@gmail.com A Bill 555-1675 bill.drowning@hotmail.com A Camilla 555-2912 camilla.infusarum@skynet.be R Fabius 555-1234 fabius.undevicesimus@ucb.edu F Martin 555-6480 martin.codicibus@hotmail.com A Jean-Paul 555-2127 jeanpaul.campanorum@nyu.edu R The subexpressions of a Boolean operator in a pattern can be constant regular expres‐ sions, comparisons, or any other awk expressions. Range patterns are not expressions, so they cannot appear inside Boolean patterns. Likewise, the special patterns BEGIN, END, BEGINFILE, and ENDFILE, which never match any input record, are not expressions and cannot appear inside Boolean patterns. The precedence of the different operators that can appear in patterns is described in “Operator Precedence (How Operators Nest)” on page 136. Specifying Record Ranges with Patterns A range pattern is made of two patterns separated by a comma, in the form ‘begpat, endpat’. It is used to match ranges of consecutive input records. The first pattern, Pattern Elements | 143 begpat, controls where the range begins, while endpat controls where the pattern ends. For example, the following: awk '$1 == "on", $1 == "off"' myfile prints every record in myfile between ‘on’/‘off’ pairs, inclusive. A range pattern starts out by matching begpat against every input record. When a record matches begpat, the range pattern is turned on, and the range pattern matches this record as well. As long as the range pattern stays turned on, it automatically matches every input record read. The range pattern also matches endpat against every input record; when this succeeds, the range pattern is turned off again for the following record. Then the range pattern goes back to checking begpat against each record. The record that turns on the range pattern and the one that turns it off both match the range pattern. If you don’t want to operate on these records, you can write if statements in the rule’s action to distinguish them from the records you are interested in. It is possible for a pattern to be turned on and off by the same record. If the record satisfies both conditions, then the action is executed for just that record. For example, suppose there is text between two identical markers (e.g., the ‘%’ symbol), each on its own line, that should be ignored. A first attempt would be to combine a range pattern that describes the delimited text with the next statement (not discussed yet, see “The next Statement” on page 157). This causes awk to skip any further processing of the current record and start over again with the next input record. Such a program looks like this: /^%$/,/^%$/ { next } { print } This program fails because the range pattern is both turned on and turned off by the first line, which just has a ‘%’ on it. To accomplish this task, write the program in the following manner, using a flag: /^%$/ { skip = ! skip; next } skip == 1 { next } # skip lines with `skip' set In a range pattern, the comma (‘,’) has the lowest precedence of all the operators (i.e., it is evaluated last). Thus, the following program attempts to combine a range pattern with another, simpler test: echo Yes | awk '/1/,/2/ || /Yes/' The intent of this program is ‘(/1/,/2/) || /Yes/’. However, awk interprets this as ‘/ 1/, (/2/ || /Yes/)’. This cannot be changed or worked around; range patterns do not combine with other patterns: $ echo Yes | gawk '(/1/,/2/) || /Yes/' error→ gawk: cmd. line:1: (/1/,/2/) || /Yes/ error→ gawk: cmd. line:1: ^ syntax error 144 | Chapter 7: Patterns, Actions, and Variables As a minor point of interest, although it is poor style, POSIX allows you to put a newline after the comma in a range pattern. (d.c.) The BEGIN and END Special Patterns All the patterns described so far are for matching input records. The BEGIN and END special patterns are different. They supply startup and cleanup actions for awk programs. BEGIN and END rules must have actions; there is no default action for these rules because there is no current record when they run. BEGIN and END rules are often referred to as “BEGIN and END blocks” by longtime awk programmers. Startup and cleanup actions A BEGIN rule is executed once only, before the first input record is read. Likewise, an END rule is executed once only, after all the input is read. For example: $ awk ' > BEGIN { print "Analysis of \"li\"" } > /li/ { ++n } > END { print "\"li\" appears in", n, "records." }' mail-list Analysis of "li" "li" appears in 4 records. This program finds the number of records in the input file mail-list that contain the string ‘li’. The BEGIN rule prints a title for the report. There is no need to use the BEGIN rule to initialize the counter n to zero, as awk does this automatically (see “Variables” on page 115). The second rule increments the variable n every time a record containing the pattern ‘li’ is read. The END rule prints the value of n at the end of the run. The special patterns BEGIN and END cannot be used in ranges or with Boolean operators (indeed, they cannot be used with any operators). An awk program may have multiple BEGIN and/or END rules. They are executed in the order in which they appear: all the BEGIN rules at startup and all the END rules at termination. BEGIN and END rules may be intermixed with other rules. This feature was added in the 1987 version of awk and is included in the POSIX standard. The original (1978) version of awk required the BEGIN rule to be placed at the beginning of the program, the END rule to be placed at the end, and only allowed one of each. This is no longer required, but it is a good idea to follow this template in terms of program organization and readability. Multiple BEGIN and END rules are useful for writing library functions, because each li‐ brary file can have its own BEGIN and/or END rule to do its own initialization and/or cleanup. The order in which library functions are named on the command line controls the order in which their BEGIN and END rules are executed. Therefore, you have to be careful when writing such rules in library files so that the order in which they are Pattern Elements | 145 1. The original version of awk kept reading and ignoring input until the end of the file was seen. executed doesn’t matter. See “Command-Line Options” on page 22 for more informa‐ tion on using library functions. See Chapter 10 for a number of useful library functions. If an awk program has only BEGIN rules and no other rules, then the program exits after the BEGIN rules are run.1 However, if an END rule exists, then the input is read, even if there are no other rules in the program. This is necessary in case the END rule checks the FNR and NR variables. Input/output from BEGIN and END rules There are several (sometimes subtle) points to be aware of when doing I/O from a BEGIN or END rule. The first has to do with the value of $0 in a BEGIN rule. Because BEGIN rules are executed before any input is read, there simply is no input record, and therefore no fields, when executing BEGIN rules. References to $0 and the fields yield a null string or zero, depending upon the context. One way to give $0 a real value is to execute a getline command without a variable (see “Explicit Input with getline” on page 77). Another way is simply to assign a value to $0. The second point is similar to the first, but from the other direction. Traditionally, due largely to implementation issues, $0 and NF were undefined inside an END rule. The POSIX standard specifies that NF is available in an END rule. It contains the number of fields from the last input record. Most probably due to an oversight, the standard does not say that $0 is also preserved, although logically one would think that it should be. In fact, all of BWK awk, mawk, and gawk preserve the value of $0 for use in END rules. Be aware, however, that some other implementations and many older versions of Unix awk do not. The third point follows from the first two. The meaning of ‘print’ inside a BEGIN or END rule is the same as always: ‘print $0’. If $0 is the null string, then this prints an empty record. Many longtime awk programmers use an unadorned ‘print’ in BEGIN and END rules, to mean ‘print ""’, relying on $0 being null. Although one might generally get away with this in BEGIN rules, it is a very bad idea in END rules, at least in gawk. It is also poor style, because if an empty line is needed in the output, the program should print one explicitly. Finally, the next and nextfile statements are not allowed in a BEGIN rule, because the implicit read-a-record-and-match-against-the-rules loop has not started yet. Similarly, those statements are not valid in an END rule, because all the input has been read. (See “The next Statement” on page 157 and “The nextfile Statement” on page 158.) 146 | Chapter 7: Patterns, Actions, and Variables The BEGINFILE and ENDFILE Special Patterns This section describes a gawk-specific feature. Two special kinds of rule, BEGINFILE and ENDFILE, give you “hooks” into gawk’s command-line file processing loop. As with the BEGIN and END rules (see the previous section), all BEGINFILE rules in a program are merged, in the order they are read by gawk, and all ENDFILE rules are merged as well. The body of the BEGINFILE rules is executed just before gawk reads the first record from a file. FILENAME is set to the name of the current file, and FNR is set to zero. The BEGINFILE rule provides you the opportunity to accomplish two tasks that would otherwise be difficult or impossible to perform: • You can test if the file is readable. Normally, it is a fatal error if a file named on the command line cannot be opened for reading. However, you can bypass the fatal error and move on to the next file on the command line. You do this by checking if the ERRNO variable is not the empty string; if so, then gawk was not able to open the file. In this case, your program can execute the nextfile statement (see “The nextfile Statement” on page 158). This causes gawk to skip the file entirely. Otherwise, gawk exits with the usual fatal error. • If you have written extensions that modify the record handling (by inserting an “input parser”; see “Customized input parsers” on page 405), you can invoke them at this point, before gawk has started processing the file. (This is a very advanced feature, currently used only by the gawkextlib project.) The ENDFILE rule is called when gawk has finished processing the last record in an input file. For the last input file, it will be called before any END rules. The ENDFILE rule is executed even for empty input files. Normally, when an error occurs when reading input in the normal input-processing loop, the error is fatal. However, if an ENDFILE rule is present, the error becomes non- fatal, and instead ERRNO is set. This makes it possible to catch and process I/O errors at the level of the awk program. The next statement (see “The next Statement” on page 157) is not allowed inside either a BEGINFILE or an ENDFILE rule. The nextfile statement is allowed only inside a BEGIN FILE rule, not inside an ENDFILE rule. The getline statement (see “Explicit Input with getline” on page 77) is restricted inside both BEGINFILE and ENDFILE: only redirected forms of getline are allowed. Pattern Elements | 147 BEGINFILE and ENDFILE are gawk extensions. In most other awk implementations, or if gawk is in compatibility mode (see “Command-Line Options” on page 22), they are not special. The Empty Pattern An empty (i.e., nonexistent) pattern is considered to match every input record. For example, the program: awk '{ print $1 }' mail-list prints the first field of every record. Using Shell Variables in Programs awk programs are often used as components in larger programs written in shell. For example, it is very common to use a shell variable to hold a pattern that the awk program searches for. There are two ways to get the value of the shell variable into the body of the awk program. A common method is to use shell quoting to substitute the variable’s value into the program inside the script. For example, consider the following program: printf "Enter search pattern: " read pattern awk "/$pattern/ "'{ nmatches++ } END { print nmatches, "found" }' /path/to/data The awk program consists of two pieces of quoted text that are concatenated together to form the program. The first part is double-quoted, which allows substitution of the pattern shell variable inside the quotes. The second part is single-quoted. Variable substitution via quoting works, but can potentially be messy. It requires a good understanding of the shell’s quoting rules (see “Shell Quoting Issues” on page 8), and it’s often difficult to correctly match up the quotes when reading the program. A better method is to use awk’s variable assignment feature (see “Assigning variables on the command line” on page 116) to assign the shell variable’s value to an awk variable. Then use dynamic regexps to match the pattern (see “Using Dynamic Regexps” on page 49). The following shows how to redo the previous example using this technique: printf "Enter search pattern: " read pattern awk -v pat="$pattern" '$0 ~ pat { nmatches++ } END { print nmatches, "found" }' /path/to/data Now, the awk program is just one single-quoted string. The assignment ‘-v pat="$pat tern"’ still requires double quotes, in case there is whitespace in the value of 148 | Chapter 7: Patterns, Actions, and Variables $pattern. The awk variable pat could be named pattern too, but that would be more confusing. Using a variable also provides more flexibility, as the variable can be used anywhere inside the program—for printing, as an array subscript, or for any other use —without requiring the quoting tricks at every point in the program. Actions An awk program or script consists of a series of rules and function definitions inter‐ spersed. (Functions are described later. See “User-Defined Functions” on page 220.) A rule contains a pattern and an action, either of which (but not both) may be omitted. The purpose of the action is to tell awk what to do once a match for the pattern is found. Thus, in outline, an awk program generally looks like this: [pattern] { action } pattern [{ action }] … function name(args) { … } … An action consists of one or more awk statements, enclosed in braces (‘{…}’). Each statement specifies one thing to do. The statements are separated by newlines or semi‐ colons. The braces around an action must be used even if the action contains only one statement, or if it contains no statements at all. However, if you omit the action entirely, omit the braces as well. An omitted action is equivalent to ‘{ print $0 }’: /foo/ { } match foo, do nothing --- empty action /foo/ match foo, print the record --- omitted action The following types of statements are supported in awk: Expressions Call functions or assign values to variables (see Chapter 6). Executing this kind of statement simply computes the value of the expression. This is useful when the expression has side effects (see “Assignment Expressions” on page 122). Control statements Specify the control flow of awk programs. The awk language gives you C-like con‐ structs (if, for, while, and do) as well as a few special ones (see “Control Statements in Actions” on page 150). Compound statements Enclose one or more statements in braces. A compound statement is used in order to put several statements together in the body of an if, while, do, or for statement. Actions | 149 Input statements Use the getline command (see “Explicit Input with getline” on page 77). Also supplied in awk are the next statement (see “The next Statement” on page 157) and the nextfile statement (see “The nextfile Statement” on page 158). Output statements Such as print and printf. See Chapter 5. Deletion statements For deleting array elements. See “The delete Statement” on page 184. Control Statements in Actions Control statements, such as if, while, and so on, control the flow of execution in awk programs. Most of awk’s control statements are patterned after similar statements in C. All the control statements start with special keywords, such as if and while, to distin‐ guish them from simple expressions. Many control statements contain other statements. For example, the if statement contains another statement that may or may not be ex‐ ecuted. The contained statement is called the body. To include more than one statement in the body, group them into a single compound statement with braces, separating them with newlines or semicolons. The if-else Statement The if-else statement is awk’s decision-making statement. It looks like this: if (condition) then-body [else else-body] The condition is an expression that controls what the rest of the statement does. If the condition is true, then-body is executed; otherwise, else-body is executed. The else part of the statement is optional. The condition is considered false if its value is zero or the null string; otherwise, the condition is true. Refer to the following: if (x % 2 == 0) print "x is even" else print "x is odd" In this example, if the expression ‘x % 2 == 0’ is true (i.e., if the value of x is evenly divisible by two), then the first print statement is executed; otherwise, the second print statement is executed. If the else keyword appears on the same line as then-body and then-body is not a compound statement (i.e., not surrounded by braces), then a semi‐ colon must separate then-body from the else. To illustrate this, the previous example can be rewritten as: 150 | Chapter 7: Patterns, Actions, and Variables if (x % 2 == 0) print "x is even"; else print "x is odd" If the ‘;’ is left out, awk can’t interpret the statement and it produces a syntax error. Don’t actually write programs this way, because a human reader might fail to see the else if it is not the first thing on its line. The while Statement In programming, a loop is a part of a program that can be executed two or more times in succession. The while statement is the simplest looping statement in awk. It repeatedly executes a statement as long as a condition is true. For example: while (condition) body body is a statement called the body of the loop, and condition is an expression that controls how long the loop keeps running. The first thing the while statement does is test the condition. If the condition is true, it executes the statement body. After body has been executed, condition is tested again, and if it is still true, body executes again. This process repeats until the condition is no longer true. If the condition is initially false, the body of the loop never executes and awk continues with the statement following the loop. This example prints the first three fields of each record, one per line: awk ' { i = 1 while (i <= 3) { print $i i++ } }' inventory-shipped The body of this loop is a compound statement enclosed in braces, containing two statements. The loop works in the following manner: first, the value of i is set to one. Then, the while statement tests whether i is less than or equal to three. This is true when i equals one, so the ith field is printed. Then the ‘i++’ increments the value of i and the loop repeats. The loop terminates when i reaches four. A newline is not required between the condition and the body; however, using one makes the program clearer unless the body is a compound statement or else is very simple. The newline after the open brace that begins the compound statement is not required either, but the program is harder to read without it. The do-while Statement The do loop is a variation of the while looping statement. The do loop executes the body once and then repeats the body as long as the condition is true. It looks like this: Control Statements in Actions | 151 do body while (condition) Even if the condition is false at the start, the body executes at least once (and only once, unless executing body makes condition true). Contrast this with the corresponding while statement: while (condition) body This statement does not execute the body even once if the condition is false to begin with. The following is an example of a do statement: { i = 1 do { print $0 i++ } while (i <= 10) } This program prints each input record 10 times. However, it isn’t a very realistic example, because in this case an ordinary while would do just as well. This situation reflects actual experience; only occasionally is there a real use for a do statement. The for Statement The for statement makes it more convenient to count iterations of a loop. The general form of the for statement looks like this: for (initialization; condition; increment) body The initialization, condition, and increment parts are arbitrary awk expressions, and body stands for any awk statement. The for statement starts by executing initialization. Then, as long as the condi tion is true, it repeatedly executes body and then increment. Typically, initializa tion sets a variable to either zero or one, increment adds one to it, and condition compares it against the desired number of iterations. For example: awk ' { for (i = 1; i <= 3; i++) print $i }' inventory-shipped This prints the first three fields of each input record, with one field per line. 152 | Chapter 7: Patterns, Actions, and Variables It isn’t possible to set more than one variable in the initialization part without using a multiple assignment statement such as ‘x = y = 0’. This makes sense only if all the initial values are equal. (But it is possible to initialize additional variables by writing their assignments as separate statements preceding the for loop.) The same is true of the increment part. Incrementing additional variables requires separate statements at the end of the loop. The C compound expression, using C’s com‐ ma operator, is useful in this context, but it is not supported in awk. Most often, increment is an increment expression, as in the previous example. But this is not required; it can be any expression whatsoever. For example, the following state‐ ment prints all the powers of two between 1 and 100: for (i = 1; i <= 100; i *= 2) print i If there is nothing to be done, any of the three expressions in the parentheses following the for keyword may be omitted. Thus, ‘for (; x > 0;)’ is equivalent to ‘while (x > 0)’. If the condition is omitted, it is treated as true, effectively yielding an infinite loop (i.e., a loop that never terminates). In most cases, a for loop is an abbreviation for a while loop, as shown here: initialization while (condition) { body increment } The only exception is when the continue statement (see “The continue Statement” on page 156) is used inside the loop. Changing a for statement to a while statement in this way can change the effect of the continue statement inside the loop. The awk language has a for statement in addition to a while statement because a for loop is often both less work to type and more natural to think of. Counting the number of iterations is very common in loops. It can be easier to think of this counting as part of looping rather than as something to do inside the loop. There is an alternative version of the for loop, for iterating over all the indices of an array: for (i in array) do something with array[i] See “Scanning All Elements of an Array” on page 178 for more information on this version of the for loop. Control Statements in Actions | 153 The switch Statement This section describes a gawk-specific feature. If gawk is in compatibility mode (see “Command-Line Options” on page 22), it is not available. The switch statement allows the evaluation of an expression and the execution of state‐ ments based on a case match. Case statements are checked for a match in the order they are defined. If no suitable case is found, the default section is executed, if supplied. Each case contains a single constant, be it numeric, string, or regexp. The switch expression is evaluated, and then each case’s constant is compared against the result in turn. The type of constant determines the comparison: numeric or string do the usual comparisons. A regexp constant does a regular expression match against the string value of the original expression. The general form of the switch statement looks like this: switch (expression) { case value or regular expression: case-body default: default-body } Control flow in the switch statement works as it does in C. Once a match to a given case is made, the case statement bodies execute until a break, continue, next, next file, or exit is encountered, or the end of the switch statement itself. For example: while ((c = getopt(ARGC, ARGV, "aksx")) != -1) { switch (c) { case "a": # report size of all files all_files = TRUE; break case "k": BLOCK_SIZE = 1024 # 1K block size break case "s": # do sums only sum_only = TRUE break case "x": # don't cross filesystems fts_flags = or(fts_flags, FTS_XDEV) break case "?": default: usage() break } } 154 | Chapter 7: Patterns, Actions, and Variables Note that if none of the statements specified here halt execution of a matched case statement, execution falls through to the next case until execution halts. In this example, the case for "?" falls through to the default case, which is to call a function named usage(). (The getopt() function being called here is described in “Processing Command-Line Options” on page 259.) The break Statement The break statement jumps out of the innermost for, while, or do loop that encloses it. The following example finds the smallest divisor of any integer, and also identifies prime numbers: # find smallest divisor of num { num = $1 for (divisor = 2; divisor * divisor <= num; divisor++) { if (num % divisor == 0) break } if (num % divisor == 0) printf "Smallest divisor of %d is %d\n", num, divisor else printf "%d is prime\n", num } When the remainder is zero in the first if statement, awk immediately breaks out of the containing for loop. This means that awk proceeds immediately to the statement fol‐ lowing the loop and continues processing. (This is very different from the exit state‐ ment, which stops the entire awk program. See “The exit Statement” on page 159.) The following program illustrates how the condition of a for or while statement could be replaced with a break inside an if: # find smallest divisor of num { num = $1 for (divisor = 2; ; divisor++) { if (num % divisor == 0) { printf "Smallest divisor of %d is %d\n", num, divisor break } if (divisor * divisor > num) { printf "%d is prime\n", num break } } } Control Statements in Actions | 155 The break statement is also used to break out of the switch statement. This is discussed in “The switch Statement” on page 154. The break statement has no meaning when used outside the body of a loop or switch. However, although it was never documented, historical implementations of awk treated the break statement outside of a loop as if it were a next statement (see “The next Statement” on page 157). (d.c.) Recent versions of BWK awk no longer allow this usage, nor does gawk. The continue Statement Similar to break, the continue statement is used only inside for, while, and do loops. It skips over the rest of the loop body, causing the next cycle around the loop to begin immediately. Contrast this with break, which jumps out of the loop altogether. The continue statement in a for loop directs awk to skip the rest of the body of the loop and resume execution with the increment-expression of the for statement. The fol‐ lowing program illustrates this fact: BEGIN { for (x = 0; x <= 20; x++) { if (x == 5) continue printf "%d ", x } print "" } This program prints all the numbers from 0 to 20—except for 5, for which the printf is skipped. Because the increment ‘x++’ is not skipped, x does not remain stuck at 5. Contrast the for loop from the previous example with the following while loop: BEGIN { x = 0 while (x <= 20) { if (x == 5) continue printf "%d ", x x++ } print "" } This program loops forever once x reaches 5, because the increment (‘x++’) is never reached. The continue statement has no special meaning with respect to the switch statement, nor does it have any meaning when used outside the body of a loop. Historical versions of awk treated a continue statement outside a loop the same way they treated a break 156 | Chapter 7: Patterns, Actions, and Variables statement outside a loop: as if it were a next statement (discussed in the following section). (d.c.) Recent versions of BWK awk no longer work this way, nor does gawk. The next Statement The next statement forces awk to immediately stop processing the current record and go on to the next record. This means that no further rules are executed for the current record, and the rest of the current rule’s action isn’t executed. Contrast this with the effect of the getline function (see “Explicit Input with getline” on page 77). That also causes awk to read the next record immediately, but it does not alter the flow of control in any way (i.e., the rest of the current action executes with a new input record). At the highest level, awk program execution is a loop that reads an input record and then tests each rule’s pattern against it. If you think of this loop as a for statement whose body contains the rules, then the next statement is analogous to a continue statement. It skips to the end of the body of this implicit loop and executes the increment (which reads another record). For example, suppose an awk program works only on records with four fields, and it shouldn’t fail when given bad input. To avoid complicating the rest of the program, write a “weed out” rule near the beginning, in the following manner: NF != 4 { printf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) > "/dev/stderr" next } Because of the next statement, the program’s subsequent rules won’t see the bad record. The error message is redirected to the standard error output stream, as error messages should be. For more detail, see “Special Filenames in gawk” on page 104. If the next statement causes the end of the input to be reached, then the code in any END rules is executed. See “The BEGIN and END Special Patterns” on page 145. The next statement is not allowed inside BEGINFILE and ENDFILE rules. See “The BE‐ GINFILE and ENDFILE Special Patterns” on page 147. According to the POSIX standard, the behavior is undefined if the next statement is used in a BEGIN or END rule. gawk treats it as a syntax error. Although POSIX does not disallow it, most other awk implementations don’t allow the next statement inside func‐ tion bodies (see “User-Defined Functions” on page 220). Just as with any other next statement, a next statement inside a function body reads the next record and starts processing it with the first rule in the program. Control Statements in Actions | 157 The nextfile Statement The nextfile statement is similar to the next statement. However, instead of aban‐ doning processing of the current record, the nextfile statement instructs awk to stop processing the current datafile. Upon execution of the nextfile statement, FILENAME is updated to the name of the next datafile listed on the command line, FNR is reset to 1, and processing starts over with the first rule in the program. If the nextfile statement causes the end of the input to be reached, then the code in any END rules is executed. An exception to this is when nextfile is invoked during execution of any statement in an END rule; in this case, it causes the program to stop immediately. See “The BEGIN and END Special Patterns” on page 145. The nextfile statement is useful when there are many datafiles to process but it isn’t necessary to process every record in every file. Without nextfile, in order to move on to the next datafile, a program would have to continue scanning the unwanted records. The nextfile statement accomplishes this much more efficiently. In gawk, execution of nextfile causes additional things to happen: any ENDFILE rules are executed if gawk is not currently in an END or BEGINFILE rule, ARGIND is incremented, and any BEGINFILE rules are executed. (ARGIND hasn’t been introduced yet. See “Prede‐ fined Variables” on page 160.) With gawk, nextfile is useful inside a BEGINFILE rule to skip over a file that would otherwise cause gawk to exit with a fatal error. In this case, ENDFILE rules are not exe‐ cuted. See “The BEGINFILE and ENDFILE Special Patterns” on page 147. Although it might seem that ‘close(FILENAME)’ would accomplish the same as next file, this isn’t true. close() is reserved for closing files, pipes, and coprocesses that are opened with redirections. It is not related to the main processing that awk does with the files listed in ARGV. For many years, nextfile was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard. See the Austin Group website. The current version of BWK awk and mawk also support nextfile. However, they don’t allow the nextfile statement inside function bodies (see “User-Defined Functions” on page 220). gawk does; a nextfile inside a function body reads the next record and starts processing it with the first rule in the program, just as any other nextfile statement. 158 | Chapter 7: Patterns, Actions, and Variables The exit Statement The exit statement causes awk to immediately stop executing the current rule and to stop processing input; any remaining input is ignored. The exit statement is written as follows: exit [return code] When an exit statement is executed from a BEGIN rule, the program stops processing everything immediately. No input records are read. However, if an END rule is present, as part of executing the exit statement, the END rule is executed (see “The BEGIN and END Special Patterns” on page 145). If exit is used in the body of an END rule, it causes the program to stop immediately. An exit statement that is not part of a BEGIN or END rule stops the execution of any further automatic rules for the current record, skips reading any remaining input re‐ cords, and executes the END rule if there is one. gawk also skips any ENDFILE rules; they do not execute. In such a case, if you don’t want the END rule to do its job, set a variable to a nonzero value before the exit statement and check that variable in the END rule. See “Asser‐ tions” on page 245 for an example that does this. If an argument is supplied to exit, its value is used as the exit status code for the awk process. If no argument is supplied, exit causes awk to return a “success” status. In the case where an argument is supplied to a first exit statement, and then exit is called a second time from an END rule with no argument, awk uses the previously supplied exit value. (d.c.) See “gawk’s Exit Status” on page 34 for more information. For example, suppose an error condition occurs that is difficult or impossible to handle. Conventionally, programs report this by exiting with a nonzero status. An awk program can do this using an exit statement with a nonzero argument, as shown in the following example: BEGIN { if (("date" | getline date_now) <= 0) { print "Can't get system date" > "/dev/stderr" exit 1 } print "current date is", date_now close("date") } For full portability, exit values should be between zero and 126, in‐ clusive. Negative values, and values of 127 or greater, may not pro‐ duce consistent results across different operating systems. Control Statements in Actions | 159 Predefined Variables Most awk variables are available to use for your own purposes; they never change unless your program assigns values to them, and they never affect anything unless your pro‐ gram examines them. However, a few variables in awk have special built-in meanings. awk examines some of these automatically, so that they enable you to tell awk how to do certain things. Others are set automatically by awk, so that they carry information from the internal workings of awk to your program. This section documents all of gawk’s predefined variables, most of which are also doc‐ umented in the chapters describing their areas of activity. Built-in Variables That Control awk The following is an alphabetical list of variables that you can change to control how awk does certain things. The variables that are specific to gawk are marked with a pound sign (‘#’). These variables are gawk extensions. In other awk implementations or if gawk is in compatibility mode (see “Command-Line Options” on page 22), they are not special. (Any exceptions are noted in the description of each variable.) BINMODE # On non-POSIX systems, this variable specifies use of binary mode for all I/O. Numeric values of one, two, or three specify that input files, output files, or all files, respectively, should use binary I/O. A numeric value less than zero is treated as zero, and a numeric value greater than three is treated as three. Alternatively, string values of "r" or "w" specify that input files and output files, respectively, should use binary I/O. A string value of "rw" or "wr" indicates that all files should use binary I/O. Any other string value is treated the same as "rw", but causes gawk to generate a warning message. BINMODE is described in more detail in “Using gawk on PC operating sys‐ tems” on page 477. mawk (see “Other Freely Available awk Implementations” on page 485) also supports this variable, but only using numeric values. CONVFMT A string that controls the conversion of numbers to strings (see “Conversion of Strings and Numbers” on page 117). It works by being passed, in effect, as the first argument to the sprintf() function (see “String-Manipulation Functions” on page 194). Its default value is "%.6g". CONVFMT was introduced by the POSIX standard. FIELDWIDTHS # A space-separated list of columns that tells gawk how to split input with fixed columnar boundaries. Assigning a value to FIELDWIDTHS overrides the use of FS 160 | Chapter 7: Patterns, Actions, and Variables 2. In POSIX awk, newline does not count as whitespace. and FPAT for field splitting. See “Reading Fixed-Width Data” on page 71 for more information. FPAT # A regular expression (as a string) that tells gawk to create the fields based on text that matches the regular expression. Assigning a value to FPAT overrides the use of FS and FIELDWIDTHS for field splitting. See “Defining Fields by Content” on page 73 for more information. FS The input field separator (see “Specifying How Fields Are Separated” on page 65). The value is a single-character string or a multicharacter regular expression that matches the separations between fields in an input record. If the value is the null string (""), then each character in the record becomes a separate field. (This be‐ havior is a gawk extension. POSIX awk does not specify the behavior when FS is the null string. Nonetheless, some other versions of awk also treat "" specially.) The default value is " ", a string consisting of a single space. As a special exception, this value means that any sequence of spaces, TABs, and/or newlines is a single separator.2 It also causes spaces, TABs, and newlines at the beginning and end of a record to be ignored. You can set the value of FS on the command line using the -F option: awk -F, 'program' input-files If gawk is using FIELDWIDTHS or FPAT for field splitting, assigning a value to FS causes gawk to return to the normal, FS-based field splitting. An easy way to do this is to simply say ‘FS = FS’, perhaps with an explanatory comment. IGNORECASE # If IGNORECASE is nonzero or non-null, then all string comparisons and all regular expression matching are case-independent. This applies to regexp matching with ‘~’ and ‘!~’, the gensub(), gsub(), index(), match(), patsplit(), split(), and sub() functions, record termination with RS, and field splitting with FS and FPAT. However, the value of IGNORECASE does not affect array subscripting and it does not affect field splitting when using a single-character field separator. See “Case Sensi‐ tivity in Matching” on page 52. LINT # When this variable is true (nonzero or non-null), gawk behaves as if the --lint command-line option is in effect (see “Command-Line Options” on page 22). With a value of "fatal", lint warnings become fatal errors. With a value of "invalid", Predefined Variables | 161 only warnings about things that are actually invalid are issued. (This is not fully implemented yet.) Any other true value prints nonfatal warnings. Assigning a false value to LINT turns off the lint warnings. This variable is a gawk extension. It is not special in other awk implementations. Unlike with the other special variables, changing LINT does affect the production of lint warnings, even if gawk is in compatibility mode. Much as the --lint and --traditional options independently control different aspects of gawk’s behavior, the control of lint warnings during program execution is independent of the flavor of awk being executed. OFMT A string that controls the conversion of numbers to strings (see “Conversion of Strings and Numbers” on page 117) for printing with the print statement. It works by being passed as the first argument to the sprintf() function (see “String- Manipulation Functions” on page 194). Its default value is "%.6g". Earlier versions of awk used OFMT to specify the format for converting numbers to strings in general expressions; this is now done by CONVFMT. OFS The output field separator (see “Output Separators” on page 91). It is output be‐ tween the fields printed by a print statement. Its default value is " ", a string consisting of a single space. ORS The output record separator. It is output at the end of every print statement. Its default value is "\n", the newline character. (See “Output Separators” on page 91.) PREC # The working precision of arbitrary-precision floating-point numbers, 53 bits by default (see “Setting the Precision” on page 385). ROUNDMODE # The rounding mode to use for arbitrary-precision arithmetic on numbers, by de‐ fault "N" (roundTiesToEven in the IEEE 754 standard; see “Setting the Rounding Mode” on page 387). RS The input record separator. Its default value is a string containing a single newline character, which means that an input record consists of a single line of text. It can also be the null string, in which case records are separated by runs of blank lines. If it is a regexp, records are separated by matches of the regexp in the input text. (See “How Input Is Split into Records” on page 55.) 162 | Chapter 7: Patterns, Actions, and Variables The ability for RS to be a regular expression is a gawk extension. In most other awk implementations, or if gawk is in compatibility mode (see “Command-Line Op‐ tions” on page 22), just the first character of RS’s value is used. SUBSEP The subscript separator. It has the default value of "\034" and is used to separate the parts of the indices of a multidimensional array. Thus, the expression ‘foo["A", "B"]’ really accesses foo["A\034B"] (see “Multidimensional Arrays” on page 185). TEXTDOMAIN # Used for internationalization of programs at the awk level. It sets the default text domain for specially marked string constants in the source text, as well as for the dcgettext(), dcngettext(), and bindtextdomain() functions (see Chapter 13). The default value of TEXTDOMAIN is "messages". Built-in Variables That Convey Information The following is an alphabetical list of variables that awk sets automatically on certain occasions in order to provide information to your program. The variables that are specific to gawk are marked with a pound sign (‘#’). These variables are gawk extensions. In other awk implementations or if gawk is in compatibility mode (see “Command-Line Options” on page 22), they are not special: ARGC, ARGV The command-line arguments available to awk programs are stored in an array called ARGV. ARGC is the number of command-line arguments present. See “Other Command-Line Arguments” on page 29. Unlike most awk arrays, ARGV is indexed from 0 to ARGC − 1. In the following example: $ awk 'BEGIN { > for (i = 0; i < ARGC; i++) > print ARGV[i] > }' inventory-shipped mail-list awk inventory-shipped mail-list ARGV[0] contains ‘awk’, ARGV[1] contains ‘inventory-shipped’, and ARGV[2] con‐ tains ‘mail-list’. The value of ARGC is three, one more than the index of the last element in ARGV, because the elements are numbered from zero. The names ARGC and ARGV, as well as the convention of indexing the array from 0 to ARGC − 1, are derived from the C language’s method of accessing command- line arguments. Predefined Variables | 163 The value of ARGV[0] can vary from system to system. Also, you should note that the program text is not included in ARGV, nor are any of awk’s command-line options. See “Using ARGC and ARGV” on page 170 for information about how awk uses these variables. (d.c.) ARGIND # The index in ARGV of the current file being processed. Every time gawk opens a new datafile for processing, it sets ARGIND to the index in ARGV of the filename. When gawk is processing the input files, ‘FILENAME == ARGV[ARGIND]’ is always true. This variable is useful in file processing; it allows you to tell how far along you are in the list of datafiles as well as to distinguish between successive instances of the same filename on the command line. While you can change the value of ARGIND within your awk program, gawk auto‐ matically sets it to a new value when it opens the next file. ENVIRON An associative array containing the values of the environment. The array indices are the environment variable names; the elements are the values of the particular environment variables. For example, ENVIRON["HOME"] might be "/home/ arnold". Changing this array does not affect the environment passed on to any programs that awk may spawn via redirection or the system() function. (In a future version of gawk, it may do so.) Some operating systems may not have environment variables. On such systems, the ENVIRON array is empty (except for ENVIRON["AWKPATH"] and ENVIRON["AWKLIB PATH"]; see “The AWKPATH Environment Variable” on page 31 and “The AW‐ KLIBPATH Environment Variable” on page 32). ERRNO # If a system error occurs during a redirection for getline, during a read for get line, or during a close() operation, then ERRNO contains a string describing the error. In addition, gawk clears ERRNO before opening each command-line input file. This enables checking if the file is readable inside a BEGINFILE pattern (see “The BE‐ GINFILE and ENDFILE Special Patterns” on page 147). Otherwise, ERRNO works similarly to the C variable errno. Except for the case just mentioned, gawk never clears it (sets it to zero or ""). Thus, you should only expect its value to be meaningful when an I/O operation returns a failure value, such as getline returning −1. You are, of course, free to clear it yourself before doing an I/O operation. 164 | Chapter 7: Patterns, Actions, and Variables 3. Some early implementations of Unix awk initialized FILENAME to "-", even if there were datafiles to be processed. This behavior was incorrect and should not be relied upon in your programs. FILENAME The name of the current input file. When no datafiles are listed on the command line, awk reads from the standard input and FILENAME is set to "-". FILENAME changes each time a new file is read (see Chapter 4). Inside a BEGIN rule, the value of FILENAME is "", because there are no input files being processed yet.3 (d.c.) Note, though, that using getline (see “Explicit Input with getline” on page 77) inside a BEGIN rule can give FILENAME a value. FNR The current record number in the current file. awk increments FNR each time it reads a new record (see “How Input Is Split into Records” on page 55). awk resets FNR to zero each time it starts a new input file. NF The number of fields in the current input record. NF is set each time a new record is read, when a new field is created, or when $0 changes (see “Examining Fields” on page 60). Unlike most of the variables described in this subsection, assigning a value to NF has the potential to affect awk’s internal workings. In particular, assignments to NF can be used to create fields in or remove fields from the current record. See “Changing the Contents of a Field” on page 62. FUNCTAB # An array whose indices and corresponding values are the names of all the built-in, user-defined, and extension functions in the program. Attempting to use the delete statement with the FUNCTAB array causes a fatal error. Any attempt to assign to an ele‐ ment of FUNCTAB also causes a fatal error. NR The number of input records awk has processed since the beginning of the program’s execution (see “How Input Is Split into Records” on page 55). awk increments NR each time it reads a new record. Predefined Variables | 165 PROCINFO # The elements of this array provide access to information about the running awk program. The following elements (listed alphabetically) are guaranteed to be available: PROCINFO["egid"] The value of the getegid() system call. PROCINFO["euid"] The value of the geteuid() system call. PROCINFO["FS"] This is "FS" if field splitting with FS is in effect, "FIELDWIDTHS" if field splitting with FIELDWIDTHS is in effect, or "FPAT" if field matching with FPAT is in effect. PROCINFO["identifiers"] A subarray, indexed by the names of all identifiers used in the text of the awk program. An identifier is simply the name of a variable (be it scalar or array), built-in function, user-defined function, or extension function. For each iden‐ tifier, the value of the element is one of the following: "array" The identifier is an array. "builtin" The identifier is a built-in function. "extension" The identifier is an extension function loaded via @load or -l. "scalar" The identifier is a scalar. "untyped" The identifier is untyped (could be used as a scalar or an array; gawk doesn’t know yet). "user" The identifier is a user-defined function. The values indicate what gawk knows about the identifiers after it has finished parsing the program; they are not updated while the program runs. PROCINFO["gid"] The value of the getgid() system call. PROCINFO["pgrpid"] The process group ID of the current process. 166 | Chapter 7: Patterns, Actions, and Variables PROCINFO["pid"] The process ID of the current process. PROCINFO["ppid"] The parent process ID of the current process. PROCINFO["sorted_in"] If this element exists in PROCINFO, its value controls the order in which array indices will be processed by ‘for (indx in array)’ loops. This is an advanced feature, so we defer the full description until later; see “Scanning All Elements of an Array” on page 178. PROCINFO["strftime"] The default time format string for strftime(). Assigning a new value to this element changes the default. See “Time Functions” on page 211. PROCINFO["uid"] The value of the getuid() system call. PROCINFO["version"] The version of gawk. The following additional elements in the array are available to provide information about the MPFR and GMP libraries if your version of gawk supports arbitrary- precision arithmetic (see Chapter 15): PROCINFO["mpfr_version"] The version of the GNU MPFR library. PROCINFO["gmp_version"] The version of the GNU MP library. PROCINFO["prec_max"] The maximum precision supported by MPFR. PROCINFO["prec_min"] The minimum precision required by MPFR. The following additional elements in the array are available to provide information about the version of the extension API, if your version of gawk supports dynamic loading of extension functions (see Chapter 16): PROCINFO["api_major"] The major version of the extension API. PROCINFO["api_minor"] The minor version of the extension API. Predefined Variables | 167 On some systems, there may be elements in the array, "group1" through "groupN" for some N. N is the number of supplementary groups that the process has. Use the in operator to test for these elements (see “Referring to an Array Element” on page 175). The PROCINFO array has the following additional uses: • It may be used to provide a timeout when reading from any open input file, pipe, or coprocess. See “Reading Input with a Timeout” on page 85 for more information. • It may be used to cause coprocesses to communicate over pseudo-ttys instead of through two-way pipes; this is discussed further in “Two-Way Communi‐ cations with Another Process” on page 334. RLENGTH The length of the substring matched by the match() function (see “String- Manipulation Functions” on page 194). RLENGTH is set by invoking the match() func‐ tion. Its value is the length of the matched string, or −1 if no match is found. RSTART The start index in characters of the substring that is matched by the match() func‐ tion (see “String-Manipulation Functions” on page 194). RSTART is set by invoking the match() function. Its value is the position of the string where the matched substring starts, or zero if no match was found. RT # The input text that matched the text denoted by RS, the record separator. It is set every time a record is read. SYMTAB # An array whose indices are the names of all defined global variables and arrays in the program. SYMTAB makes gawk’s symbol table visible to the awk programmer. It is built as gawk parses the program and is complete before the program starts to run. The array may be used for indirect access to read or write the value of a variable: foo = 5 SYMTAB["foo"] = 4 print foo # prints 4 The isarray() function (see “Getting Type Information” on page 219) may be used to test if an element in SYMTAB is an array. Also, you may not use the delete statement with the SYMTAB array. You may use an index for SYMTAB that is not a predefined identifier: 168 | Chapter 7: Patterns, Actions, and Variables 4. Not to mention difficult implementation issues. SYMTAB["xxx"] = 5 print SYMTAB["xxx"] This works as expected: in this case SYMTAB acts just like a regular array. The only difference is that you can’t then delete SYMTAB["xxx"]. The SYMTAB array is more interesting than it looks. Andrew Schorr points out that it effectively gives awk data pointers. Consider his example: # Indirect multiply of any variable by amount, return result function multiply(variable, amount) { return SYMTAB[variable] *= amount } In order to avoid severe time-travel paradoxes,4 neither FUNC TAB nor SYMTAB is available as an element within the SYMTAB array. Changing NR and FNR awk increments NR and FNR each time it reads a record, instead of setting them to the absolute value of the number of records read. This means that a program can change these variables and their new values are incremented for each record. (d.c.) The fol‐ lowing example shows this: $ echo '1 > 2 > 3 > 4' | awk 'NR == 2 { NR = 17 } > { print NR }' 1 17 18 19 Before FNR was added to the awk language (see “Major Changes Between V7 and SVR3.1” on page 457), many awk programs used this feature to track the number of records in a file by resetting NR to zero when FILENAME changed. Predefined Variables | 169 Using ARGC and ARGV “Built-in Variables That Convey Information” on page 163 presented the following program describing the information contained in ARGC and ARGV: $ awk 'BEGIN { > for (i = 0; i < ARGC; i++) > print ARGV[i] > }' inventory-shipped mail-list awk inventory-shipped mail-list In this example, ARGV[0] contains ‘awk’, ARGV[1] contains ‘inventory-shipped’, and ARGV[2] contains ‘mail-list’. Notice that the awk program is not entered in ARGV. The other command-line options, with their arguments, are also not entered. This includes variable assignments done with the -v option (see “Command-Line Options” on page 22). Normal variable assignments on the command line are treated as arguments and do show up in the ARGV array. Given the following program in a file named showargs.awk: BEGIN { printf "A=%d, B=%d\n", A, B for (i = 0; i < ARGC; i++) printf "\tARGV[%d] = %s\n", i, ARGV[i] } END { printf "A=%d, B=%d\n", A, B } Running it produces the following: $ awk -v A=1 -f showargs.awk B=2 /dev/null A=1, B=0 ARGV[0] = awk ARGV[1] = B=2 ARGV[2] = /dev/null A=1, B=2 A program can alter ARGC and the elements of ARGV. Each time awk reaches the end of an input file, it uses the next element of ARGV as the name of the next input file. By storing a different string there, a program can change which files are read. Use "-" to represent the standard input. Storing additional elements and incrementing ARGC causes addi‐ tional files to be read. If the value of ARGC is decreased, that eliminates input files from the end of the list. By recording the old value of ARGC elsewhere, a program can treat the eliminated arguments as something other than filenames. To eliminate a file from the middle of the list, store the null string ("") into ARGV in place of the file’s name. As a special feature, awk ignores filenames that have been replaced 170 | Chapter 7: Patterns, Actions, and Variables with the null string. Another option is to use the delete statement to remove elements from ARGV (see “The delete Statement” on page 184). All of these actions are typically done in the BEGIN rule, before actual processing of the input begins. See “Splitting a Large File into Pieces” on page 289 and “Duplicating Output into Multiple Files” on page 290 for examples of each way of removing elements from ARGV. To actually get options into an awk program, end the awk options with -- and then supply the awk program’s options, in the following manner: awk -f myprog.awk -- -v -q file1 file2 … The following fragment processes ARGV in order to examine, and then remove, the pre‐ viously mentioned command-line options: BEGIN { for (i = 1; i < ARGC; i++) { if (ARGV[i] == "-v") verbose = 1 else if (ARGV[i] == "-q") debug = 1 else if (ARGV[i] ~ /^-./) { e = sprintf("%s: unrecognized option -- %c", ARGV[0], substr(ARGV[i], 2, 1)) print e > "/dev/stderr" } else break delete ARGV[i] } } Ending the awk options with -- isn’t necessary in gawk. Unless --posix has been speci‐ fied, gawk silently puts any unrecognized options into ARGV for the awk program to deal with. As soon as it sees an unknown option, gawk stops looking for other options that it might otherwise recognize. The previous command line with gawk would be: gawk -f myprog.awk -q -v file1 file2 … Because -q is not a valid gawk option, it and the following -v are passed on to the awk program. (See “Processing Command-Line Options” on page 259 for an awk library func‐ tion that parses command-line options.) When designing your program, you should choose options that don’t conflict with gawk’s, because it will process any options that it accepts before passing the rest of the command line on to your program. Using ‘#!’ with the -E option may help (see “Exe‐ cutable awk Programs” on page 6 and “Command-Line Options” on page 22). Predefined Variables | 171 Summary • Pattern–action pairs make up the basic elements of an awk program. Patterns are either normal expressions, range expressions, or regexp constants; one of the special keywords BEGIN, END, BEGINFILE, or ENDFILE; or empty. The action executes if the current record matches the pattern. Empty (missing) patterns match all records. • I/O from BEGIN and END rules has certain constraints. This is also true, only more so, for BEGINFILE and ENDFILE rules. The latter two give you “hooks” into gawk’s file processing, allowing you to recover from a file that otherwise would cause a fatal error (such as a file that cannot be opened). • Shell variables can be used in awk programs by careful use of shell quoting. It is easier to pass a shell variable into awk by using the -v option and an awk variable. • Actions consist of statements enclosed in curly braces. Statements are built up from expressions, control statements, compound statements, input and output state‐ ments, and deletion statements. • The control statements in awk are if-else, while, for, and do-while. gawk adds the switch statement. There are two flavors of for statement: one for performing general looping, and the other for iterating through an array. • break and continue let you exit early or start the next iteration of a loop (or get out of a switch). • next and nextfile let you read the next record and start over at the top of your program or skip to the next input file and start over, respectively. • The exit statement terminates your program. When executed from an action (or function body), it transfers control to the END statements. From an END statement body, it exits immediately. You may pass an optional numeric value to be used as awk’s exit status. • Some predefined variables provide control over awk, mainly for I/O. Other variables convey information from awk to your program. • ARGC and ARGV make the command-line arguments available to your program. Ma‐ nipulating them from a BEGIN rule lets you control how awk will process the pro‐ vided datafiles. 172 | Chapter 7: Patterns, Actions, and Variables CHAPTER 8 Arrays in awk An array is a table of values called elements. The elements of an array are distinguished by their indices. Indices may be either numbers or strings. This chapter describes how arrays work in awk, how to use array elements, how to scan through every element in an array, and how to remove array elements. It also describes how awk simulates multidimensional arrays, as well as some of the less obvious points about array usage. The chapter moves on to discuss gawk’s facility for sorting arrays, and ends with a brief description of gawk’s ability to support true arrays of arrays. The Basics of Arrays This section presents the basics: working with elements in arrays one at a time, and traversing all of the elements in an array. Introduction to Arrays Doing linear scans over an associative array is like trying to club someone to death with a loaded Uzi. —Larry Wall The awk language provides one-dimensional arrays for storing groups of related strings or numbers. Every awk array must have a name. Array names have the same syntax as variable names; any valid variable name would also be a valid array name. But one name cannot be used in both ways (as an array and as a variable) in the same awk program. Arrays in awk superficially resemble arrays in other programming languages, but there are fundamental differences. In awk, it isn’t necessary to specify the size of an array before starting to use it. Additionally, any number or string, not just consecutive inte‐ gers, may be used as an array index. 173 1. The ordering will vary among awk implementations, which typically use hash tables to store array elements and values. In most other languages, arrays must be declared before use, including a specification of how many elements or components they contain. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. Usually, an index in the array must be a positive integer. For example, the index zero specifies the first element in the array, which is actually stored at the beginning of the block of memory. Index one specifies the second element, which is stored in memory right after the first element, and so on. It is impossible to add more elements to the array, because it has room only for as many elements as given in the declaration. (Some languages allow arbitrary starting and ending indices—e.g., ‘15 .. 27’—but the size of the array is still fixed when the array is declared.) A contiguous array of four elements might look like Figure 8-1, conceptually, if the element values are eight, "foo", "", and 30. Figure 8-1. A contiguous array Only the values are stored; the indices are implicit from the order of the values. Here, eight is the value at index zero, because eight appears in the position with zero elements before it. Arrays in awk are different—they are associative. This means that each array is a col‐ lection of pairs—an index and its corresponding array element value: Index Value 3 30 1 "foo" 0 8 2 "" The pairs are shown in jumbled order because their order is irrelevant.1 One advantage of associative arrays is that new pairs can be added at any time. For example, suppose a tenth element is added to the array whose value is "number ten". The result is: 174 | Chapter 8: Arrays in awk Index Value 10 "number ten" 3 30 1 "foo" 0 8 2 "" Now the array is sparse, which just means some indices are missing. It has elements 0– 3 and 10, but doesn’t have elements 4, 5, 6, 7, 8, or 9. Another consequence of associative arrays is that the indices don’t have to be positive integers. Any number, or even a string, can be an index. For example, the following is an array that translates words from English to French: Index Value "dog" "chien" "cat" "chat" "one" "un" 1 "un" Here we decided to translate the number one in both spelled-out and numeric form— thus illustrating that a single array can have both numbers and strings as indices. (In fact, array subscripts are always strings. There are some subtleties to how numbers work when used as array subscripts; this is discussed in more detail in “Using Numbers to Subscript Arrays” on page 182.) Here, the number 1 isn’t double-quoted, because awk automatically converts it to a string. The value of IGNORECASE has no effect upon array subscripting. The identical string value used to store an array element must be used to retrieve it. When awk creates an array (e.g., with the split() built-in function), that array’s indices are consecutive in‐ tegers starting at one. (See “String-Manipulation Functions” on page 194.) awk’s arrays are efficient—the time to access an element is independent of the number of elements in the array. Referring to an Array Element The principal way to use an array is to refer to one of its elements. An array reference is an expression as follows: array[index-expression] Here, array is the name of an array. The expression index-expression is the index of the desired element of the array. The Basics of Arrays | 175 The value of the array reference is the current value of that array element. For example, foo[4.3] is an expression referencing the element of array foo at index ‘4.3’. A reference to an array element that has no recorded value yields a value of "", the null string. This includes elements that have not been assigned any value as well as elements that have been deleted (see “The delete Statement” on page 184). A reference to an element that does not exist automatically creates that array element, with the null string as its value. (In some cases, this is unfortunate, because it might waste memory inside awk.) Novice awk programmers often make the mistake of checking if an element exists by checking if the value is empty: # Check if "foo" exists in a: Incorrect! if (a["foo"] != "") … This is incorrect for two reasons. First, it creates a["foo"] if it didn’t exist before! Second, it is valid (if a bit unusual) to set an array ele‐ ment equal to the empty string. To determine whether an element exists in an array at a certain index, use the following expression: indx in array This expression tests whether the particular index indx exists, without the side effect of creating that element if it is not present. The expression has the value one (true) if array[indx] exists and zero (false) if it does not exist. (We use indx here, because ‘index’ is the name of a built-in function.) For example, this statement tests whether the array frequencies contains the index ‘2’: if (2 in frequencies) print "Subscript 2 is present." Note that this is not a test of whether the array frequencies contains an element whose value is two. There is no way to do that except to scan all the elements. Also, this does not create frequencies[2], while the following (incorrect) alternative does: if (frequencies[2] != "") print "Subscript 2 is present." Assigning Array Elements Array elements can be assigned values just like awk variables: array[index-expression] = value 176 | Chapter 8: Arrays in awk array is the name of an array. The expression index-expression is the index of the element of the array that is assigned a value. The expression value is the value to assign to that element of the array. Basic Array Example The following program takes a list of lines, each beginning with a line number, and prints them out in order of line number. The line numbers are not in order when they are first read—instead, they are scrambled. This program sorts the lines by making an array using the line numbers as subscripts. The program then prints out the lines in sorted order of their numbers. It is a very simple program and gets confused upon encountering repeated numbers, gaps, or lines that don’t begin with a number: { if ($1 > max) max = $1 arr[$1] = $0 } END { for (x = 1; x <= max; x++) print arr[x] } The first rule keeps track of the largest line number seen so far; it also stores each line into the array arr, at an index that is the line’s number. The second rule runs after all the input has been read, to print out all the lines. When this program is run with the following input: 5 I am the Five man 2 Who are you? The new number two! 4 . . . And four on the floor 1 Who is number one? 3 I three you. Its output is: 1 Who is number one? 2 Who are you? The new number two! 3 I three you. 4 . . . And four on the floor 5 I am the Five man If a line number is repeated, the last line with a given number overrides the others. Gaps in the line numbers can be handled with an easy improvement to the program’s END rule, as follows: The Basics of Arrays | 177 END { for (x = 1; x <= max; x++) if (x in arr) print arr[x] } Scanning All Elements of an Array In programs that use arrays, it is often necessary to use a loop that executes once for each element of an array. In other languages, where arrays are contiguous and indices are limited to positive integers, this is easy: all the valid indices can be found by counting from the lowest index up to the highest. This technique won’t do the job in awk, because any number or string can be an array index. So awk has a special kind of for statement for scanning an array: for (var in array) body This loop executes body once for each index in array that the program has previously used, with the variable var set to that index. The following program uses this form of the for statement. The first rule scans the input records and notes which words appear (at least once) in the input, by storing a one into the array used with the word as the index. The second rule scans the elements of used to find all the distinct words that appear in the input. It prints each word that is more than 10 characters long and also prints the number of such words. See “String- Manipulation Functions” on page 194 for more information on the built-in function length(). # Record a 1 for each word that is used at least once { for (i = 1; i <= NF; i++) used[$i] = 1 } # Find number of distinct words more than 10 characters long END { for (x in used) { if (length(x) > 10) { ++num_long_words print x } } print num_long_words, "words longer than 10 characters" } See “Generating Word-Usage Counts” on page 306 for a more detailed example of this type. 178 | Chapter 8: Arrays in awk The order in which elements of the array are accessed by this statement is determined by the internal arrangement of the array elements within awk and in standard awk cannot be controlled or changed. This can lead to problems if new elements are added to array by statements in the loop body; it is not predictable whether the for loop will reach them. Similarly, changing var inside the loop may produce strange results. It is best to avoid such things. As a point of information, gawk sets up the list of elements to be iterated over before the loop starts, and does not change it. But not all awk versions do so. Consider this program, named loopcheck.awk: BEGIN { a["here"] = "here" a["is"] = "is" a["a"] = "a" a["loop"] = "loop" for (i in a) { j++ a[j] = j print i } } Here is what happens when run with gawk (and mawk): $ gawk -f loopcheck.awk here loop a is Contrast this to BWK awk: $ nawk -f loopcheck.awk loop here is a 1 Using Predefined Array Scanning Orders with gawk This subsection describes a feature that is specific to gawk. By default, when a for loop traverses an array, the order is undefined, meaning that the awk implementation determines the order in which the array is traversed. This order is usually based on the internal implementation of arrays and will vary from one version of awk to the next. The Basics of Arrays | 179 Often, though, you may wish to do something simple, such as “traverse the array by comparing the indices in ascending order,” or “traverse the array by comparing the values in descending order.” gawk provides two mechanisms that give you this control: • Set PROCINFO["sorted_in"] to one of a set of predefined values. We describe this now. • Set PROCINFO["sorted_in"] to the name of a user-defined function to use for comparison of array elements. This advanced feature is described later in “Con‐ trolling Array Traversal and Array Sorting” on page 328. The following special values for PROCINFO["sorted_in"] are available: "@unsorted" Array elements are processed in arbitrary order, which is the default awk behavior. "@ind_str_asc" Order by indices in ascending order, compared as strings; this is the most basic sort. (Internally, array indices are always strings, so with ‘a[2*5] = 1’ the index is "10" rather than numeric 10.) "@ind_num_asc" Order by indices in ascending order, but force them to be treated as numbers in the process. Any index with a non-numeric value will end up positioned as if it were zero. "@val_type_asc" Order by element values in ascending order (rather than by indices). Ordering is by the type assigned to the element (see “Variable Typing and Comparison Expres‐ sions” on page 128). All numeric values come before all string values, which in turn come before all subarrays. (Subarrays have not been described yet; see “Arrays of Arrays” on page 187.) "@val_str_asc" Order by element values in ascending order (rather than by indices). Scalar values are compared as strings. Subarrays, if present, come out last. "@val_num_asc" Order by element values in ascending order (rather than by indices). Scalar values are compared as numbers. Subarrays, if present, come out last. When numeric values are equal, the string values are used to provide an ordering: this guarantees 180 | Chapter 8: Arrays in awk 2. When two elements compare as equal, the C qsort() function does not guarantee that they will maintain their original relative order after sorting. Using the string value to provide a unique ordering when the numeric values are equal ensures that gawk behaves consistently across different environments. consistent results across different versions of the C qsort() function,2 which gawk uses internally to perform the sorting. "@ind_str_desc" Like "@ind_str_asc", but the string indices are ordered from high to low. "@ind_num_desc" Like "@ind_num_asc", but the numeric indices are ordered from high to low. "@val_type_desc" Like "@val_type_asc", but the element values, based on type, are ordered from high to low. Subarrays, if present, come out first. "@val_str_desc" Like "@val_str_asc", but the element values, treated as strings, are ordered from high to low. Subarrays, if present, come out first. "@val_num_desc" Like "@val_num_asc", but the element values, treated as numbers, are ordered from high to low. Subarrays, if present, come out first. The array traversal order is determined before the for loop starts to run. Changing PROCINFO["sorted_in"] in the loop body does not affect the loop. For example: $ gawk ' > BEGIN { > a[4] = 4 > a[3] = 3 > for (i in a) > print i, a[i] > }' 4 4 3 3 $ gawk ' > BEGIN { > PROCINFO["sorted_in"] = "@ind_str_asc" > a[4] = 4 > a[3] = 3 > for (i in a) > print i, a[i] > }' 3 3 4 4 The Basics of Arrays | 181 When sorting an array by element values, if a value happens to be a subarray then it is considered to be greater than any string or numeric value, regardless of what the sub‐ array itself contains, and all subarrays are treated as being equal to each other. Their order relative to each other is determined by their index strings. Here are some additional things to bear in mind about sorted array traversal: • The value of PROCINFO["sorted_in"] is global. That is, it affects all array traversal for loops. If you need to change it within your own code, you should see if it’s defined and save and restore the value: … if ("sorted_in" in PROCINFO) { save_sorted = PROCINFO["sorted_in"] PROCINFO["sorted_in"] = "@val_str_desc" # or whatever } … if (save_sorted) PROCINFO["sorted_in"] = save_sorted • As already mentioned, the default array traversal order is represented by "@unsorted". You can also get the default behavior by assigning the null string to PROCINFO["sorted_in"] or by just deleting the "sorted_in" element from the PROCINFO array with the delete statement. (The delete statement hasn’t been de‐ scribed yet; see “The delete Statement” on page 184.) In addition, gawk provides built-in functions for sorting arrays; see “Sorting Array Val‐ ues and Indices with gawk” on page 333. Using Numbers to Subscript Arrays An important aspect to remember about arrays is that array subscripts are always strings. When a numeric value is used as a subscript, it is converted to a string value before being used for subscripting (see “Conversion of Strings and Numbers” on page 117). This means that the value of the predefined variable CONVFMT can affect how your program accesses elements of an array. For example: xyz = 12.153 data[xyz] = 1 CONVFMT = "%2.2f" if (xyz in data) printf "%s is in data\n", xyz else printf "%s is not in data\n", xyz This prints ‘12.15 is not in data’. The first statement gives xyz a numeric value. Assigning to data[xyz] subscripts data with the string value "12.153" (using the de‐ fault conversion value of CONVFMT, "%.6g"). Thus, the array element data["12.153"] is 182 | Chapter 8: Arrays in awk assigned the value one. The program then changes the value of CONVFMT. The test ‘(xyz in data)’ generates a new string value from xyz—this time "12.15"—because the value of CONVFMT only allows two significant digits. This test fails, because "12.15" is different from "12.153". According to the rules for conversions (see “Conversion of Strings and Numbers” on page 117), integer values always convert to strings as integers, no matter what the value of CONVFMT may happen to be. So the usual case of the following works: for (i = 1; i <= maxsub; i++) do something with array[i] The “integer values always convert to strings as integers” rule has an additional conse‐ quence for array indexing. Octal and hexadecimal constants (covered in “Octal and hexadecimal numbers” on page 112) are converted internally into numbers, and their original form is forgotten. This means, for example, that array[17], array[021], and array[0x11] all refer to the same element! As with many things in awk, the majority of the time things work as you would expect them to. But it is useful to have a precise knowledge of the actual rules, as they can sometimes have a subtle effect on your programs. Using Uninitialized Variables as Subscripts Suppose it’s necessary to write a program to print the input data in reverse order. A reasonable attempt to do so (with some test data) might look like this: $ echo 'line 1 > line 2 > line 3' | awk '{ l[lines] = $0; ++lines } > END { > for (i = lines - 1; i >= 0; i--) > print l[i] > }' line 3 line 2 Unfortunately, the very first line of input data did not appear in the output! Upon first glance, we would think that this program should have worked. The variable lines is uninitialized, and uninitialized variables have the numeric value zero. So, awk should have printed the value of l[0]. The issue here is that subscripts for awk arrays are always strings. Uninitialized variables, when used as strings, have the value "", not zero. Thus, ‘line 1’ ends up stored in l[""]. The following version of the program works correctly: { l[lines++] = $0 } END { Using Uninitialized Variables as Subscripts | 183 for (i = lines - 1; i >= 0; i--) print l[i] } Here, the ‘++’ forces lines to be numeric, thus making the “old value” numeric zero. This is then converted to "0" as the array subscript. Even though it is somewhat unusual, the null string ("") is a valid array subscript. (d.c.) gawk warns about the use of the null string as a subscript if --lint is provided on the command line (see “Command-Line Options” on page 22). The delete Statement To remove an individual element of an array, use the delete statement: delete array[index-expression] Once an array element has been deleted, any value the element once had is no longer available. It is as if the element had never been referred to or been given a value. The following is an example of deleting elements in an array: for (i in frequencies) delete frequencies[i] This example removes all the elements from the array frequencies. Once an element is deleted, a subsequent for statement to scan the array does not report that element and using the in operator to check for the presence of that element returns zero (i.e., false): delete foo[4] if (4 in foo) print "This will never be printed" It is important to note that deleting an element is not the same as assigning it a null value (the empty string, ""). For example: foo[4] = "" if (4 in foo) print "This is printed, even though foo[4] is empty" It is not an error to delete an element that does not exist. However, if --lint is provided on the command line (see “Command-Line Options” on page 22), gawk issues a warning message when an element that is not in the array is deleted. All the elements of an array may be deleted with a single statement by leaving off the subscript in the delete statement, as follows: delete array Using this version of the delete statement is about three times more efficient than the equivalent loop that deletes each element one at a time. 184 | Chapter 8: Arrays in awk 3. Thanks to Michael Brennan for pointing this out. This form of the delete statement is also supported by BWK awk and mawk, as well as by a number of other implementations. For many years, using delete without a subscript was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard. See the Austin Group website. The following statement provides a portable but nonobvious way to clear out an array:3 split("", array) The split() function (see “String-Manipulation Functions” on page 194) clears out the target array first. This call asks it to split apart the null string. Because there is no data to split out, the function simply clears the array and then returns. Deleting all the elements from an array does not change its type; you cannot clear an array and then use the array’s name as a scalar (i.e., a regular variable). For example, the following does not work: a[1] = 3 delete a a = 3 Multidimensional Arrays A multidimensional array is an array in which an element is identified by a sequence of indices instead of a single index. For example, a two-dimensional array requires two indices. The usual way (in many languages, including awk) to refer to an element of a two-dimensional array named grid is with grid[x,y]. Multidimensional arrays are supported in awk through concatenation of indices into one string. awk converts the indices into strings (see “Conversion of Strings and Num‐ bers” on page 117) and concatenates them together, with a separator between them. This creates a single string that describes the values of the separate indices. The com‐ bined string is used as a single index into an ordinary, one-dimensional array. The separator used is the value of the built-in variable SUBSEP. For example, suppose we evaluate the expression ‘foo[5,12] = "value"’ when the value of SUBSEP is "@". The numbers 5 and 12 are converted to strings and concatenated with Multidimensional Arrays | 185 an ‘@’ between them, yielding "5@12"; thus, the array element foo["5@12"] is set to "value". Once the element’s value is stored, awk has no record of whether it was stored with a single index or a sequence of indices. The two expressions ‘foo[5,12]’ and ‘foo[5 SUBSEP 12]’ are always equivalent. The default value of SUBSEP is the string "\034", which contains a nonprinting character that is unlikely to appear in an awk program or in most input data. The usefulness of choosing an unlikely character comes from the fact that index values that contain a string matching SUBSEP can lead to combined strings that are ambiguous. Suppose that SUBSEP is "@"; then ‘foo["a@b", "c"]’ and ‘foo["a", "b@c"]’ are indistinguishable because both are actually stored as ‘foo["a@b@c"]’. To test whether a particular index sequence exists in a multidimensional array, use the same operator (in) that is used for single-dimensional arrays. Write the whole sequence of indices in parentheses, separated by commas, as the left operand: if ((subscript1, subscript2, …) in array) … Here is an example that treats its input as a two-dimensional array of fields; it rotates this array 90 degrees clockwise and prints the result. It assumes that all lines have the same number of elements: { if (max_nf < NF) max_nf = NF max_nr = NR for (x = 1; x <= NF; x++) vector[x, NR] = $x } END { for (x = 1; x <= max_nf; x++) { for (y = max_nr; y >= 1; --y) printf("%s ", vector[x, y]) printf("\n") } } When given the input: 1 2 3 4 5 6 2 3 4 5 6 1 3 4 5 6 1 2 4 5 6 1 2 3 the program produces the following output: 186 | Chapter 8: Arrays in awk 4 3 2 1 5 4 3 2 6 5 4 3 1 6 5 4 2 1 6 5 3 2 1 6 Scanning Multidimensional Arrays There is no special for statement for scanning a “multidimensional” array. There cannot be one, because, in truth, awk does not have multidimensional arrays or elements—there is only a multidimensional way of accessing an array. However, if your program has an array that is always accessed as multidimensional, you can get the effect of scanning it by combining the scanning for statement (see “Scanning All Elements of an Array” on page 178) with the built-in split() function (see “String- Manipulation Functions” on page 194). It works in the following manner: for (combined in array) { split(combined, separate, SUBSEP) … } This sets the variable combined to each concatenated combined index in the array, and splits it into the individual indices by breaking it apart where the value of SUBSEP appears. The individual indices then become the elements of the array separate. Thus, if a value is previously stored in array[1, "foo"], then an element with index "1\034foo" exists in array. (Recall that the default value of SUBSEP is the character with code 034.) Sooner or later, the for statement finds that index and does an iteration with the variable combined set to "1\034foo". Then the split() function is called as follows: split("1\034foo", separate, "\034") The result is to set separate[1] to "1" and separate[2] to "foo". Presto! The original sequence of separate indices is recovered. Arrays of Arrays gawk goes beyond standard awk’s multidimensional array access and provides true ar‐ rays of arrays. Elements of a subarray are referred to by their own indices enclosed in square brackets, just like the elements of the main array. For example, the following creates a two-element subarray at index 1 of the main array a: a[1][1] = 1 a[1][2] = 2 Arrays of Arrays | 187 This simulates a true two-dimensional array. Each subarray element can contain another subarray as a value, which in turn can hold other arrays as well. In this way, you can create arrays of three or more dimensions. The indices can be any awk expressions, including scalars separated by commas (i.e., a regular awk simulated multidimensional subscript). So the following is valid in gawk: a[1][3][1, "name"] = "barney" Each subarray and the main array can be of different length. In fact, the elements of an array or its subarray do not all have to have the same type. This means that the main array and any of its subarrays can be nonrectangular, or jagged in structure. You can assign a scalar value to the index 4 of the main array a, even though a[1] is itself an array and not a scalar: a[4] = "An element in a jagged array" The terms dimension, row, and column are meaningless when applied to such an array, but we will use “dimension” henceforth to imply the maximum number of indices needed to refer to an existing element. The type of any element that has already been assigned cannot be changed by assigning a value of a different type. You have to first delete the current element, which effectively makes gawk forget about the element at that index: delete a[4] a[4][5][6][7] = "An element in a four-dimensional array" This removes the scalar value from index 4 and then inserts a three-level nested subarray containing a scalar. You can also delete an entire subarray or subarray of subarrays: delete a[4][5] a[4][5] = "An element in subarray a[4]" But recall that you can not delete the main array a and then use it as a scalar. The built-in functions that take array arguments can also be used with subarrays. For example, the following code fragment uses length() (see “String-Manipulation Func‐ tions” on page 194) to determine the number of elements in the main array a and its subarrays: print length(a), length(a[1]), length(a[1][3]) This results in the following output for our main array a: 2, 3, 1 The ‘subscript in array’ expression (see “Referring to an Array Element” on page 175) works similarly for both regular awk-style arrays and arrays of arrays. For example, the tests ‘1 in a’, ‘3 in a[1]’, and ‘(1, "name") in a[1][3]’ all evaluate to one (true) for our array a. 188 | Chapter 8: Arrays in awk The ‘for (item in array)’ statement (see “Scanning All Elements of an Array” on page 178) can be nested to scan all the elements of an array of arrays if it is rectangular in structure. In order to print the contents (scalar values) of a two-dimensional array of arrays (i.e., in which each first-level element is itself an array, not necessarily of the same length), you could use the following code: for (i in array) for (j in array[i]) print array[i][j] The isarray() function (see “Getting Type Information” on page 219) lets you test if an array element is itself an array: for (i in array) { if (isarray(array[i]) { for (j in array[i]) { print array[i][j] } } else print array[i] } If the structure of a jagged array of arrays is known in advance, you can often devise workarounds using control statements. For example, the following code prints the el‐ ements of our main array a: for (i in a) { for (j in a[i]) { if (j == 3) { for (k in a[i][j]) print a[i][j][k] } else print a[i][j] } } See “Traversing Arrays of Arrays” on page 273 for a user-defined function that “walks” an arbitrarily dimensioned array of arrays. Recall that a reference to an uninitialized array element yields a value of "", the null string. This has one important implication when you intend to use a subarray as an argument to a function, as illustrated by the following example: $ gawk 'BEGIN { split("a b c d", b[1]); print b[1][1] }' error→ gawk: cmd. line:1: fatal: split: second argument is not an array The way to work around this is to first force b[1] to be an array by creating an arbitrary index: $ gawk 'BEGIN { b[1][1] = ""; split("a b c d", b[1]); print b[1][1] }' a Arrays of Arrays | 189 Summary • Standard awk provides one-dimensional associative arrays (arrays indexed by string values). All arrays are associative; numeric indices are converted automatically to strings. • Array elements are referenced as array[indx]. Referencing an element creates it if it did not exist previously. • The proper way to see if an array has an element with a given index is to use the in operator: ‘indx in array’. • Use ‘for (indx in array) …’ to scan through all the individual elements of an array. In the body of the loop, indx takes on the value of each element’s index in turn. • The order in which a ‘for (indx in array)’ loop traverses an array is undefined in POSIX awk and varies among implementations. gawk lets you control the order by assigning special predefined values to PROCINFO["sorted_in"]. • Use ‘delete array[indx]’ to delete an individual element. To delete all of the el‐ ements in an array, use ‘delete array’. This latter feature has been a common extension for many years and is now standard, but may not be supported by all commercial versions of awk. • Standard awk simulates multidimensional arrays by separating subscript values with commas. The values are concatenated into a single string, separated by the value of SUBSEP. The fact that such a subscript was created in this way is not retained; thus, changing SUBSEP may have unexpected consequences. You can use ‘(sub1, sub2, …) in array’ to see if such a multidimensional subscript exists in array. • gawk provides true arrays of arrays. You use a separate set of square brackets for each dimension in such an array: data[row][col], for example. Array elements may thus be either scalar values (number or string) or other arrays. • Use the isarray() built-in function to determine if an array element is itself a subarray. 190 | Chapter 8: Arrays in awk CHAPTER 9 Functions This chapter describes awk’s built-in functions, which fall into three categories: numeric, string, and I/O. gawk provides additional groups of functions to work with values that represent time, do bit manipulation, sort arrays, provide type information, and inter‐ nationalize and localize programs. Besides the built-in functions, awk has provisions for writing new functions that the rest of a program can use. The second part of this chapter describes these user-defined functions. Finally, we explore indirect function calls, a gawk-specific extension that lets you determine at runtime what function is to be called. Built-in Functions Built-in functions are always available for your awk program to call. This section defines all the built-in functions in awk; some of these are mentioned in other sections but are summarized here for your convenience. Calling Built-in Functions To call one of awk’s built-in functions, write the name of the function followed by ar‐ guments in parentheses. For example, ‘atan2(y + z, 1)’ is a call to the function atan2() and has two arguments. Whitespace is ignored between the built-in function name and the opening parenthesis, but nonetheless it is good practice to avoid using whitespace there. User-defined func‐ tions do not permit whitespace in this way, and it is easier to avoid mistakes by following a simple convention that always works—no whitespace after a function name. Each built-in function accepts a certain number of arguments. In some cases, arguments can be omitted. The defaults for omitted arguments vary from function to function and 191 are described under the individual functions. In some awk implementations, extra arguments given to built-in functions are ignored. However, in gawk, it is a fatal error to give extra arguments to a built-in function. When a function is called, expressions that create the function’s actual parameters are evaluated completely before the call is performed. For example, in the following code fragment: i = 4 j = sqrt(i++) the variable i is incremented to the value five before sqrt() is called with a value of four for its actual parameter. The order of evaluation of the expressions used for the function’s parameters is undefined. Thus, avoid writing programs that assume that pa‐ rameters are evaluated from left to right or from right to left. For example: i = 5 j = atan2(++i, i *= 2) If the order of evaluation is left to right, then i first becomes six, and then 12, and atan2() is called with the two arguments six and 12. But if the order of evaluation is right to left, i first becomes 10, then 11, and atan2() is called with the two arguments 11 and 10. Numeric Functions The following list describes all of the built-in functions that work with numbers. Op‐ tional parameters are enclosed in square brackets ([ ]): atan2(y, x) Return the arctangent of y / x in radians. You can use ‘pi = atan2(0, -1)’ to retrieve the value of π. cos(x) Return the cosine of x, with x in radians. exp(x) Return the exponential of x (e ^ x) or report an error if x is out of range. The range of values x can have depends on your machine’s floating-point representation. int(x) Return the nearest integer to x, located between x and zero and truncated toward zero. For example, int(3) is 3, int(3.9) is 3, int(-3.9) is −3, and int(-3) is −3 as well. 192 | Chapter 9: Functions 1. The C version of rand() on many Unix systems is known to produce fairly poor sequences of random numbers. However, nothing requires that an awk implementation use the C rand() to implement the awk version of rand(). In fact, gawk uses the BSD random() function, which is considerably better than rand(), to produce random numbers. 2. mawk uses a different seed each time. log(x) Return the natural logarithm of x, if x is positive; otherwise, return NaN (“not a number”) on IEEE 754 systems. Additionally, gawk prints a warning message when x is negative. rand() Return a random number. The values of rand() are uniformly distributed between zero and one. The value could be zero but is never one.1 Often random integers are needed instead. Following is a user-defined function that can be used to obtain a random nonnegative integer less than n: function randint(n) { return int(n * rand()) } The multiplication produces a random number greater than zero and less than n. Using int(), this result is made into an integer between zero and n − 1, inclusive. The following example uses a similar function to produce random integers between one and n. This program prints a new random number for each input record: # Function to roll a simulated die. function roll(n) { return 1 + int(rand() * n) } # Roll 3 six-sided dice and # print total number of points. { printf("%d points\n", roll(6) + roll(6) + roll(6)) } In most awk implementations, including gawk, rand() starts generating numbers from the same starting number, or seed, each time you run awk.2 Thus, a program generates the same results each time you run it. The numbers are random with‐ in one awk run but predictable from run to run. This is con‐ venient for debugging, but if you want a program to do dif‐ ferent things each time it is used, you must change the seed to a value that is different in each run. To do this, use srand(). Built-in Functions | 193 3. Computer-generated random numbers really are not truly random. They are technically known as pseudorandom. This means that although the numbers in a sequence appear to be random, you can in fact generate the same sequence of random numbers over and over again. sin(x) Return the sine of x, with x in radians. sqrt(x) Return the positive square root of x. gawk prints a warning message if x is negative. Thus, sqrt(4) is 2. srand([x]) Set the starting point, or seed, for generating random numbers to the value x. Each seed value leads to a particular sequence of random numbers.3 Thus, if the seed is set to the same value a second time, the same sequence of random numbers is produced again. Different awk implementations use different random- number generators internally. Don’t expect the same awk program to produce the same series of random numbers when executed by different versions of awk. If the argument x is omitted, as in ‘srand()’, then the current date and time of day are used for a seed. This is the way to get random numbers that are truly unpredictable. The return value of srand() is the previous seed. This makes it easy to keep track of the seeds in case you need to consistently reproduce sequences of random numbers. POSIX does not specify the initial seed; it differs among awk implementations. String-Manipulation Functions The functions in this section look at or change the text of one or more strings. gawk understands locales (see “Where You Are Makes a Difference” on page 138) and does all string processing in terms of characters, not bytes. This distinction is particularly important to understand for locales where one character may be represented by multiple bytes. Thus, for example, length() returns the number of characters in a string, and not the number of bytes used to represent those characters. Similarly, index() works with character indices, and not byte indices. 194 | Chapter 9: Functions A number of functions deal with indices into strings. For these func‐ tions, the first character of a string is at position (index) one. This is different from C and the languages descended from it, where the first character is at position zero. You need to remember this when do‐ ing index calculations, particularly if you are used to C. In the following list, optional parameters are enclosed in square brackets ([ ]). Several functions perform string substitution; the full discussion is provided in the description of the sub() function, which comes toward the end, because the list is presented alphabetically. Those functions that are specific to gawk are marked with a pound sign (‘#’). They are not available in compatibility mode (see “Command-Line Options” on page 22): asort(source [, dest [, how ] ]) # asorti(source [, dest [, how ] ]) # These two functions are similar in behavior, so they are described together. The following description ignores the third argument, how, as it requires understanding features that we have not discussed yet. Thus, the discussion here is a deliberate simplification. (We do provide all the details later on; see “Sorting Array Values and Indices with gawk” on page 333 for the full story.) Both functions return the number of elements in the array source. For asort(), gawk sorts the values of source and replaces the indices of the sorted values of source with sequential integers starting with one. If the optional array dest is specified, then source is duplicated into dest. dest is then sorted, leaving the in‐ dices of source unchanged. When comparing strings, IGNORECASE affects the sorting (see “Sorting Array Values and Indices with gawk” on page 333). If the source array contains subarrays as values (see “Arrays of Arrays” on page 187), they will come last, after all scalar values. Subarrays are not recursively sorted. For example, if the contents of a are as follows: a["last"] = "de" a["first"] = "sac" a["middle"] = "cul" A call to asort(): asort(a) results in the following contents of a: Built-in Functions | 195 a[1] = "cul" a[2] = "de" a[3] = "sac" The asorti() function works similarly to asort(); however, the indices are sorted, instead of the values. Thus, in the previous example, starting with the same initial set of indices and values in a, calling ‘asorti(a)’ would yield: a[1] = "first" a[2] = "last" a[3] = "middle" gensub(regexp, replacement, how [, target]) # Search the target string target for matches of the regular expression regexp. If how is a string beginning with ‘g’ or ‘G’ (short for “global”), then replace all matches of regexp with replacement. Otherwise, how is treated as a number indicating which match of regexp to replace. If no target is supplied, use $0. It returns the modified string as the result of the function and the original target string is not changed. gensub() is a general substitution function. Its purpose is to provide more features than the standard sub() and gsub() functions. gensub() provides an additional feature that is not available in sub() or gsub(): the ability to specify components of a regexp in the replacement text. This is done by using parentheses in the regexp to mark the components and then specifying ‘\N’ in the replacement text, where N is a digit from 1 to 9. For example: $ gawk ' > BEGIN { > a = "abc def" > b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a) > print b > }' def abc As with sub(), you must type two backslashes in order to get one into the string. In the replacement text, the sequence ‘\0’ represents the entire matched text, as does the character ‘&’. The following example shows how you can use the third argument to control which match of the regexp should be changed: $ echo a b c a b c | > gawk '{ print gensub(/a/, "AA", 2) }' a b c AA b c In this case, $0 is the default target string. gensub() returns the new string as its result, which is passed directly to print for printing. 196 | Chapter 9: Functions If the how argument is a string that does not begin with ‘g’ or ‘G’, or if it is a number that is less than or equal to zero, only one substitution is performed. If how is zero, gawk issues a warning message. If regexp does not match target, gensub()’s return value is the original unchanged value of target. gsub(regexp, replacement [, target]) Search target for all of the longest, leftmost, nonoverlapping matching substrings it can find and replace them with replacement. The ‘g’ in gsub() stands for “global,” which means replace everywhere. For example: { gsub(/Britain/, "United Kingdom"); print } replaces all occurrences of the string ‘Britain’ with ‘United Kingdom’ for all input records. The gsub() function returns the number of substitutions made. If the variable to search and alter (target) is omitted, then the entire input record ($0) is used. As in sub(), the characters ‘&’ and ‘\’ are special, and the third argument must be assignable. index(in, find) Search the string in for the first occurrence of the string find, and return the po‐ sition in characters where that occurrence begins in the string in. Consider the following example: $ awk 'BEGIN { print index("peanut", "an") }' 3 If find is not found, index() returns zero. With BWK awk and gawk, it is a fatal error to use a regexp constant for find. Other implementations allow it, simply treating the regexp constant as an expression meaning ‘$0 ~ /regexp/’. (d.c.) length([string]) Return the number of characters in string. If string is a number, the length of the digit string representing that number is returned. For example, length("abcde") is five. By contrast, length(15 * 35) works out to three. In this example, 15 ⋅ 35 = 525, and 525 is then converted to the string "525", which has three characters. If no argument is supplied, length() returns the length of $0. Built-in Functions | 197 In older versions of awk, the length() function could be called without any parentheses. Doing so is considered poor practice, although the 2008 POSIX standard explicitly allows it, to sup‐ port historical practice. For programs to be maximally portable, always supply the parentheses. If length() is called with a variable that has not been used, gawk forces the variable to be a scalar. Other implementations of awk leave the variable without a type. (d.c.) Consider: $ gawk 'BEGIN { print length(x) ; x[1] = 1 }' 0 error→ gawk: fatal: attempt to use scalar `x' as array $ nawk 'BEGIN { print length(x) ; x[1] = 1 }' 0 If --lint has been specified on the command line, gawk issues a warning about this. With gawk and several other awk implementations, when given an array argument, the length() function returns the number of elements in the array. (c.e.) This is less useful than it might seem at first, as the array is not guaranteed to be indexed from one to the number of elements in it. If --lint is provided on the command line (see “Command-Line Options” on page 22), gawk warns that passing an array argument is not portable. If --posix is supplied, using an array argument is a fatal error (see Chapter 8). match(string, regexp [, array]) Search string for the longest, leftmost substring matched by the regular expression regexp and return the character position (index) at which that substring begins (one, if it starts at the beginning of string). If no match is found, return zero. The regexp argument may be either a regexp constant (/…/) or a string constant ("…"). In the latter case, the string is treated as a regexp to be matched. See “Using Dynamic Regexps” on page 49 for a discussion of the difference between the two forms, and the implications for writing your program correctly. The order of the first two arguments is the opposite of most other string functions that work with regular expressions, such as sub() and gsub(). It might help to remember that for match(), the order is the same as for the ‘~’ operator: ‘string ~ regexp’. The match() function sets the predefined variable RSTART to the index. It also sets the predefined variable RLENGTH to the length in characters of the matched sub‐ string. If no match is found, RSTART is set to zero, and RLENGTH to −1. 198 | Chapter 9: Functions For example: { if ($1 == "FIND") regex = $2 else { where = match($0, regex) if (where != 0) print "Match of", regex, "found at", where, "in", $0 } } This program looks for lines that match the regular expression stored in the variable regex. This regular expression can be changed. If the first word on a line is ‘FIND’, regex is changed to be the second word on that line. Therefore, if given: FIND ru+n My program runs but not very quickly FIND Melvin JF+KM This line is property of Reality Engineering Co. Melvin was here. awk prints: Match of ru+n found at 12 in My program runs Match of Melvin found at 1 in Melvin was here. If array is present, it is cleared, and then the zeroth element of array is set to the entire portion of string matched by regexp. If regexp contains parentheses, the integer-indexed elements of array are set to contain the portion of string matching the corresponding parenthesized subexpression. For example: $ echo foooobazbarrrrr | > gawk '{ match($0, /(fo+).+(bar*)/, arr) > print arr[1], arr[2] }' foooo barrrrr In addition, multidimensional subscripts are available providing the start index and length of each matched subexpression: $ echo foooobazbarrrrr | > gawk '{ match($0, /(fo+).+(bar*)/, arr) > print arr[1], arr[2] > print arr[1, "start"], arr[1, "length"] > print arr[2, "start"], arr[2, "length"] > }' foooo barrrrr 1 5 9 7 Built-in Functions | 199 There may not be subscripts for the start and index for every parenthesized sub‐ expression, because they may not all have matched text; thus, they should be tested for with the in operator (see “Referring to an Array Element” on page 175). The array argument to match() is a gawk extension. In compatibility mode (see “Command-Line Options” on page 22), using a third argument is a fatal error. patsplit(string, array [, fieldpat [, seps ] ]) # Divide string into pieces defined by fieldpat and store the pieces in array and the separator strings in the seps array. The first piece is stored in array[1], the second piece in array[2], and so forth. The third argument, fieldpat, is a regexp describing the fields in string (just as FPAT is a regexp describing the fields in input records). It may be either a regexp constant or a string. If fieldpat is omitted, the value of FPAT is used. patsplit() returns the number of elements created. seps[i] is the separator string between array[i] and array[i+1]. Any leading separator will be in seps[0]. The patsplit() function splits strings into pieces in a manner similar to the way input lines are split into fields using FPAT (see “Defining Fields by Content” on page 73). Before splitting the string, patsplit() deletes any previously existing elements in the arrays array and seps. split(string, array [, fieldsep [, seps ] ]) Divide string into pieces separated by fieldsep and store the pieces in array and the separator strings in the seps array. The first piece is stored in array[1], the second piece in array[2], and so forth. The string value of the third argument, fieldsep, is a regexp describing where to split string (much as FS can be a regexp describing where to split input records). If fieldsep is omitted, the value of FS is used. split() returns the number of elements created. seps is a gawk extension, with seps[i] being the separator string between array[i] and array[i+1]. If fieldsep is a single space, then any leading whitespace goes into seps[0] and any trailing whitespace goes into seps[n], where n is the return value of split() (i.e., the number of elements in array). The split() function splits strings into pieces in a manner similar to the way input lines are split into fields. For example: split("cul-de-sac", a, "-", seps) splits the string "cul-de-sac" into three fields using ‘-’ as the separator. It sets the contents of the array a as follows: 200 | Chapter 9: Functions a[1] = "cul" a[2] = "de" a[3] = "sac" and sets the contents of the array seps as follows: seps[1] = "-" seps[2] = "-" The value returned by this call to split() is three. As with input field splitting, when the value of fieldsep is " ", leading and trailing whitespace is ignored in values assigned to the elements of array but not in seps, and the elements are separated by runs of whitespace. Also, as with input field splitting, if fieldsep is the null string, each individual character in the string is split into its own array element. (c.e.) Note, however, that RS has no effect on the way split() works. Even though ‘RS = ""’ causes the newline character to also be an input field separator, this does not affect how split() splits strings. Modern implementations of awk, including gawk, allow the third argument to be a regexp constant (/…/) as well as a string. (d.c.) The POSIX standard allows this as well. See “Using Dynamic Regexps” on page 49 for a discussion of the difference between using a string constant or a regexp constant, and the implications for writ‐ ing your program correctly. Before splitting the string, split() deletes any previously existing elements in the arrays array and seps. If string is null, the array has no elements. (So this is a portable way to delete an entire array with one statement. See “The delete Statement” on page 184.) If string does not match fieldsep at all (but is not null), array has one element only. The value of that element is the original string. In POSIX mode (see “Command-Line Options” on page 22), the fourth argument is not allowed. sprintf(format, expression1, …) Return (without printing) the string that printf would have printed out with the same arguments (see “Using printf Statements for Fancier Printing” on page 93). For example: pival = sprintf("pi = %.2f (approx.)", 22/7) assigns the string ‘pi = 3.14 (approx.)’ to the variable pival. Built-in Functions | 201 4. Unless you use the --non-decimal-data option, which isn’t recommended. See “Allowing Nondecimal Input Data” on page 327 for more information. 5. Note that this means that the record will first be regenerated using the value of OFS if any fields have been changed, and that the fields will be updated after the substitution, even if the operation is a “no-op” such as ‘sub(/^/, "")’. strtonum(str) # Examine str and return its numeric value. If str begins with a leading ‘0’, strto num() assumes that str is an octal number. If str begins with a leading ‘0x’ or ‘0X’, strtonum() assumes that str is a hexadecimal number. For example: $ echo 0x11 | > gawk '{ printf "%d\n", strtonum($1) }' 17 Using the strtonum() function is not the same as adding zero to a string value; the automatic coercion of strings to numbers works only for decimal data, not for octal or hexadecimal.4 Note also that strtonum() uses the current locale’s decimal point for recognizing numbers (see “Where You Are Makes a Difference” on page 138). sub(regexp, replacement [, target]) Search target, which is treated as a string, for the leftmost, longest substring matched by the regular expression regexp. Modify the entire string by replacing the matched text with replacement. The modified string becomes the new value of target. Return the number of substitutions made (zero or one). The regexp argument may be either a regexp constant (/…/) or a string constant ("…"). In the latter case, the string is treated as a regexp to be matched. See “Using Dynamic Regexps” on page 49 for a discussion of the difference between the two forms, and the implications for writing your program correctly. This function is peculiar because target is not simply used to compute a value, and not just any expression will do—it must be a variable, field, or array element so that sub() can store a modified value there. If this argument is omitted, then the default is to use and alter $0.5 For example: str = "water, water, everywhere" sub(/at/, "ith", str) sets str to ‘wither, water, everywhere’, by replacing the leftmost longest occur‐ rence of ‘at’ with ‘ith’. If the special character ‘&’ appears in replacement, it stands for the precise substring that was matched by regexp. (If the regexp can match more than one string, then this precise substring may vary.) For example: 202 | Chapter 9: Functions 6. This is different from C and C++, in which the first character is number zero. { sub(/candidate/, "& and his wife"); print } changes the first occurrence of ‘candidate’ to ‘candidate and his wife’ on each input line. Here is another example: $ awk 'BEGIN { > str = "daabaaa" > sub(/a+/, "C&C", str) > print str > }' dCaaCbaaa This shows how ‘&’ can represent a nonconstant string and also illustrates the “left‐ most, longest” rule in regexp matching (see “How Much Text Matches?” on page 48). The effect of this special character (‘&’) can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write ‘\\&’ in a string constant to include a literal ‘&’ in the replacement. For example, the following shows how to replace the first ‘|’ on each line with an ‘&’: { sub(/\|/, "\\&"); print } As mentioned, the third argument to sub() must be a variable, field, or array ele‐ ment. Some versions of awk allow the third argument to be an expression that is not an lvalue. In such a case, sub() still searches for the pattern and returns zero or one, but the result of the substitution (if any) is thrown away because there is no place to put it. Such versions of awk accept expressions like the following: sub(/USA/, "United States", "the USA and Canada") For historical compatibility, gawk accepts such erroneous code. However, using any other nonchangeable object as the third parameter causes a fatal error and your program will not run. Finally, if the regexp is not a regexp constant, it is converted into a string, and then the value of that string is treated as the regexp to match. substr(string, start [, length ]) Return a length-character-long substring of string, starting at character number start. The first character of a string is character number one.6 For example, substr("washington", 5, 3) returns "ing". If length is not present, substr() returns the whole suffix of string that begins at character number start. For example, substr("washington", 5) returns Built-in Functions | 203 "ington". The whole suffix is also returned if length is greater than the number of characters remaining in the string, counting from character start. If start is less than one, substr() treats it as if it was one. (POSIX doesn’t specify what to do in this case: BWK awk acts this way, and therefore gawk does too.) If start is greater than the number of characters in the string, substr() returns the null string. Similarly, if length is present but less than or equal to zero, the null string is returned. The string returned by substr() cannot be assigned. Thus, it is a mistake to attempt to change a portion of a string, as shown in the following example: string = "abcdef" # try to get "abCDEf", won't work substr(string, 3, 3) = "CDE" It is also a mistake to use substr() as the third argument of sub() or gsub(): gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG (Some commercial versions of awk treat substr() as assignable, but doing so is not portable.) If you need to replace bits and pieces of a string, combine substr() with string concatenation, in the following manner: string = "abcdef" … string = substr(string, 1, 2) "CDE" substr(string, 6) tolower(string) Return a copy of string, with each uppercase character in the string replaced with its corresponding lowercase character. Nonalphabetic characters are left un‐ changed. For example, tolower("MiXeD cAsE 123") returns "mixed case 123". toupper(string) Return a copy of string, with each lowercase character in the string replaced with its corresponding uppercase character. Nonalphabetic characters are left un‐ changed. For example, toupper("MiXeD cAsE 123") returns "MIXED CASE 123". 204 | Chapter 9: Functions Matching the Null String In awk, the ‘*’ operator can match the null string. This is particularly important for the sub(), gsub(), and gensub() functions. For example: $ echo abc | awk '{ gsub(/m*/, "X"); print }' XaXbXcX Although this makes a certain amount of sense, it can be surprising. More about ‘\’ and ‘&’ with sub(), gsub(), and gensub() This subsubsection has been reported to cause headaches. You might want to skip it upon first reading. When using sub(), gsub(), or gensub(), and trying to get literal backslashes and am‐ persands into the replacement text, you need to remember that there are several levels of escape processing going on. First, there is the lexical level, which is when awk reads your program and builds an internal copy of it to execute. Then there is the runtime level, which is when awk actually scans the replacement string to determine what to generate. At both levels, awk looks for a defined set of characters that can come after a backslash. At the lexical level, it looks for the escape sequences listed in “Escape Sequences” on page 40. Thus, for every ‘\’ that awk processes at the runtime level, you must type two backslashes at the lexical level. When a character that is not valid for an escape sequence follows the ‘\’, BWK awk and gawk both simply remove the initial ‘\’ and put the next character into the string. Thus, for example, "a\qb" is treated as "aqb". At the runtime level, the various functions handle sequences of ‘\’ and ‘&’ differently. The situation is (sadly) somewhat complex. Historically, the sub() and gsub() functions treated the two-character sequence ‘\&’ specially; this sequence was replaced in the generated text with a single ‘&’. Any other ‘\’ within the replacement string that did not precede an ‘&’ was passed through unchanged. This is illustrated in Table 9-1. Table 9-1. Historical escape sequence processing for sub() and gsub() You type sub() sees sub() generates \& & The matched text \\& \& A literal ‘&’ Built-in Functions | 205 You type sub() sees sub() generates \\\& \& A literal ‘&’ \\\\& \\& A literal ‘\&’ \\\\\& \\& A literal ‘\&’ \\\\\\& \\\& A literal ‘\\&’ \\q \q A literal ‘\q’ This table shows the lexical-level processing, where an odd number of backslashes be‐ comes an even number at the runtime level, as well as the runtime processing done by sub(). (For the sake of simplicity, the rest of the following tables only show the case of even numbers of backslashes entered at the lexical level.) The problem with the historical approach is that there is no way to get a literal ‘\’ followed by the matched text. Several editions of the POSIX standard attempted to fix this problem but weren’t suc‐ cessful. The details are irrelevant at this point in time. At one point, the gawk maintainer submitted proposed text for a revised standard that reverts to rules that correspond more closely to the original existing practice. The pro‐ posed rules have special cases that make it possible to produce a ‘\’ preceding the matched text. This is shown in Table 9-2. Table 9-2. Gawk rules for sub() and backslash You type sub() sees sub() generates \\\\\\& \\\& A literal ‘\&’ \\\\& \\& A literal ‘\’, followed by the matched text \\& \& A literal ‘&’ \\q \q A literal ‘\q’ \\\\ \\ \\ In a nutshell, at the runtime level, there are now three special sequences of characters (‘\\\&’, ‘\\&’, and ‘\&’) whereas historically there was only one. However, as in the his‐ torical case, any ‘\’ that is not part of one of these three sequences is not special and appears in the output literally. gawk 3.0 and 3.1 follow these rules for sub() and gsub(). The POSIX standard took much longer to be revised than was expected. In addition, the gawk maintainer’s pro‐ posal was lost during the standardization process. The final rules are somewhat simpler. The results are similar except for one case. 206 | Chapter 9: Functions 7. This was rather naive of him, despite there being a note in this section indicating that the next major version would move to the POSIX rules. The POSIX rules state that ‘\&’ in the replacement string produces a literal ‘&’, ‘\\’ pro‐ duces a literal ‘\’, and ‘\’ followed by anything else is not special; the ‘\’ is placed straight into the output. These rules are presented in Table 9-3. Table 9-3. POSIX rules for sub() and gsub() You type sub() sees sub() generates \\\\\\& \\\& A literal ‘\&’ \\\\& \\& A literal ‘\’, followed by the matched text \\& \& A literal ‘&’ \\q \q A literal ‘\q’ \\\\ \\ \ The only case where the difference is noticeable is the last one: ‘\\\\’ is seen as ‘\\’ and produces ‘\’ instead of ‘\\’. Starting with version 3.1.4, gawk followed the POSIX rules when --posix was specified (see “Command-Line Options” on page 22). Otherwise, it continued to follow the pro‐ posed rules, as that had been its behavior for many years. When version 4.0.0 was released, the gawk maintainer made the POSIX rules the default, breaking well over a decade’s worth of backward compatibility.7 Needless to say, this was a bad idea, and as of version 4.0.1, gawk resumed its historical behavior, and only follows the POSIX rules when --posix is given. The rules for gensub() are considerably simpler. At the runtime level, whenever gawk sees a ‘\’, if the following character is a digit, then the text that matched the corresponding parenthesized subexpression is placed in the generated output. Otherwise, no matter what character follows the ‘\’, it appears in the generated text and the ‘\’ does not, as shown in Table 9-4. Table 9-4. Escape sequence processing for gensub() You type gensub() sees gensub() generates & & The matched text \\& \& A literal ‘&’ \\\\ \\ A literal ‘\’ \\\\& \\& A literal ‘\’, then the matched text \\\\\\& \\\& A literal ‘\&’ \\q \q A literal ‘q’ Built-in Functions | 207 Because of the complexity of the lexical- and runtime-level processing and the special cases for sub() and gsub(), we recommend the use of gawk and gensub() when you have to do substitutions. Input/Output Functions The following functions relate to input/output (I/O). Optional parameters are enclosed in square brackets ([ ]): close(filename [, how]) Close the file filename for input or output. Alternatively, the argument may be a shell command that was used for creating a coprocess, or for redirecting to or from a pipe; then the coprocess or pipe is closed. See “Closing Input and Output Redi‐ rections” on page 105 for more information. When closing a coprocess, it is occasionally useful to first close one end of the two- way pipe and then to close the other. This is done by providing a second argument to close(). This second argument (how) should be one of the two string values "to" or "from", indicating which end of the pipe to close. Case in the string does not matter. See “Two-Way Communications with Another Process” on page 334, which discusses this feature in more detail and gives an example. Note that the second argument to close() is a gawk extension; it is not available in compatibility mode (see “Command-Line Options” on page 22). fflush([filename]) Flush any buffered output associated with filename, which is either a file opened for writing or a shell command for redirecting output to a pipe or coprocess. Many utility programs buffer their output (i.e., they save information to write to a disk file or the screen in memory until there is enough for it to be worthwhile to send the data to the output device). This is often more efficient than writing every little bit of information as soon as it is ready. However, sometimes it is necessary to force a program to flush its buffers (i.e., write the information to its destination, even if a buffer is not full). This is the purpose of the fflush() function—gawk also buffers its output, and the fflush() function forces gawk to flush its buffers. Brian Kernighan added fflush() to his awk in April 1992. For two decades, it was a common extension. In December 2012, it was accepted for inclusion into the POSIX standard. See the Austin Group website. POSIX standardizes fflush() as follows: if there is no argument, or if the argument is the null string (""), then awk flushes the buffers for all open output files and pipes. 208 | Chapter 9: Functions 8. A program is interactive if the standard output is connected to a terminal device. On modern systems, this means your keyboard and screen. Prior to version 4.0.2, gawk would flush only the standard out‐ put if there was no argument, and flush all output files and pipes if the argument was the null string. This was changed in order to be compatible with Brian Kernighan’s awk, in the hope that standardizing this feature in POSIX would then be easier (which indeed proved to be the case). With gawk, you can use ‘fflush("/dev/stdout")’ if you wish to flush only the standard output. fflush() returns zero if the buffer is successfully flushed; otherwise, it returns a nonzero value. (gawk returns −1.) In the case where all buffers are flushed, the return value is zero only if all buffers were flushed successfully. Otherwise, it is −1, and gawk warns about the problem filename. gawk also issues a warning message if you attempt to flush a file or pipe that was opened for reading (such as with getline), or if filename is not an open file, pipe, or coprocess. In such a case, fflush() returns −1, as well. Interactive Versus Noninteractive Buffering As a side point, buffering issues can be even more confusing if your program is interactive (i.e., communicating with a user sitting at a keyboard).8 Interactive programs generally line buffer their output (i.e., they write out every line). Noninteractive programs wait until they have a full buffer, which may be many lines of output. Here is an example of the difference: $ awk '{ print $1 + $2 }' 1 1 2 2 3 5 Ctrl-d Each line of output is printed immediately. Compare that behavior with this example: $ awk '{ print $1 + $2 }' | cat 1 1 2 3 Ctrl-d 2 5 Built-in Functions | 209 Here, no output is printed until after the Ctrl-d is typed, because it is all buffered and sent down the pipe to cat in one shot. system(command) Execute the operating system command command and then return to the awk pro‐ gram. Return command ’s exit status. For example, if the following fragment of code is put in your awk program: END { system("date | mail -s 'awk run done' root") } the system administrator is sent mail when the awk program finishes processing input and begins its end-of-input processing. Note that redirecting print or printf into a pipe is often enough to accomplish your task. If you need to run many commands, it is more efficient to simply print them down a pipeline to the shell: while (more stuff to do) print command | "/bin/sh" close("/bin/sh") However, if your awk program is interactive, system() is useful for running large self-contained programs, such as a shell or an editor. Some operating systems cannot implement the system() function. system() causes a fatal error if it is not supported. When --sandbox is specified, the system() function is disabled (see “Command-Line Options” on page 22). Controlling Output Buffering with system() The fflush() function provides explicit control over output buffering for individual files and pipes. However, its use is not portable to many older awk implementations. An alternative method to flush output buffers is to call system() with a null string as its argument: system("") # flush output gawk treats this use of the system() function as a special case and is smart enough not to run a shell (or other command interpreter) with the empty command. Therefore, 210 | Chapter 9: Functions 9. The GNU date utility can also do many of the things described here. Its use may be preferable for simple time-related operations in shell scripts. with gawk, this idiom is not only useful, it is also efficient. Although this method should work with other awk implementations, it does not necessarily avoid starting an unnec‐ essary shell. (Other implementations may only flush the buffer associated with the stan‐ dard output and not necessarily all buffered output.) If you think about what a programmer expects, it makes sense that system() should flush any pending output. The following program: BEGIN { print "first print" system("echo system echo") print "second print" } must print: first print system echo second print and not: system echo first print second print If awk did not flush its buffers before calling system(), you would see the latter (unde‐ sirable) output. Time Functions awk programs are commonly used to process log files containing timestamp informa‐ tion, indicating when a particular log record was written. Many programs log their timestamps in the form returned by the time() system call, which is the number of seconds since a particular epoch. On POSIX-compliant systems, it is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds. All known POSIX- compliant systems support timestamps from 0 through 231 − 1, which is sufficient to represent times through 2038-01-19 03:14:07 UTC. Many systems support a wider range of timestamps, including negative timestamps that represent times before the epoch. In order to make it easier to process such log files and to produce useful reports, gawk provides the following functions for working with timestamps. They are gawk exten‐ sions; they are not specified in the POSIX standard.9 However, recent versions of mawk Built-in Functions | 211 10. Occasionally there are minutes in a year with a leap second, which is why the seconds can go up to 60. (see “Other Freely Available awk Implementations” on page 485) also support these func‐ tions. Optional parameters are enclosed in square brackets ([ ]): mktime(datespec) Turn datespec into a timestamp in the same form as is returned by systime(). It is similar to the function of the same name in ISO C. The argument, datespec, is a string of the form "YYYY MM DD HH MM SS [DST]". The string consists of six or seven numbers representing, respectively, the full year including century, the month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to 59, the second from 0 to 60,10 and an optional daylight-savings flag. The values of these numbers need not be within the ranges specified; for example, an hour of −1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding year 1 and year −1 preceding year 0. The time is assumed to be in the local time zone. If the daylight-savings flag is positive, the time is assumed to be daylight savings time; if zero, the time is assumed to be standard time; and if negative (the default), mktime() attempts to determine whether daylight savings time is in effect for the specified time. If datespec does not contain enough elements or if the resulting time is out of range, mktime() returns −1. strftime([format [, timestamp [, utc-flag] ] ]) Format the time specified by timestamp based on the contents of the format string and return the result. It is similar to the function of the same name in ISO C. If utc-flag is present and is either nonzero or non-null, the value is formatted as UTC (Coordinated Universal Time, formerly GMT or Greenwich Mean Time). Otherwise, the value is formatted for the local time zone. The timestamp is in the same format as the value returned by the systime() function. If no timestamp argument is supplied, gawk uses the current time of day as the timestamp. Without a format argument, strftime() uses the value of PROCINFO["strftime"] as the format string (see “Predefined Variables” on page 160). The default string value is "%a %b %e %H:%M:%S %Z %Y". This format string produces output that is equivalent to that of the date utility. You can assign a new value to PROCINFO["strftime"] to change the default format; see the following list for the various format directives. systime() Return the current time as the number of seconds since the system epoch. On POSIX systems, this is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a different number on other systems. 212 | Chapter 9: Functions 11. Unfortunately, not every system’s strftime() necessarily supports all of the conversions listed here. The systime() function allows you to compare a timestamp from a log file with the current time of day. In particular, it is easy to determine how long ago a particular record was logged. It also allows you to produce log records using the “seconds since the epoch” format. The mktime() function allows you to convert a textual representation of a date and time into a timestamp. This makes it easy to do before/after comparisons of dates and times, particularly when dealing with date and time data coming from an external source, such as a log file. The strftime() function allows you to easily turn a timestamp into human-readable information. It is similar in nature to the sprintf() function (see “String-Manipulation Functions” on page 194), in that it copies nonformat specification characters verbatim to the returned string, while substituting date and time values for format specifications in the format string. strftime() is guaranteed by the 1999 ISO C standard11 to support the following date format specifications: %a The locale’s abbreviated weekday name. %A The locale’s full weekday name. %b The locale’s abbreviated month name. %B The locale’s full month name. %c The locale’s “appropriate” date and time representation. (This is ‘%A %B %d %T %Y’ in the "C" locale.) %C The century part of the current year. This is the year divided by 100 and truncated to the next lower integer. %d The day of the month as a decimal number (01–31). %D Equivalent to specifying ‘%m/%d/%y’. Built-in Functions | 213 %e The day of the month, padded with a space if it is only one digit. %F Equivalent to specifying ‘%Y-%m-%d’. This is the ISO 8601 date format. %g The year modulo 100 of the ISO 8601 week number, as a decimal number (00–99). For example, January 1, 2012, is in week 53 of 2011. Thus, the year of its ISO 8601 week number is 2011, even though its year is 2012. Similarly, December 31, 2012, is in week 1 of 2013. Thus, the year of its ISO week number is 2013, even though its year is 2012. %G The full year of the ISO week number, as a decimal number. %h Equivalent to ‘%b’. %H The hour (24-hour clock) as a decimal number (00–23). %I The hour (12-hour clock) as a decimal number (01–12). %j The day of the year as a decimal number (001–366). %m The month as a decimal number (01–12). %M The minute as a decimal number (00–59). %n A newline character (ASCII LF). %p The locale’s equivalent of the AM/PM designations associated with a 12-hour clock. %r The locale’s 12-hour clock time. (This is ‘%I:%M:%S %p’ in the "C" locale.) %R Equivalent to specifying ‘%H:%M’. %S The second as a decimal number (00–60). 214 | Chapter 9: Functions %t A TAB character. %T Equivalent to specifying ‘%H:%M:%S’. %u The weekday as a decimal number (1–7). Monday is day one. %U The week number of the year (with the first Sunday as the first day of week one) as a decimal number (00–53). %V The week number of the year (with the first Monday as the first day of week one) as a decimal number (01–53). The method for determining the week number is as specified by ISO 8601. (To wit: if the week containing January 1 has four or more days in the new year, then it is week one; otherwise it is week 53 of the previous year and the next week is week one.) %w The weekday as a decimal number (0–6). Sunday is day zero. %W The week number of the year (with the first Monday as the first day of week one) as a decimal number (00–53). %x The locale’s “appropriate” date representation. (This is ‘%A %B %d %Y’ in the "C" locale.) %X The locale’s “appropriate” time representation. (This is ‘%T’ in the "C" locale.) %y The year modulo 100 as a decimal number (00–99). %Y The full year as a decimal number (e.g., 2015). %z The time zone offset in ‘+HHMM’ format (e.g., the format necessary to produce RFC 822/RFC 1036 date headers). %Z The time zone name or abbreviation; no characters if no time zone is determinable. Built-in Functions | 215 12. If you don’t understand any of this, don’t worry about it; these facilities are meant to make it easier to “in‐ ternationalize” programs. Other internationalization features are described in Chapter 13. 13. This is because ISO C leaves the behavior of the C version of strftime() undefined and gawk uses the system’s version of strftime() if it’s there. Typically, the conversion specifier either does not appear in the returned string or appears literally. %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy “Alternative representations” for the specifications that use only the second letter (‘%c’, ‘%C’, and so on).12 (These facilitate compliance with the POSIX date utility.) %% A literal ‘%’. If a conversion specifier is not one of those just listed, the behavior is undefined.13 For systems that are not yet fully standards-compliant, gawk supplies a copy of strftime() from the GNU C Library. It supports all of the just-listed format specifi‐ cations. If that version is used to compile gawk (see Appendix B), then the following additional format specifications are available: %k The hour (24-hour clock) as a decimal number (0–23). Single-digit numbers are padded with a space. %l The hour (12-hour clock) as a decimal number (1–12). Single-digit numbers are padded with a space. %s The time as a decimal timestamp in seconds since the epoch. Additionally, the alternative representations are recognized but their normal represen‐ tations are used. The following example is an awk implementation of the POSIX date utility. Normally, the date utility prints the current date and time of day in a well-known format. However, if you provide an argument to it that begins with a ‘+’, date copies nonformat specifier characters to the standard output and interprets the current time according to the format specifiers in the string. For example: $ date '+Today is %A, %B %d, %Y.' Today is Monday, September 22, 2014. Here is the gawk version of the date utility. It has a shell “wrapper” to handle the -u option, which requires that date run as if the time zone is set to UTC: 216 | Chapter 9: Functions #! /bin/sh # # date --- approximate the POSIX 'date' command case $1 in -u) TZ=UTC0 # use UTC export TZ shift ;; esac gawk 'BEGIN { format = PROCINFO["strftime"] exitval = 0 if (ARGC > 2) exitval = 1 else if (ARGC == 2) { format = ARGV[1] if (format ~ /^\+/) format = substr(format, 2) # remove leading + } print strftime(format) exit exitval }' "$@" Bit-Manipulation Functions I can explain it for you, but I can’t understand it for you. —Anonymous Many languages provide the ability to perform bitwise operations on two integer num‐ bers. In other words, the operation is performed on each successive pair of bits in the operands. Three common operations are bitwise AND, OR, and XOR. The operations are described in Table 9-5. Table 9-5. Bitwise operations Bit operator AND OR XOR Operands 0 1 0 1 0 1 0 0 0 0 1 0 1 1 0 1 1 1 1 0 As you can see, the result of an AND operation is 1 only when both bits are 1. The result of an OR operation is 1 if either bit is 1. The result of an XOR operation is 1 if either bit is 1, but not both. The next operation is the complement; the complement of 1 is 0 and the complement of 0 is 1. Thus, this operation “flips” all the bits of a given value. Built-in Functions | 217 14. This example shows that zeros come in on the left side. For gawk, this is always true, but in some languages, it’s possible to have the left side fill with ones. Finally, two other common operations are to shift the bits left or right. For example, if you have a bit string ‘10111001’ and you shift it right by three bits, you end up with ‘00010111’.14 If you start over again with ‘10111001’ and shift it left by three bits, you end up with ‘11001000’. The following list describes gawk’s built-in functions that implement the bitwise operations. Optional parameters are enclosed in square brackets ([ ]): and(v1, v2 [, …]) Return the bitwise AND of the arguments. There must be at least two. compl(val) Return the bitwise complement of val. lshift(val, count) Return the value of val, shifted left by count bits. or(v1, v2 [, …]) Return the bitwise OR of the arguments. There must be at least two. rshift(val, count) Return the value of val, shifted right by count bits. xor(v1, v2 [, …]) Return the bitwise XOR of the arguments. There must be at least two. For all of these functions, first the double-precision floating-point value is converted to the widest C unsigned integer type, then the bitwise operation is performed. If the result cannot be represented exactly as a C double, leading nonzero bits are removed one by one until it can be represented exactly. The result is then converted back into a C double. (If you don’t understand this paragraph, don’t worry about it.) Here is a user-defined function (see “User-Defined Functions” on page 220) that illustrates the use of these functions: # bits2str --- turn a byte into readable ones and zeros function bits2str(bits, data, mask) { if (bits == 0) return "0" mask = 1 for (; bits != 0; bits = rshift(bits, 1)) data = (and(bits, mask) ? "1" : "0") data 218 | Chapter 9: Functions while ((length(data) % 8) != 0) data = "0" data return data } BEGIN { printf "123 = %s\n", bits2str(123) printf "0123 = %s\n", bits2str(0123) printf "0x99 = %s\n", bits2str(0x99) comp = compl(0x99) printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp) shift = lshift(0x99, 2) printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) shift = rshift(0x99, 2) printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) } This program produces the following output when run: $ gawk -f testbits.awk 123 = 01111011 0123 = 01010011 0x99 = 10011001 compl(0x99) = 0xffffff66 = 11111111111111111111111101100110 lshift(0x99, 2) = 0x264 = 0000001001100100 rshift(0x99, 2) = 0x26 = 00100110 The bits2str() function turns a binary number into a string. Initializing mask to one creates a binary value where the rightmost bit is set to one. Using this mask, the function repeatedly checks the rightmost bit. ANDing the mask with the value indicates whether the rightmost bit is one or not. If so, a "1" is concatenated onto the front of the string. Otherwise, a "0" is added. The value is then shifted right by one bit and the loop con‐ tinues until there are no more one bits. If the initial value is zero, it returns a simple "0". Otherwise, at the end, it pads the value with zeros to represent multiples of 8-bit quantities. This is typical in modern computers. The main code in the BEGIN rule shows the difference between the decimal and octal values for the same numbers (see “Octal and hexadecimal numbers” on page 112), and then demonstrates the results of the compl(), lshift(), and rshift() functions. Getting Type Information gawk provides a single function that lets you distinguish an array from a scalar variable. This is necessary for writing code that traverses every element of an array of arrays (see “Arrays of Arrays” on page 187). isarray(x) Return a true value if x is an array. Otherwise, return false. Built-in Functions | 219 isarray() is meant for use in two circumstances. The first is when traversing a multi‐ dimensional array: you can test if an element is itself an array or not. The second is inside the body of a user-defined function (not discussed yet; see “User-Defined Func‐ tions” on page 220), to test if a parameter is an array or not. Using isarray() at the global level to test variables makes no sense. Because you are the one writing the program, you are supposed to know if your variables are arrays or not. And in fact, due to the way gawk works, if you pass the name of a variable that has not been previously used to isarray(), gawk ends up turning it into a scalar. String-Translation Functions gawk provides facilities for internationalizing awk programs. These include the func‐ tions described in the following list. The descriptions here are purposely brief. See Chapter 13 for the full story. Optional parameters are enclosed in square brackets ([ ]): bindtextdomain(directory [, domain]) Set the directory in which gawk will look for message translation files, in case they will not or cannot be placed in the “standard” locations (e.g., during testing). It returns the directory in which domain is “bound.” The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then bindtextdomain() returns the current binding for the given domain. dcgettext(string [, domain [, category] ]) Return the translation of string in text domain domain for locale category catego ry. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES". dcngettext(string1, string2, number [, domain [, category] ]) Return the plural form used for number of the translation of string1 and string2 in text domain domain for locale category category. string1 is the English singular variant of a message, and string2 is the English plural variant of the same message. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES". User-Defined Functions Complicated awk programs can often be simplified by defining your own functions. User-defined functions can be called just like built-in ones (see “Function Calls” on page 134), but it is up to you to define them (i.e., to tell awk what they should do). 220 | Chapter 9: Functions Function Definition Syntax It’s entirely fair to say that the awk syntax for local variable definitions is appallingly awful. —Brian Kernighan Definitions of functions can appear anywhere between the rules of an awk program. Thus, the general form of an awk program is extended to include sequences of rules and user-defined function definitions. There is no need to put the definition of a function before all uses of the function. This is because awk reads the entire program before starting to execute any of it. The definition of a function named name looks like this: function name([parameter-list]) { body-of-function } Here, name is the name of the function to define. A valid function name is like a valid variable name: a sequence of letters, digits, and underscores that doesn’t start with a digit. Here too, only the 52 upper- and lowercase English letters may be used in a func‐ tion name. Within a single awk program, any particular name can only be used as a variable, array, or function. parameter-list is an optional list of the function’s arguments and local variable names, separated by commas. When the function is called, the argument names are used to hold the argument values given in the call. A function cannot have two parameters with the same name, nor may it have a param‐ eter with the same name as the function itself. According to the POSIX standard, function parameters cannot have the same name as one of the special predefined variables (see “Pre‐ defined Variables” on page 160), nor may a function parameter have the same name as another function. Not all versions of awk enforce these restrictions. gawk always en‐ forces the first restriction. With --posix (see “Command-Line Op‐ tions” on page 22), it also enforces the second restriction. Local variables act like the empty string if referenced where a string value is required, and like zero if referenced where a numeric value is required. This is the same as the behavior of regular variables that have never been assigned a value. (There is more to understand about local variables; see “Functions and Their Effects on Variable Typ‐ ing” on page 230.) User-Defined Functions | 221 The body-of-function consists of awk statements. It is the most important part of the definition, because it says what the function should actually do. The argument names exist to give the body a way to talk about the arguments; local variables exist to give the body places to keep temporary values. Argument names are not distinguished syntactically from local variable names. Instead, the number of arguments supplied when the function is called determines how many argument variables there are. Thus, if three argument values are given, the first three names in parameter-list are arguments and the rest are local variables. It follows that if the number of arguments is not the same in all calls to the function, some of the names in parameter-list may be arguments on some occasions and local variables on others. Another way to think of this is that omitted arguments default to the null string. Usually when you write a function, you know how many names you intend to use for arguments and how many you intend to use as local variables. It is conventional to place some extra space between the arguments and the local variables, in order to document how your function is supposed to be used. During execution of the function body, the arguments and local variable values hide, or shadow, any variables of the same names used in the rest of the program. The shad‐ owed variables are not accessible in the function definition, because there is no way to name them while their names have been taken away for the arguments and local vari‐ ables. All other variables used in the awk program can be referenced or set normally in the function’s body. The arguments and local variables last only as long as the function body is executing. Once the body finishes, you can once again access the variables that were shadowed while the function was running. The function body can contain expressions that call functions. They can even call this function, either directly or by way of another function. When this happens, we say the function is recursive. The act of a function calling itself is called recursion. All the built-in functions return a value to their caller. User-defined functions can do so also, using the return statement, which is described in detail in “The return State‐ ment” on page 229. Many of the subsequent examples in this section use the return statement. In many awk implementations, including gawk, the keyword function may be abbre‐ viated func. (c.e.) However, POSIX only specifies the use of the keyword function. This actually has some practical implications. If gawk is in POSIX-compatibility mode (see “Command-Line Options” on page 22), then the following statement does not define a function: func foo() { a = sqrt($1) ; print a } 222 | Chapter 9: Functions 15. This program won’t actually run, because foo() is undefined. Instead, it defines a rule that, for each record, concatenates the value of the variable ‘func’ with the return value of the function ‘foo.’ If the resulting string is non-null, the action is executed. This is probably not what is desired. (awk accepts this input as syn‐ tactically valid, because functions may be used before they are defined in awk programs.15) To ensure that your awk programs are portable, always use the keyword function when defining a function. Function Definition Examples Here is an example of a user-defined function, called myprint(), that takes a number and prints it in a specific format: function myprint(num) { printf "%6.3g\n", num } To illustrate, here is an awk rule that uses our myprint() function: $3 > 0 { myprint($3) } This program prints, in our special format, all the third fields that contain a positive number in our input. Therefore, when given the following input: 1.2 3.4 5.6 7.8 9.10 11.12 -13.14 15.16 17.18 19.20 21.22 23.24 this program, using our function to format the results, prints: 5.6 21.2 This function deletes all the elements in an array (recall that the extra whitespace sig‐ nifies the start of the local variable list): function delarray(a, i) { for (i in a) delete a[i] } When working with arrays, it is often necessary to delete all the elements in an array and start over with a new list of elements (see “The delete Statement” on page 184). Instead of having to repeat this loop everywhere that you need to clear out an array, your program can just call delarray(). (This guarantees portability. The use of ‘delete User-Defined Functions | 223 16. Late in 2012. array’ to delete the contents of an entire array is a relatively recent16 addition to the POSIX standard.) The following is an example of a recursive function. It takes a string as an input pa‐ rameter and returns the string in reverse order. Recursive functions must always have a test that stops the recursion. In this case, the recursion terminates when the input string is already empty: function rev(str) { if (str == "") return "" return (rev(substr(str, 2)) substr(str, 1, 1)) } If this function is in a file named rev.awk, it can be tested this way: $ echo "Don't Panic!" | > gawk -e '{ print rev($0) }' -f rev.awk !cinaP t'noD The C ctime() function takes a timestamp and returns it as a string, formatted in a well- known fashion. The following example uses the built-in strftime() function (see “Time Functions” on page 211) to create an awk version of ctime(): # ctime.awk # # awk version of C ctime(3) function function ctime(ts, format) { format = "%a %b %e %H:%M:%S %Z %Y" if (ts == 0) ts = systime() # use current time as default return strftime(format, ts) } You might think that ctime() could use PROCINFO["strftime"] for its format string. That would be a mistake, because ctime() is supposed to return the time formatted in a standard fashion, and user-level code could have changed PROCINFO["strftime"]. Calling User-Defined Functions Calling a function means causing the function to run and do its job. A function call is an expression and its value is the value returned by the function. 224 | Chapter 9: Functions Writing a function call A function call consists of the function name followed by the arguments in parentheses. awk expressions are what you write in the call for the arguments. Each time the call is executed, these expressions are evaluated, and the values become the actual arguments. For example, here is a call to foo() with three arguments (the first being a string concatenation): foo(x y, "lose", 4 * z) Whitespace characters (spaces and TABs) are not allowed between the function name and the opening parenthesis of the argument list. If you write whitespace by mistake, awk might think that you mean to concatenate a variable with an expression in parentheses. How‐ ever, it notices that you used a function name and not a variable name, and reports an error. Controlling variable scope Unlike in many languages, there is no way to make a variable local to a { … } block in awk, but you can make a variable local to a function. It is good practice to do so whenever a variable is needed only in that function. To make a variable local to a function, simply declare the variable as an argument after the actual function arguments (see “Function Definition Syntax” on page 221). Look at the following example, where variable i is a global variable used by both functions foo() and bar(): function bar() { for (i = 0; i < 3; i++) print "bar's i=" i } function foo(j) { i = j + 1 print "foo's i=" i bar() print "foo's i=" i } BEGIN { i = 10 print "top's i=" i foo(0) print "top's i=" i } User-Defined Functions | 225 Running this script produces the following, because the i in functions foo() and bar() and at the top level refer to the same variable instance: top's i=10 foo's i=1 bar's i=0 bar's i=1 bar's i=2 foo's i=3 top's i=3 If you want i to be local to both foo() and bar(), do as follows (the extra space before i is a coding convention to indicate that i is a local variable, not an argument): function bar( i) { for (i = 0; i < 3; i++) print "bar's i=" i } function foo(j, i) { i = j + 1 print "foo's i=" i bar() print "foo's i=" i } BEGIN { i = 10 print "top's i=" i foo(0) print "top's i=" i } Running the corrected script produces the following: top's i=10 foo's i=1 bar's i=0 bar's i=1 bar's i=2 foo's i=1 top's i=10 Besides scalar values (strings and numbers), you may also have local arrays. By using a parameter name as an array, awk treats it as an array, and it is local to the function. In addition, recursive calls create new arrays. Consider this example: function some_func(p1, a) { if (p1++ > 3) return 226 | Chapter 9: Functions a[p1] = p1 some_func(p1) printf("At level %d, index %d %s found in a\n", p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not") printf("At level %d, index %d %s found in a\n", p1, p1, p1 in a ? "is" : "is not") print "" } BEGIN { some_func(1) } When run, this program produces the following output: At level 4, index 3 is not found in a At level 4, index 4 is found in a At level 3, index 2 is not found in a At level 3, index 3 is found in a At level 2, index 1 is not found in a At level 2, index 2 is found in a Passing function arguments by value or by reference In awk, when you declare a function, there is no way to declare explicitly whether the arguments are passed by value or by reference. Instead, the passing convention is determined at runtime when the function is called, according to the following rule: if the argument is an array variable, then it is passed by reference. Otherwise, the argument is passed by value. Passing an argument by value means that when a function is called, it is given a copy of the value of this argument. The caller may use a variable as the expression for the ar‐ gument, but the called function does not know this—it only knows what value the argument had. For example, if you write the following code: foo = "bar" z = myfunc(foo) then you should not think of the argument to myfunc() as being “the variable foo.” Instead, think of the argument as the string value "bar". If the function myfunc() alters the values of its local variables, this has no effect on any other variables. Thus, if my func() does this: function myfunc(str) { print str User-Defined Functions | 227 str = "zzz" print str } to change its first argument variable str, it does not change the value of foo in the caller. The role of foo in calling myfunc() ended when its value ("bar") was computed. If str also exists outside of myfunc(), the function body cannot alter this outer value, because it is shadowed during the execution of myfunc() and cannot be seen or changed from there. However, when arrays are the parameters to functions, they are not copied. Instead, the array itself is made available for direct manipulation by the function. This is usually termed call by reference. Changes made to an array parameter inside the body of a function are visible outside that function. Changing an array parameter inside a function can be very danger‐ ous if you do not watch what you are doing. For example: function changeit(array, ind, nvalue) { array[ind] = nvalue } BEGIN { a[1] = 1; a[2] = 2; a[3] = 3 changeit(a, 2, "two") printf "a[1] = %s, a[2] = %s, a[3] = %s\n", a[1], a[2], a[3] } prints ‘a[1] = 1, a[2] = two, a[3] = 3’, because changeit() stores "two" in the second element of a. Some awk implementations allow you to call a function that has not been defined. They only report a problem at runtime, when the program actually tries to call the function. For example: BEGIN { if (0) foo() else bar() } function bar() { … } # note that `foo' is not defined Because the ‘if’ statement will never be true, it is not really a problem that foo() has not been defined. Usually, though, it is a problem if a program calls an undefined function. 228 | Chapter 9: Functions If --lint is specified (see “Command-Line Options” on page 22), gawk reports calls to undefined functions. Some awk implementations generate a runtime error if you use either the next statement or the nextfile statement (see “The next Statement” on page 157, and “The nextfile Statement” on page 158) inside a user-defined function. gawk does not have this limitation. The return Statement As seen in several earlier examples, the body of a user-defined function can contain a return statement. This statement returns control to the calling part of the awk program. It can also be used to return a value for use in the rest of the awk program. It looks like this: return [expression] The expression part is optional. Due most likely to an oversight, POSIX does not define what the return value is if you omit the expression. Technically speaking, this makes the returned value undefined, and therefore unpredictable. In practice, though, all versions of awk simply return the null string, which acts like zero if used in a numeric context. A return statement without an expression is assumed at the end of every function definition. So, if control reaches the end of the function body, then technically the func‐ tion returns an unpredictable value. In practice, it returns the empty string. awk does not warn you if you use the return value of such a function. Sometimes, you want to write a function for what it does, not for what it returns. Such a function corresponds to a void function in C, C++, or Java, or to a procedure in Ada. Thus, it may be appropriate to not return any value; simply bear in mind that you should not be using the return value of such a function. The following is an example of a user-defined function that returns a value for the largest number among the elements of an array: function maxelt(vec, i, ret) { for (i in vec) { if (ret == "" || vec[i] > ret) ret = vec[i] } return ret } You call maxelt() with one argument, which is an array name. The local variables i and ret are not intended to be arguments; there is nothing to stop you from passing more User-Defined Functions | 229 than one argument to maxelt() but the results would be strange. The extra space before i in the function parameter list indicates that i and ret are local variables. You should follow this convention when defining functions. The following program uses the maxelt() function. It loads an array, calls maxelt(), and then reports the maximum number in that array: function maxelt(vec, i, ret) { for (i in vec) { if (ret == "" || vec[i] > ret) ret = vec[i] } return ret } # Load all fields of each record into nums. { for(i = 1; i <= NF; i++) nums[NR, i] = $i } END { print maxelt(nums) } Given the following input: 1 5 23 8 16 44 3 5 2 8 26 256 291 1396 2962 100 -6 467 998 1101 99385 11 0 225 the program reports (predictably) that 99,385 is the largest value in the array. Functions and Their Effects on Variable Typing awk is a very fluid language. It is possible that awk can’t tell if an identifier represents a scalar variable or an array until runtime. Here is an annotated sample program: function foo(a) { a[1] = 1 # parameter is an array } BEGIN { b = 1 foo(b) # invalid: fatal type mismatch foo(x) # x uninitialized, becomes an array dynamically 230 | Chapter 9: Functions x = 1 # now not allowed, runtime error } In this example, the first call to foo() generates a fatal error, so awk will not report the second error. If you comment out that call, though, then awk does report the second error. Usually, such things aren’t a big issue, but it’s worth being aware of them. Indirect Function Calls This section describes an advanced, gawk-specific extension. Often, you may wish to defer the choice of function to call until runtime. For example, you may have different kinds of records, each of which should be processed differently. Normally, you would have to use a series of if-else statements to decide which function to call. By using indirect function calls, you can specify the name of the function to call as a string variable, and then call the function. Let’s look at an example. Suppose you have a file with your test scores for the classes you are taking, and you wish to get the sum and the average of your test scores. The first field is the class name. The following fields are the functions to call to process the data, up to a “marker” field ‘data:’. Following the marker, to the end of the record, are the various numeric test scores. Here is the initial file: Biology_101 sum average data: 87.0 92.4 78.5 94.9 Chemistry_305 sum average data: 75.2 98.3 94.7 88.2 English_401 sum average data: 100.0 95.6 87.1 93.4 To process the data, you might write initially: { class = $1 for (i = 2; $i != "data:"; i++) { if ($i == "sum") sum() # processes the whole record else if ($i == "average") average() … # and so on } } This style of programming works, but can be awkward. With indirect function calls, you tell gawk to use the value of a variable as the name of the function to call. Indirect Function Calls | 231 The syntax is similar to that of a regular function call: an identifier immediately followed by an opening parenthesis, any arguments, and then a closing parenthesis, with the addition of a leading ‘@’ character: the_func = "sum" result = @the_func() # calls the sum() function Here is a full program that processes the previously shown data, using indirect function calls: # indirectcall.awk --- Demonstrate indirect function calls # average --- return the average of the values in fields $first - $last function average(first, last, sum, i) { sum = 0; for (i = first; i <= last; i++) sum += $i return sum / (last - first + 1) } # sum --- return the sum of the values in fields $first - $last function sum(first, last, ret, i) { ret = 0; for (i = first; i <= last; i++) ret += $i return ret } These two functions expect to work on fields; thus, the parameters first and last indicate where in the fields to start and end. Otherwise, they perform the expected computations and are not unusual: # For each record, print the class name and the requested statistics { class_name = $1 gsub(/_/, " ", class_name) # Replace _ with spaces # find start for (i = 1; i <= NF; i++) { if ($i == "data:") { start = i + 1 break } } printf("%s:\n", class_name) 232 | Chapter 9: Functions for (i = 2; $i != "data:"; i++) { the_function = $i printf("\t%s: <%s>\n", $i, @the_function(start, NF) "") } print "" } This is the main processing for each record. It prints the class name (with underscores replaced with spaces). It then finds the start of the actual data, saving it in start. The last part of the code loops through each function name (from $2 up to the marker, ‘data:’), calling the function named by the field. The indirect function call itself occurs as a parameter in the call to printf. (The printf format string uses ‘%s’ as the format specifier so that we can use functions that return strings, as well as numbers. Note that the result from the indirect call is concatenated with the empty string, in order to force it to be a string value.) Here is the result of running the program: $ gawk -f indirectcall.awk class_data1 Biology 101: sum: <352.8> average: <88.2> Chemistry 305: sum: <356.4> average: <89.1> English 401: sum: <376.1> average: <94.025> The ability to use indirect function calls is more powerful than you may think at first. The C and C++ languages provide “function pointers,” which are a mechanism for calling a function chosen at runtime. One of the most well-known uses of this ability is the C qsort() function, which sorts an array using the famous “quicksort” algorithm (see the Wikipedia article for more information). To use this function, you supply a pointer to a comparison function. This mechanism allows you to sort arbitrary data in an arbitrary fashion. We can do something similar using gawk, like this: # quicksort.awk --- Quicksort algorithm, with user-supplied # comparison function # quicksort --- C.A.R. Hoare's quicksort algorithm. See Wikipedia # or almost any algorithms or computer science text. function quicksort(data, left, right, less_than, i, last) { if (left >= right) # do nothing if array contains fewer return # than two elements Indirect Function Calls | 233 quicksort_swap(data, left, int((left + right) / 2)) last = left for (i = left + 1; i <= right; i++) if (@less_than(data[i], data[left])) quicksort_swap(data, ++last, i) quicksort_swap(data, left, last) quicksort(data, left, last - 1, less_than) quicksort(data, last + 1, right, less_than) } # quicksort_swap --- helper function for quicksort, should really be inline function quicksort_swap(data, i, j, temp) { temp = data[i] data[i] = data[j] data[j] = temp } The quicksort() function receives the data array, the starting and ending indices to sort (left and right), and the name of a function that performs a “less than” compar‐ ison. It then implements the quicksort algorithm. To make use of the sorting function, we return to our previous example. The first thing to do is write some comparison functions: # num_lt --- do a numeric less than comparison function num_lt(left, right) { return ((left + 0) < (right + 0)) } # num_ge --- do a numeric greater than or equal to comparison function num_ge(left, right) { return ((left + 0) >= (right + 0)) } The num_ge() function is needed to perform a descending sort; when used to perform a “less than” test, it actually does the opposite (greater than or equal to), which yields data sorted in descending order. Next comes a sorting function. It is parameterized with the starting and ending field numbers and the comparison function. It builds an array with the data and calls quick sort() appropriately, and then formats the results as a single string: # do_sort --- sort the data according to `compare' # and return it as a string function do_sort(first, last, compare, data, i, retval) 234 | Chapter 9: Functions { delete data for (i = 1; first <= last; first++) { data[i] = $first i++ } quicksort(data, 1, i-1, compare) retval = data[1] for (i = 2; i in data; i++) retval = retval " " data[i] return retval } Finally, the two sorting functions call do_sort(), passing in the names of the two comparison functions: # sort --- sort the data in ascending order and return it as a string function sort(first, last) { return do_sort(first, last, "num_lt") } # rsort --- sort the data in descending order and return it as a string function rsort(first, last) { return do_sort(first, last, "num_ge") } Here is an extended version of the datafile: Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9 Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2 English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4 Finally, here are the results when the enhanced program is run: $ gawk -f quicksort.awk -f indirectcall.awk class_data2 Biology 101: sum: <352.8> average: <88.2> sort: <78.5 87.0 92.4 94.9> rsort: <94.9 92.4 87.0 78.5> Chemistry 305: sum: <356.4> average: <89.1> sort: <75.2 88.2 94.7 98.3> rsort: <98.3 94.7 88.2 75.2> English 401: sum: <376.1> Indirect Function Calls | 235 17. This may change in a future version; recheck the documentation that comes with your version of gawk to see if it has. average: <94.025> sort: <87.1 93.4 95.6 100.0> rsort: <100.0 95.6 93.4 87.1> Another example where indirect functions calls are useful can be found in processing arrays. This is described in “Traversing Arrays of Arrays” on page 273. Remember that you must supply a leading ‘@’ in front of an indirect function call. Starting with version 4.1.2 of gawk, indirect function calls may also be used with built- in functions and with extension functions (see Chapter 16). The only thing you cannot do is pass a regular expression constant to a built-in function through an indirect func‐ tion call.17 gawk does its best to make indirect function calls efficient. For example, in the following case: for (i = 1; i <= n; i++) @the_func() gawk looks up the actual function to call only once. Summary • awk provides built-in functions and lets you define your own functions. • POSIX awk provides three kinds of built-in functions: numeric, string, and I/O. gawk provides functions that sort arrays, work with values representing time, do bit manipulation, determine variable type (array versus scalar), and internationalize and localize programs. gawk also provides several extensions to some of standard functions, typically in the form of additional arguments. • Functions accept zero or more arguments and return a value. The expressions that provide the argument values are completely evaluated before the function is called. Order of evaluation is not defined. The return value can be ignored. • The handling of backslash in sub() and gsub() is not simple. It is more straight‐ forward in gawk’s gensub() function, but that function still requires care in its use. • User-defined functions provide important capabilities but come with some syn‐ tactic inelegancies. In a function call, there cannot be any space between the func‐ tion name and the opening left parenthesis of the argument list. Also, there is no provision for local variables, so the convention is to add extra parameters, and to separate them visually from the real parameters by extra whitespace. 236 | Chapter 9: Functions • User-defined functions may call other user-defined (and built-in) functions and may call themselves recursively. Function parameters “hide” any global variables of the same names. You cannot use the name of a reserved variable (such as ARGC) as the name of a parameter in user-defined functions. • Scalar values are passed to user-defined functions by value. Array parameters are passed by reference; any changes made by the function to array parameters are thus visible after the function has returned. • Use the return statement to return from a user-defined function. An optional ex‐ pression becomes the function’s return value. Only scalar values may be returned by a function. • If a variable that has never been used is passed to a user-defined function, how that function treats the variable can set its nature: either scalar or array. • gawk provides indirect function calls using a special syntax. By setting a variable to the name of a function, you can determine at runtime what function will be called at that point in the program. This is equivalent to function pointers in C and C++. Summary | 237 PART II Problem Solving with awk Part II shows how to use awk and gawk for problem solving. There is lots of code here for you to read and learn from. It contains the following chapters: • Chapter 10, A Library of awk Functions • Chapter 11, Practical awk Programs 1. Sadly, over 35 years later, many of the lessons taught by this book have yet to be learned by a vast number of practicing programmers. CHAPTER 10 A Library of awk Functions “User-Defined Functions” on page 220 describes how to write your own awk functions. Writing functions is important, because it allows you to encapsulate algorithms and program tasks in a single place. It simplifies programming, making program develop‐ ment more manageable and making programs more readable. In their seminal 1976 book, Software Tools,1 Brian Kernighan and P.J. Plauger wrote: Good Programming is not learned from generalities, but by seeing how significant pro‐ grams can be made clean, easy to read, easy to maintain and modify, human-engineered, efficient and reliable, by the application of common sense and good programming prac‐ tices. Careful study and imitation of good programs leads to better writing. In fact, they felt this idea was so important that they placed this statement on the cover of their book. Because we believe strongly that their statement is correct, this chapter and Chapter 11 provide a good-sized body of code for you to read and, we hope, to learn from. This chapter presents a library of useful awk functions. Many of the sample programs presented later in this book use these functions. The functions are presented here in a progression from simple to complex. “Extracting Programs from Texinfo Source Files” on page 309 presents a program that you can use to extract the source code for these example library functions and programs from the Texinfo source for this book. (This has already been done as part of the gawk distribution.) 241 2. The effects are not identical. Output of the transformed record will be in all lowercase, while IGNORECASE preserves the original contents of the input record. The programs in this chapter and in Chapter 11 freely use gawk-specific features. Re‐ writing these programs for different implementations of awk is pretty straightforward: • Diagnostic error messages are sent to /dev/stderr. Use ‘| "cat 1>&2"’ instead of ‘> "/dev/stderr"’ if your system does not have a /dev/stderr, or if you cannot use gawk. • A number of programs use nextfile (see “The nextfile Statement” on page 158) to skip any remaining input in the input file. • Finally, some of the programs choose to ignore upper- and lowercase distinctions in their input. They do so by assigning one to IGNORECASE. You can achieve almost the same effect2 by adding the following rule to the beginning of the program: # ignore case { $0 = tolower($0) } Also, verify that all regexp and string constants used in comparisons use only low‐ ercase letters. Naming Library Function Global Variables Due to the way the awk language evolved, variables are either global (usable by the entire program) or local (usable just by a specific function). There is no intermediate state analogous to static variables in C. Library functions often need to have global variables that they can use to preserve state information between calls to the function—for example, getopt()’s variable _opti (see “Processing Command-Line Options” on page 259). Such variables are called private, as the only functions that need to use them are the ones in the library. When writing a library function, you should try to choose names for your private vari‐ ables that will not conflict with any variables used by either another library function or a user’s main program. For example, a name like i or j is not a good choice, because user programs often use variable names like these for their own purposes. The example programs shown in this chapter all start the names of their private variables with an underscore (‘_’). Users generally don’t use leading underscores in their variable names, so this convention immediately decreases the chances that the variable names will be accidentally shared with the user’s program. In addition, several of the library functions use a prefix that helps indicate what function or set of functions use the variables—for example, _pw_byname() in the user database 242 | Chapter 10: A Library of awk Functions 3. Although all the library routines could have been rewritten to use this convention, this was not done, in order to show how our own awk programming style has evolved and to provide some basis for this discussion. 4. gawk’s --dump-variables command-line option is useful for verifying this. routines (see “Reading the User Database” on page 264). This convention is recommended, as it even further decreases the chance of inadvertent conflict among variable names. Note that this convention is used equally well for variable names and for private function names.3 As a final note on variable naming, if a function makes global variables available for use by a main program, it is a good convention to start those variables’ names with a capital letter—for example, getopt()’s Opterr and Optind variables (see “Processing Command-Line Options” on page 259). The leading capital letter indicates that it is global, while the fact that the variable name is not all capital letters indicates that the variable is not one of awk’s predefined variables, such as FS. It is also important that all variables in library functions that do not need to save state are, in fact, declared local.4 If this is not done, the variables could accidentally be used in the user’s program, leading to bugs that are very difficult to track down: function lib_func(x, y, l1, l2) { … # some_var should be local but by oversight is not use variable some_var … } A different convention, common in the Tcl community, is to use a single associative array to hold the values needed by the library function(s), or “package.” This significantly decreases the number of actual global names in use. For example, the functions described in “Reading the User Database” on page 264 might have used array elements PW_data["inited"], PW_data["total"], PW_data["count"], and PW_data["awklib"], instead of _pw_inited, _pw_awklib, _pw_total, and _pw_count. The conventions presented in this section are exactly that: conventions. You are not required to write your programs this way—we merely recommend that you do so. General Programming This section presents a number of functions that are of general programming use. General Programming | 243 Converting Strings to Numbers The strtonum() function (see “String-Manipulation Functions” on page 194) is a gawk extension. The following function provides an implementation for other versions of awk: # mystrtonum --- convert string to number function mystrtonum(str, ret, n, i, k, c) { if (str ~ /^0[0-7]*$/) { # octal n = length(str) ret = 0 for (i = 1; i <= n; i++) { c = substr(str, i, 1) # index() returns 0 if c not in string, # includes c == "0" k = index("1234567", c) ret = ret * 8 + k } } else if (str ~ /^0[xX][[:xdigit:]]+$/) { # hexadecimal str = substr(str, 3) # lop off leading 0x n = length(str) ret = 0 for (i = 1; i <= n; i++) { c = substr(str, i, 1) c = tolower(c) # index() returns 0 if c not in string, # includes c == "0" k = index("123456789abcdef", c) ret = ret * 16 + k } } else if (str ~ \ /^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) { # decimal number, possibly floating point ret = str + 0 } else ret = "NOT-A-NUMBER" return ret } # BEGIN { # gawk test harness # a[1] = "25" # a[2] = ".31" # a[3] = "0123" # a[4] = "0xdeadBEEF" # a[5] = "123.45" 244 | Chapter 10: A Library of awk Functions # a[6] = "1.e3" # a[7] = "1.32" # a[8] = "1.32E2" # # for (i = 1; i in a; i++) # print a[i], strtonum(a[i]), mystrtonum(a[i]) # } The function first looks for C-style octal numbers (base 8). If the input string matches a regular expression describing octal numbers, then mystrtonum() loops through each character in the string. It sets k to the index in "1234567" of the current octal digit. The return value will either be the same number as the digit, or zero if the character is not there, which will be true for a ‘0’. This is safe, because the regexp test in the if ensures that only octal values are converted. Similar logic applies to the code that checks for and converts a hexadecimal value, which starts with ‘0x’ or ‘0X’. The use of tolower() simplifies the computation for finding the correct numeric value for each hexadecimal digit. Finally, if the string matches the (rather complicated) regexp for a regular decimal in‐ teger or floating-point number, the computation ‘ret = str + 0’ lets awk convert the value to a number. A commented-out test program is included, so that the function can be tested with gawk and the results compared to the the built-in strtonum() function. Assertions When writing large programs, it is often useful to know that a condition or set of con‐ ditions is true. Before proceeding with a particular computation, you make a statement about what you believe to be the case. Such a statement is known as an assertion. The C language provides an header file and corresponding assert() macro that a programmer can use to make assertions. If an assertion fails, the assert() macro arranges to print a diagnostic message describing the condition that should have been true but was not, and then it kills the program. In C, using assert() looks this: #include int myfunc(int a, double b) { assert(a <= 5 && b >= 17.1); … } If the assertion fails, the program prints a message similar to this: prog.c:5: assertion failed: a <= 5 && b >= 17.1 General Programming | 245 The C language makes it possible to turn the condition into a string for use in printing the diagnostic message. This is not possible in awk, so this assert() function also re‐ quires a string version of the condition that is being tested. Following is the function: # assert --- assert that a condition is true. Otherwise, exit. function assert(condition, string) { if (! condition) { printf("%s:%d: assertion failed: %s\n", FILENAME, FNR, string) > "/dev/stderr" _assert_exit = 1 exit 1 } } END { if (_assert_exit) exit 1 } The assert() function tests the condition parameter. If it is false, it prints a message to standard error, using the string parameter to describe the failed condition. It then sets the variable _assert_exit to one and executes the exit statement. The exit statement jumps to the END rule. If the END rule finds _assert_exit to be true, it exits immediately. The purpose of the test in the END rule is to keep any other END rules from running. When an assertion fails, the program should exit immediately. If no assertions fail, then _assert_exit is still false when the END rule is run normally, and the rest of the program’s END rules execute. For all of this to work correctly, assert.awk must be the first source file read by awk. The function can be used in a program in the following way: function myfunc(a, b) { assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1") … } If the assertion fails, you see a message similar to the following: mydata:1357: assertion failed: a <= 5 && b >= 17.1 There is a small problem with this version of assert(). An END rule is automatically added to the program calling assert(). Normally, if a program consists of just a BE GIN rule, the input files and/or standard input are not read. However, now that the program has an END rule, awk attempts to read the input datafiles or standard input (see “Startup and cleanup actions” on page 145), most likely causing the program to hang as it waits for input. 246 | Chapter 10: A Library of awk Functions There is a simple workaround to this: make sure that such a BEGIN rule always ends with an exit statement. Rounding Numbers The way printf and sprintf() (see “Using printf Statements for Fancier Printing” on page 93) perform rounding often depends upon the system’s C sprintf() subroutine. On many machines, sprintf() rounding is unbiased, which means it doesn’t always round a trailing .5 up, contrary to naive expectations. In unbiased rounding, .5 rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4. This means that if you are using a format that does rounding (e.g., "%.0f"), you should check what your system does. The following function does traditional rounding; it might be useful if your awk’s printf does unbiased rounding: # round.awk --- do normal rounding function round(x, ival, aval, fraction) { ival = int(x) # integer part, int() truncates # see if fractional part if (ival == x) # no fraction return ival # ensure no decimals if (x < 0) { aval = -x # absolute value ival = int(aval) fraction = aval - ival if (fraction >= .5) return int(x) - 1 # -2.5 --> -3 else return int(x) # -2.3 --> -2 } else { fraction = x - ival if (fraction >= .5) return ival + 1 else return ival } } # test harness # { print $0, round($0) } The Cliff Random Number Generator The Cliff random number generator is a very simple random number generator that “passes the noise sphere test for randomness by showing no structure.” It is easily pro‐ grammed, in less than 10 lines of awk code: General Programming | 247 # cliff_rand.awk --- generate Cliff random numbers BEGIN { _cliff_seed = 0.1 } function cliff_rand() { _cliff_seed = (100 * log(_cliff_seed)) % 1 if (_cliff_seed < 0) _cliff_seed = - _cliff_seed return _cliff_seed } This algorithm requires an initial “seed” of 0.1. Each new value uses the current seed as input for the calculation. If the built-in rand() function (see “Numeric Functions” on page 192) isn’t random enough, you might try using this function instead. Translating Between Characters and Numbers One commercial implementation of awk supplies a built-in function, ord(), which takes a character and returns the numeric value for that character in the machine’s character set. If the string passed to ord() has more than one character, only the first one is used. The inverse of this function is chr() (from the function of the same name in Pascal), which takes a number and returns the corresponding character. Both functions are written very nicely in awk; there is no real reason to build them into the awk interpreter: # ord.awk --- do ord and chr # Global identifiers: # _ord_: numerical values indexed by characters # _ord_init: function to initialize _ord_ BEGIN { _ord_init() } function _ord_init( low, high, i, t) { low = sprintf("%c", 7) # BEL is ascii 7 if (low == "\a") { # regular ascii low = 0 high = 127 } else if (sprintf("%c", 128 + 7) == "\a") { # ascii, mark parity low = 128 high = 255 } else { # ebcdic(!) low = 0 high = 255 } for (i = low; i <= high; i++) { t = sprintf("%c", i) 248 | Chapter 10: A Library of awk Functions 5. This is changing; many systems use Unicode, a very large character set that includes ASCII as a subset. On systems with full Unicode support, a character can occupy up to 32 bits, making simple tests such as used here prohibitively expensive. 6. ASCII has been extended in many countries to use the values from 128 to 255 for country-specific characters. If your system uses these extensions, you can simplify _ord_init() to loop from 0 to 255. _ord_[t] = i } } Some explanation of the numbers used by _ord_init() is worthwhile. The most prom‐ inent character set in use today is ASCII.5 Although an 8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only defines characters that use the values from 0 to 127.6 In the now distant past, at least one minicomputer manufacturer used ASCII, but with mark parity, meaning that the leftmost bit in the byte is always 1. This means that on those systems, characters have numeric values from 128 to 255. Finally, large mainframe systems use the EBCDIC character set, which uses all 256 values. There are other character sets in use on some older systems, but they are not really worth worrying about: function ord(str, c) { # only first character is of interest c = substr(str, 1, 1) return _ord_[c] } function chr(c) { # force c to be numeric by adding 0 return sprintf("%c", c + 0) } #### test code #### # BEGIN { # for (;;) { # printf("enter a character: ") # if (getline var <= 0) # break # printf("ord(%s) = %d\n", var, ord(var)) # } # } General Programming | 249 7. It would be nice if awk had an assignment operator for concatenation. The lack of an explicit operator for concatenation makes string operations more difficult than they really need to be. An obvious improvement to these functions is to move the code for the _ord_init function into the body of the BEGIN rule. It was written this way initially for ease of development. There is a “test program” in a BEGIN rule, to test the function. It is com‐ mented out for production use. Merging an Array into a String When doing string processing, it is often useful to be able to join all the strings in an array into one long string. The following function, join(), accomplishes this task. It is used later in several of the application programs (see Chapter 11). Good function design is important; this function needs to be general, but it should also have a reasonable default behavior. It is called with an array as well as the beginning and ending indices of the elements in the array to be merged. This assumes that the array indices are numeric—a reasonable assumption, as the array was likely created with split() (see “String-Manipulation Functions” on page 194): # join.awk --- join an array into a string function join(array, start, end, sep, result, i) { if (sep == "") sep = " " else if (sep == SUBSEP) # magic value sep = "" result = array[start] for (i = start + 1; i <= end; i++) result = result sep array[i] return result } An optional additional argument is the separator to use when joining the strings back together. If the caller supplies a nonempty value, join() uses it; if it is not supplied, it has a null value. In this case, join() uses a single space as a default separator for the strings. If the value is equal to SUBSEP, then join() joins the strings with no separator between them. SUBSEP serves as a “magic” value to indicate that there should be no separation between the component strings.7 Managing the Time of Day The systime() and strftime() functions described in “Time Functions” on page 211 provide the minimum functionality necessary for dealing with the time of day in human- 250 | Chapter 10: A Library of awk Functions readable form. Although strftime() is extensive, the control formats are not neces‐ sarily easy to remember or intuitively obvious when reading a program. The following function, getlocaltime(), populates a user-supplied array with prefor‐ matted time information. It returns a string with the current time formatted in the same way as the date utility: # getlocaltime.awk --- get the time of day in a usable format # Returns a string in the format of output of date(1) # Populates the array argument time with individual values: # time["second"] -- seconds (0 - 59) # time["minute"] -- minutes (0 - 59) # time["hour"] -- hours (0 - 23) # time["althour"] -- hours (0 - 12) # time["monthday"] -- day of month (1 - 31) # time["month"] -- month of year (1 - 12) # time["monthname"] -- name of the month # time["shortmonth"] -- short name of the month # time["year"] -- year modulo 100 (0 - 99) # time["fullyear"] -- full year # time["weekday"] -- day of week (Sunday = 0) # time["altweekday"] -- day of week (Monday = 0) # time["dayname"] -- name of weekday # time["shortdayname"] -- short name of weekday # time["yearday"] -- day of year (0 - 365) # time["time zone"] -- abbreviation of time zone name # time["ampm"] -- AM or PM designation # time["weeknum"] -- week number, Sunday first day # time["altweeknum"] -- week number, Monday first day function getlocaltime(time, ret, now, i) { # get time once, avoids unnecessary system calls now = systime() # return date(1)-style output ret = strftime("%a %b %e %H:%M:%S %Z %Y", now) # clear out target array delete time # fill in values, force numeric values to be # numeric by adding 0 time["second"] = strftime("%S", now) + 0 time["minute"] = strftime("%M", now) + 0 time["hour"] = strftime("%H", now) + 0 time["althour"] = strftime("%I", now) + 0 time["monthday"] = strftime("%d", now) + 0 time["month"] = strftime("%m", now) + 0 time["monthname"] = strftime("%B", now) time["shortmonth"] = strftime("%b", now) General Programming | 251 time["year"] = strftime("%y", now) + 0 time["fullyear"] = strftime("%Y", now) + 0 time["weekday"] = strftime("%w", now) + 0 time["altweekday"] = strftime("%u", now) + 0 time["dayname"] = strftime("%A", now) time["shortdayname"] = strftime("%a", now) time["yearday"] = strftime("%j", now) + 0 time["timezone"] = strftime("%Z", now) time["ampm"] = strftime("%p", now) time["weeknum"] = strftime("%U", now) + 0 time["altweeknum"] = strftime("%W", now) + 0 return ret } The string indices are easier to use and read than the various formats required by strftime(). The alarm program presented in “An Alarm Clock Program” on page 300 uses this function. A more general design for the getlocaltime() function would have allowed the user to supply an optional timestamp value to use instead of the current time. Reading a Whole File at Once Often, it is convenient to have the entire contents of a file available in memory as a single string. A straightforward but naive way to do that might be as follows: function readfile(file, tmp, contents) { if ((getline tmp < file) < 0) return contents = tmp while (getline tmp < file) > 0) contents = contents RT tmp close(file) return contents } This function reads from file one record at a time, building up the full contents of the file in the local variable contents. It works, but is not necessarily efficient. The following function, based on a suggestion by Denis Shirokov, reads the entire con‐ tents of the named file in one shot: # readfile.awk --- read an entire file at once function readfile(file, tmp, save_rs) { save_rs = RS RS = "^$" 252 | Chapter 10: A Library of awk Functions getline tmp < file close(file) RS = save_rs return tmp } It works by setting RS to ‘^$’, a regular expression that will never match if the file has contents. gawk reads data from the file into tmp, attempting to match RS. The match fails after each read, but fails quickly, such that gawk fills tmp with the entire contents of the file. (See “How Input Is Split into Records” on page 55 for information on RT and RS.) In the case that file is empty, the return value is the null string. Thus, calling code may use something like: contents = readfile("/some/path") if (length(contents) == 0) # file was empty … This tests the result to see if it is empty or not. An equivalent test would be ‘contents == ""’. See “Reading an Entire File” on page 450 for an extension function that also reads an entire file into memory. Quoting Strings to Pass to the Shell Michael Brennan offers the following programming pattern, which he uses frequently: #! /bin/sh awkp=' … ' input_program | awk "$awkp" | /bin/sh For example, a program of his named flac-edit has this form: $ flac-edit -song="Whoope! That's Great" file.flac It generates the following output, which is to be piped to the shell (/bin/sh): chmod +w file.flac metaflac --remove-tag=TITLE file.flac LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great' file.flac chmod -w file.flac Note the need for shell quoting. The function shell_quote() does it. SINGLE is the one- character string "'" and QSINGLE is the three-character string "\"'\"": # shell_quote --- quote an argument for passing to the shell General Programming | 253 function shell_quote(s, # parameter SINGLE, QSINGLE, i, X, n, ret) # locals { if (s == "") return "\"\"" SINGLE = "\x27" # single quote QSINGLE = "\"\x27\"" n = split(s, X, SINGLE) ret = SINGLE X[1] SINGLE for (i = 2; i <= n; i++) ret = ret QSINGLE SINGLE X[i] SINGLE return ret } Datafile Management This section presents functions that are useful for managing command-line datafiles. Noting Datafile Boundaries The BEGIN and END rules are each executed exactly once, at the beginning and end of your awk program, respectively (see “The BEGIN and END Special Patterns” on page 145). We (the gawk authors) once had a user who mistakenly thought that the BEGIN rules were executed at the beginning of each datafile and the END rules were executed at the end of each datafile. When informed that this was not the case, the user requested that we add new special patterns to gawk, named BEGIN_FILE and END_FILE, that would have the desired be‐ havior. He even supplied us the code to do so. Adding these special patterns to gawk wasn’t necessary; the job can be done cleanly in awk itself, as illustrated by the following library program. It arranges to call two user- supplied functions, beginfile() and endfile(), at the beginning and end of each da‐ tafile. Besides solving the problem in only nine(!) lines of code, it does so portably; this works with any implementation of awk: # transfile.awk # # Give the user a hook for filename transitions # # The user must supply functions beginfile() and endfile() # that each take the name of the file being started or # finished, respectively. FILENAME != _oldfilename { 254 | Chapter 10: A Library of awk Functions if (_oldfilename != "") endfile(_oldfilename) _oldfilename = FILENAME beginfile(FILENAME) } END { endfile(FILENAME) } This file must be loaded before the user’s “main” program, so that the rule it supplies is executed first. This rule relies on awk’s FILENAME variable, which automatically changes for each new datafile. The current filename is saved in a private variable, _oldfilename. If FILE NAME does not equal _oldfilename, then a new datafile is being processed and it is necessary to call endfile() for the old file. Because endfile() should only be called if a file has been processed, the program first checks to make sure that _oldfilename is not the null string. The program then assigns the current filename to _oldfilename and calls beginfile() for the file. Because, like all awk variables, _oldfilename is initialized to the null string, this rule executes correctly even for the first datafile. The program also supplies an END rule to do the final processing for the last file. Because this END rule comes before any END rules supplied in the “main” program, endfile() is called first. Once again, the value of multiple BEGIN and END rules should be clear. If the same datafile occurs twice in a row on the command line, then endfile() and beginfile() are not executed at the end of the first pass and at the beginning of the second pass. The following version solves the problem: # ftrans.awk --- handle datafile transitions # # user supplies beginfile() and endfile() functions FNR == 1 { if (_filename_ != "") endfile(_filename_) _filename_ = FILENAME beginfile(FILENAME) } END { endfile(_filename_) } “Counting Things” on page 296 shows how this library function can be used and how it simplifies writing the main program. Datafile Management | 255 So Why Does gawk Have BEGINFILE and ENDFILE? You are probably wondering, if beginfile() and endfile() functions can do the job, why does gawk have BEGINFILE and ENDFILE patterns? Good question. Normally, if awk cannot open a file, this causes an immediate fatal error. In this case, there is no way for a user-defined function to deal with the problem, as the mechanism for calling it relies on the file being open and at the first record. Thus, the main reason for BEGINFILE is to give you a “hook” to catch files that cannot be processed. ENDFILE exists for symmetry, and because it provides an easy way to do per-file cleanup processing. For more information, refer to “The BEGINFILE and ENDFILE Special Patterns” on page 147. Rereading the Current File Another request for a new built-in function was for a function that would make it possible to reread the current file. The requesting user didn’t want to have to use get line (see “Explicit Input with getline” on page 77) inside a loop. However, as long as you are not in the END rule, it is quite easy to arrange to immediately close the current input file and then start over with it from the top. For lack of a better name, we’ll call the function rewind(): # rewind.awk --- rewind the current file and start over function rewind( i) { # shift remaining arguments up for (i = ARGC; i > ARGIND; i--) ARGV[i] = ARGV[i-1] # make sure gawk knows to keep going ARGC++ # make current file next to get done ARGV[ARGIND+1] = FILENAME # do it nextfile } The rewind() function relies on the ARGIND variable (see “Built-in Variables That Con‐ vey Information” on page 163), which is specific to gawk. It also relies on the next file keyword (see “The nextfile Statement” on page 158). Because of this, you should 256 | Chapter 10: A Library of awk Functions 8. The BEGINFILE special pattern (see “The BEGINFILE and ENDFILE Special Patterns” on page 147) provides an alternative mechanism for dealing with files that can’t be opened. However, the code here provides a portable solution. not call it from an ENDFILE rule. (This isn’t necessary anyway, because gawk goes to the next file as soon as an ENDFILE rule finishes!) Checking for Readable Datafiles Normally, if you give awk a datafile that isn’t readable, it stops with a fatal error. There are times when you might want to just ignore such files and keep going.8 You can do this by prepending the following program to your awk program: # readable.awk --- library file to skip over unreadable files BEGIN { for (i = 1; i < ARGC; i++) { if (ARGV[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/ \ || ARGV[i] == "-" || ARGV[i] == "/dev/stdin") continue # assignment or standard input else if ((getline junk < ARGV[i]) < 0) # unreadable delete ARGV[i] else close(ARGV[i]) } } This works, because the getline won’t be fatal. Removing the element from ARGV with delete skips the file (because it’s no longer in the list). See also “Using ARGC and ARGV” on page 170. Because awk variable names only allow the English letters, the regular expression check purposely does not use character classes such as ‘[:alpha:]’ and ‘[:alnum:]’ (see “Us‐ ing Bracket Expressions” on page 46). Checking for Zero-Length Files All known awk implementations silently skip over zero-length files. This is a by-product of awk’s implicit read-a-record-and-match-against-the-rules loop: when awk tries to read a record from an empty file, it immediately receives an end-of-file indication, closes the file, and proceeds on to the next command-line datafile, without executing any user- level awk program code. Using gawk’s ARGIND variable (see “Predefined Variables” on page 160), it is possible to detect when an empty datafile has been skipped. Similar to the library file presented in “Noting Datafile Boundaries” on page 254, the following library file calls a function Datafile Management | 257 named zerofile() that the user must provide. The arguments passed are the filename and the position in ARGV where it was found: # zerofile.awk --- library file to process empty input files BEGIN { Argind = 0 } ARGIND > Argind + 1 { for (Argind++; Argind < ARGIND; Argind++) zerofile(ARGV[Argind], Argind) } ARGIND != Argind { Argind = ARGIND } END { if (ARGIND > Argind) for (Argind++; Argind <= ARGIND; Argind++) zerofile(ARGV[Argind], Argind) } The user-level variable Argind allows the awk program to track its progress through ARGV. Whenever the program detects that ARGIND is greater than ‘Argind + 1’, it means that one or more empty files were skipped. The action then calls zerofile() for each such file, incrementing Argind along the way. The ‘Argind != ARGIND’ rule simply keeps Argind up to date in the normal case. Finally, the END rule catches the case of any empty files at the end of the command-line arguments. Note that the test in the condition of the for loop uses the ‘<=’ operator, not ‘<’. Treating Assignments as Filenames Occasionally, you might not want awk to process command-line variable assignments (see “Assigning variables on the command line” on page 116). In particular, if you have a filename that contains an ‘=’ character, awk treats the filename as an assignment and does not process it. Some users have suggested an additional command-line option for gawk to disable command-line assignments. However, some simple programming with a library file does the trick: # noassign.awk --- library file to avoid the need for a # special option that disables command-line assignments function disable_assigns(argc, argv, i) { for (i = 1; i < argc; i++) if (argv[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/) argv[i] = ("./" argv[i]) 258 | Chapter 10: A Library of awk Functions } BEGIN { if (No_command_assign) disable_assigns(ARGC, ARGV) } You then run your program this way: awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk * The function works by looping through the arguments. It prepends ‘./’ to any argument that matches the form of a variable assignment, turning that argument into a filename. The use of No_command_assign allows you to disable command-line assignments at invocation time, by giving the variable a true value. When not set, it is initially zero (i.e., false), so the command-line arguments are left alone. Processing Command-Line Options Most utilities on POSIX-compatible systems take options on the command line that can be used to change the way a program behaves. awk is an example of such a program (see “Command-Line Options” on page 22). Often, options take arguments (i.e., data that the program needs to correctly obey the command-line option). For example, ktawk’s -F option requires a string to use as the field separator. The first occurrence on the command line of either -- or a string that does not begin with ‘-’ ends the options. Modern Unix systems provide a C function named getopt() for processing command- line arguments. The programmer provides a string describing the one-letter options. If an option requires an argument, it is followed in the string with a colon. getopt() is also passed the count and values of the command-line arguments and is called in a loop. getopt() processes the command-line arguments for option letters. Each time around the loop, it returns a single character representing the next option letter that it finds, or ‘?’ if it finds an invalid option. When it returns −1, there are no options left on the command line. When using getopt(), options that do not take arguments can be grouped together. Furthermore, options that take arguments require that the argument be present. The argument can immediately follow the option letter, or it can be a separate command- line argument. Given a hypothetical program that takes three command-line options, -a, -b, and -c, where -b requires an argument, all of the following are valid ways of invoking the program: prog -a -b foo -c data1 data2 data3 prog -ac -bfoo -- data1 data2 data3 prog -acbfoo data1 data2 data3 Processing Command-Line Options | 259 Notice that when the argument is grouped with its option, the rest of the argument is considered to be the option’s argument. In this example, -acbfoo indicates that all of the -a, -b, and -c options were supplied, and that ‘foo’ is the argument to the -b option. getopt() provides four external variables that the programmer can use: optind The index in the argument value array (argv) where the first nonoption command- line argument can be found. optarg The string value of the argument to an option. opterr Usually getopt() prints an error message when it finds an invalid option. Setting opterr to zero disables this feature. (An application might want to print its own error message.) optopt The letter representing the command-line option. The following C fragment shows how getopt() might process command-line argu‐ ments for awk: int main(int argc, char *argv[]) { ... /* print our own message */ opterr = 0; while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) { switch (c) { case 'f': /* file */ ... break; case 'F': /* field separator */ ... break; case 'v': /* variable assignment */ ... break; case 'W': /* extension */ ... break; case '?': default: usage(); break; } } 260 | Chapter 10: A Library of awk Functions 9. This function was written before gawk acquired the ability to split strings into single characters using "" as the separator. We have left it alone, as using substr() is more portable. ... } As a side point, gawk actually uses the GNU getopt_long() function to process both normal and GNU-style long options (see “Command-Line Options” on page 22). The abstraction provided by getopt() is very useful and is quite handy in awk programs as well. Following is an awk version of getopt(). This function highlights one of the greatest weaknesses in awk, which is that it is very poor at manipulating single charac‐ ters. Repeated calls to substr() are necessary for accessing individual characters (see “String-Manipulation Functions” on page 194).9 The discussion that follows walks through the code a bit at a time: # getopt.awk --- Do C library getopt(3) function in awk # External variables: # Optind -- index in ARGV of first nonoption argument # Optarg -- string value of argument to current option # Opterr -- if nonzero, print our own diagnostic # Optopt -- current option letter # Returns: # -1 at end of options # "?" for unrecognized option # a character representing the current option # Private Data: # _opti -- index in multiflag option, e.g., -abc The function starts out with comments presenting a list of the global variables it uses, what the return values are, what they mean, and any global variables that are “private” to this library function. Such documentation is essential for any program, and partic‐ ularly for library functions. The getopt() function first checks that it was indeed called with a string of options (the options parameter). If options has a zero length, getopt() immediately returns −1: function getopt(argc, argv, options, thisopt, i) { if (length(options) == 0) # no options given return -1 if (argv[Optind] == "--") { # all done Optind++ _opti = 0 return -1 Processing Command-Line Options | 261 } else if (argv[Optind] !~ /^-[^:[:space:]]/) { _opti = 0 return -1 } The next thing to check for is the end of the options. A -- ends the command-line options, as does any command-line argument that does not begin with a ‘-’. Optind is used to step through the array of command-line arguments; it retains its value across calls to getopt(), because it is a global variable. The regular expression that is used, /^-[^:[:space:]/, checks for a ‘-’ followed by anything that is not whitespace and not a colon. If the current command-line argument does not match this pattern, it is not an option, and it ends option processing. Con‐ tinuing on: if (_opti == 0) _opti = 2 thisopt = substr(argv[Optind], _opti, 1) Optopt = thisopt i = index(options, thisopt) if (i == 0) { if (Opterr) printf("%c -- invalid option\n", thisopt) > "/dev/stderr" if (_opti >= length(argv[Optind])) { Optind++ _opti = 0 } else _opti++ return "?" } The _opti variable tracks the position in the current command-line argument (argv[Op tind]). If multiple options are grouped together with one ‘-’ (e.g., -abx), it is necessary to return them to the user one at a time. If _opti is equal to zero, it is set to two, which is the index in the string of the next character to look at (we skip the ‘-’, which is at position one). The variable thisopt holds the character, obtained with substr(). It is saved in Optopt for the main program to use. If thisopt is not in the options string, then it is an invalid option. If Opterr is nonzero, getopt() prints an error message on the standard error that is similar to the message from the C version of getopt(). Because the option is invalid, it is necessary to skip it and move on to the next option character. If _opti is greater than or equal to the length of the current command-line argument, it is necessary to move on to the next argument, so Optind is incremented and _opti is reset to zero. Otherwise, Optind is left alone and _opti is merely incremented. 262 | Chapter 10: A Library of awk Functions In any case, because the option is invalid, getopt() returns "?". The main program can examine Optopt if it needs to know what the invalid option letter actually is. Continuing on: if (substr(options, i + 1, 1) == ":") { # get option argument if (length(substr(argv[Optind], _opti + 1)) > 0) Optarg = substr(argv[Optind], _opti + 1) else Optarg = argv[++Optind] _opti = 0 } else Optarg = "" If the option requires an argument, the option letter is followed by a colon in the options string. If there are remaining characters in the current command-line argument (argv[Optind]), then the rest of that string is assigned to Optarg. Otherwise, the next command-line argument is used (‘-xFOO’ versus ‘-x FOO’). In either case, _opti is reset to zero, because there are no more characters left to examine in the current command- line argument. Continuing: if (_opti == 0 || _opti >= length(argv[Optind])) { Optind++ _opti = 0 } else _opti++ return thisopt } Finally, if _opti is either zero or greater than the length of the current command-line argument, it means this element in argv is through being processed, so Optind is in‐ cremented to point to the next element in argv. If neither condition is true, then only _opti is incremented, so that the next option letter can be processed on the next call to getopt(). The BEGIN rule initializes both Opterr and Optind to one. Opterr is set to one, because the default behavior is for getopt() to print a diagnostic message upon seeing an invalid option. Optind is set to one, because there’s no reason to look at the program name, which is in ARGV[0]: BEGIN { Opterr = 1 # default is to diagnose Optind = 1 # skip ARGV[0] # test program if (_getopt_test) { while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1) printf("c = <%c>, Optarg = <%s>\n", _go_c, Optarg) printf("non-option arguments:\n") Processing Command-Line Options | 263 for (; Optind < ARGC; Optind++) printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind]) } } The rest of the BEGIN rule is a simple test program. Here are the results of two sample runs of the test program: $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x c = , Optarg = <> c = , Optarg = <> c = , Optarg = non-option arguments: ARGV[3] = ARGV[4] = <-x> $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc c = , Optarg = <> error→ x -- invalid option c = , Optarg = <> non-option arguments: ARGV[4] = ARGV[5] = In both runs, the first -- terminates the arguments to awk, so that it does not try to interpret the -a, etc., as its own options. After getopt() is through, user-level code must clear out all the ele‐ ments of ARGV from 1 to Optind, so that awk does not try to process the command-line options as filenames. Using ‘#!’ with the -E option may help avoid conflicts between your program’s options and gawk’s options, as -E causes gawk to abandon processing of further options (see “Executable awk Programs” on page 6 and “Command-Line Options” on page 22). Several of the sample programs presented in Chapter 11 use getopt() to process their arguments. Reading the User Database The PROCINFO array (see “Predefined Variables” on page 160) provides access to the current user’s real and effective user and group ID numbers, and, if available, the user’s supplementary group set. However, because these are numbers, they do not provide very useful information to the average user. There needs to be some way to find the user information associated with the user and group ID numbers. This section presents a 264 | Chapter 10: A Library of awk Functions 10. It is often the case that password information is stored in a network database. suite of functions for retrieving information from the user database. See “Reading the Group Database” on page 269 for a similar suite that retrieves information from the group database. The POSIX standard does not define the file where user information is kept. Instead, it provides the header file and several C language subroutines for obtaining user information. The primary function is getpwent(), for “get password entry.” The “pass‐ word” comes from the original user database file, /etc/passwd, which stores user in‐ formation along with the encrypted passwords (hence the name). Although an awk program could simply read /etc/passwd directly, this file may not contain complete information about the system’s set of users.10 To be sure you are able to produce a readable and complete version of the user database, it is necessary to write a small C program that calls getpwent(). getpwent() is defined as returning a pointer to a struct passwd. Each time it is called, it returns the next entry in the database. When there are no more entries, it returns NULL, the null pointer. When this happens, the C program should call endpwent() to close the database. Following is pwcat, a C program that “cats” the password database: /* * pwcat.c * * Generate a printable version of the password database. */ #include #include int main(int argc, char **argv) { struct passwd *p; while ((p = getpwent()) != NULL) printf("%s:%s:%ld:%ld:%s:%s:%s\n", p->pw_name, p->pw_passwd, (long) p->pw_uid, (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell); endpwent(); return 0; } If you don’t understand C, don’t worry about it. The output from pwcat is the user database, in the traditional /etc/passwd format of colon-separated fields. The fields are: Reading the User Database | 265 Login name The user’s login name. Encrypted password The user’s encrypted password. This may not be available on some systems. User-ID The user’s numeric user ID number. (On some systems, it’s a C long, and not an int. Thus, we cast it to long for all cases.) Group-ID The user’s numeric group ID number. (Similar comments about long versus int apply here.) Full name The user’s full name, and perhaps other information associated with the user. Home directory The user’s login (or “home”) directory (familiar to shell programmers as $HOME). Login shell The program that is run when the user logs in. This is usually a shell, such as Bash. A few lines representative of pwcat’s output are as follows: $ pwcat root:x:0:1:Operator:/:/bin/sh nobody:*:65534:65534::/: daemon:*:1:1::/: sys:*:2:2::/:/bin/csh bin:*:3:3::/bin: arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh ... With that introduction, following is a group of functions for getting user information. There are several functions here, corresponding to the C functions of the same names: # passwd.awk --- access password file information BEGIN { # tailor this to suit your system _pw_awklib = "/usr/local/libexec/awk/" } function _pw_init( oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat) { if (_pw_inited) return 266 | Chapter 10: A Library of awk Functions oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") using_fpat = (PROCINFO["FS"] == "FPAT") FS = ":" RS = "\n" pwcat = _pw_awklib "pwcat" while ((pwcat | getline) > 0) { _pw_byname[$1] = $0 _pw_byuid[$3] = $0 _pw_bycount[++_pw_total] = $0 } close(pwcat) _pw_count = 0 _pw_inited = 1 FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS else if (using_fpat) FPAT = FPAT RS = oldrs $0 = olddol0 } The BEGIN rule sets a private variable to the directory where pwcat is stored. Because it is used to help out an awk library routine, we have chosen to put it in /usr/local/ libexec/awk; however, you might want it to be in a different directory on your system. The function _pw_init() fills three copies of the user information into three associative arrays. The arrays are indexed by username (_pw_byname), by user ID number (_pw_byuid), and by order of occurrence (_pw_bycount). The variable _pw_inited is used for efficiency, as _pw_init() needs to be called only once. Because this function uses getline to read information from pwcat, it first saves the values of FS, RS, and $0. It notes in the variable using_fw whether field splitting with FIELDWIDTHS is in effect or not. Doing so is necessary, as these functions could be called from anywhere within a user’s program, and the user may have his or her own way of splitting records and fields. This makes it possible to restore the correct field-splitting mechanism later. The test can only be true for gawk. It is false if using FS or FPAT, or on some other awk implementation. The code that checks for using FPAT, using using_fpat and PROCINFO["FS"], is similar. The main part of the function uses a loop to read database lines, split the lines into fields, and then store the lines into each array as necessary. When the loop is done, _pw_init() cleans up by closing the pipeline, setting _pw_inited to one, and restoring FS (and FIELDWIDTHS or FPAT if necessary), RS, and $0. The use of _pw_count is explained shortly. Reading the User Database | 267 The getpwnam() function takes a username as a string argument. If that user is in the database, it returns the appropriate line. Otherwise, it relies on the array reference to a nonexistent element to create the element with the null string as its value: function getpwnam(name) { _pw_init() return _pw_byname[name] } Similarly, the getpwuid() function takes a user ID number argument. If that user num‐ ber is in the database, it returns the appropriate line. Otherwise, it returns the null string: function getpwuid(uid) { _pw_init() return _pw_byuid[uid] } The getpwent() function simply steps through the database, one entry at a time. It uses _pw_count to track its current position in the _pw_bycount array: function getpwent() { _pw_init() if (_pw_count < _pw_total) return _pw_bycount[++_pw_count] return "" } The endpwent() function resets _pw_count to zero, so that subsequent calls to getpwent() start over again: function endpwent() { _pw_count = 0 } A conscious design decision in this suite is that each subroutine calls _pw_init() to initialize the database arrays. The overhead of running a separate process to generate the user database, and the I/O to scan it, are only incurred if the user’s main program actually calls one of these functions. If this library file is loaded along with a user’s program, but none of the routines are ever called, then there is no extra runtime over‐ head. (The alternative is move the body of _pw_init() into a BEGIN rule, which always runs pwcat. This simplifies the code but runs an extra process that may never be needed.) In turn, calling _pw_init() is not too expensive, because the _pw_inited variable keeps the program from reading the data more than once. If you are worried about squeezing every last cycle out of your awk program, the check of _pw_inited could be moved out 268 | Chapter 10: A Library of awk Functions of _pw_init() and duplicated in all the other functions. In practice, this is not necessary, as most awk programs are I/O-bound, and such a change would clutter up the code. The id program in “Printing Out User Information” on page 287 uses these functions. Reading the Group Database Much of the discussion presented in “Reading the User Database” on page 264 applies to the group database as well. Although there has traditionally been a well-known file (/etc/group) in a well-known format, the POSIX standard only provides a set of C library routines ( and getgrent()) for accessing the information. Even though this file may exist, it may not have complete information. Therefore, as with the user database, it is necessary to have a small C program that generates the group database as its output. grcat, a C program that “cats” the group database, is as follows: /* * grcat.c * * Generate a printable version of the group database. */ #include #include int main(int argc, char **argv) { struct group *g; int i; while ((g = getgrent()) != NULL) { printf("%s:%s:%ld:", g->gr_name, g->gr_passwd, (long) g->gr_gid); for (i = 0; g->gr_mem[i] != NULL; i++) { printf("%s", g->gr_mem[i]); if (g->gr_mem[i+1] != NULL) putchar(','); } putchar('\n'); } endgrent(); return 0; } Each line in the group database represents one group. The fields are separated with colons and represent the following information: Group Name The group’s name. Reading the Group Database | 269 Group Password The group’s encrypted password. In practice, this field is never used; it is usually empty or set to ‘*’. Group ID Number The group’s numeric group ID number; the association of name to number must be unique within the file. (On some systems it’s a C long, and not an int. Thus, we cast it to long for all cases.) Group Member List A comma-separated list of usernames. These users are members of the group. Modern Unix systems allow users to be members of several groups simultaneously. If your system does, then there are elements "group1" through "groupN" in PRO CINFO for those group ID numbers. (Note that PROCINFO is a gawk extension; see “Predefined Variables” on page 160.) Here is what running grcat might produce: $ grcat wheel:*:0:arnold nogroup:*:65534: daemon:*:1: kmem:*:2: staff:*:10:arnold,miriam,andy other:*:20: … Here are the functions for obtaining information from the group database. There are several, modeled after the C library functions of the same names: # group.awk --- functions for dealing with the group file BEGIN { # Change to suit your system _gr_awklib = "/usr/local/libexec/awk/" } function _gr_init( oldfs, oldrs, olddol0, grcat, using_fw, using_fpat, n, a, i) { if (_gr_inited) return oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") using_fpat = (PROCINFO["FS"] == "FPAT") FS = ":" RS = "\n" 270 | Chapter 10: A Library of awk Functions grcat = _gr_awklib "grcat" while ((grcat | getline) > 0) { if ($1 in _gr_byname) _gr_byname[$1] = _gr_byname[$1] "," $4 else _gr_byname[$1] = $0 if ($3 in _gr_bygid) _gr_bygid[$3] = _gr_bygid[$3] "," $4 else _gr_bygid[$3] = $0 n = split($4, a, "[ \t]*,[ \t]*") for (i = 1; i <= n; i++) if (a[i] in _gr_groupsbyuser) _gr_groupsbyuser[a[i]] = gr_groupsbyuser[a[i]] " " $1 else _gr_groupsbyuser[a[i]] = $1 _gr_bycount[++_gr_count] = $0 } close(grcat) _gr_count = 0 _gr_inited++ FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS else if (using_fpat) FPAT = FPAT RS = oldrs $0 = olddol0 } The BEGIN rule sets a private variable to the directory where grcat is stored. Because it is used to help out an awk library routine, we have chosen to put it in /usr/local/ libexec/awk. You might want it to be in a different directory on your system. These routines follow the same general outline as the user database routines (see “Reading the User Database” on page 264). The _gr_inited variable is used to ensure that the database is scanned no more than once. The _gr_init() function first saves FS, RS, and $0, and then sets FS and RS to the correct values for scanning the group information. It also takes care to note whether FIELDWIDTHS or FPAT is being used, and to restore the appropriate field-splitting mechanism. The group information is stored in several associative arrays. The arrays are indexed by group name (_gr_byname), by group ID number (_gr_bygid), and by position in the database (_gr_bycount). There is an additional array indexed by username (_gr_groupsbyuser), which is a space-separated list of groups to which each user belongs. Reading the Group Database | 271 11. There is a subtle problem with the code just presented. Suppose that the first time there were no names. This code adds the names with a leading comma. It also doesn’t check that there is a $4. Unlike in the user database, it is possible to have multiple records in the database for the same group. This is common when a group has a large number of members. A pair of such entries might look like the following: tvpeople:*:101:johnny,jay,arsenio tvpeople:*:101:david,conan,tom,joan For this reason, _gr_init() looks to see if a group name or group ID number is already seen. If so, the usernames are simply concatenated onto the previous list of users.11 Finally, _gr_init() closes the pipeline to grcat, restores FS (and FIELDWIDTHS or FPAT, if necessary), RS, and $0, initializes _gr_count to zero (it is used later), and makes _gr_inited nonzero. The getgrnam() function takes a group name as its argument, and if that group exists, it is returned. Otherwise, it relies on the array reference to a nonexistent element to create the element with the null string as its value: function getgrnam(group) { _gr_init() return _gr_byname[group] } The getgrgid() function is similar; it takes a numeric group ID and looks up the in‐ formation associated with that group ID: function getgrgid(gid) { _gr_init() return _gr_bygid[gid] } The getgruser() function does not have a C counterpart. It takes a username and returns the list of groups that have the user as a member: function getgruser(user) { _gr_init() return _gr_groupsbyuser[user] } The getgrent() function steps through the database one entry at a time. It uses _gr_count to track its position in the list: function getgrent() { _gr_init() 272 | Chapter 10: A Library of awk Functions if (++_gr_count in _gr_bycount) return _gr_bycount[_gr_count] return "" } The endgrent() function resets _gr_count to zero so that getgrent() can start over again: function endgrent() { _gr_count = 0 } As with the user database routines, each function calls _gr_init() to initialize the arrays. Doing so only incurs the extra overhead of running grcat if these functions are used (as opposed to moving the body of _gr_init() into a BEGIN rule). Most of the work is in scanning the database and building the various associative arrays. The functions that the user calls are themselves very simple, relying on awk’s associative arrays to do work. The id program in “Printing Out User Information” on page 287 uses these functions. Traversing Arrays of Arrays “Arrays of Arrays” on page 187 described how gawk provides arrays of arrays. In particular, any element of an array may be either a scalar or another array. The isar ray() function (see “Getting Type Information” on page 219) lets you distinguish an array from a scalar. The following function, walk_array(), recursively traverses an array, printing the element indices and values. You call it with the array and a string representing the name of the array: function walk_array(arr, name, i) { for (i in arr) { if (isarray(arr[i])) walk_array(arr[i], (name "[" i "]")) else printf("%s[%s] = %s\n", name, i, arr[i]) } } It works by looping over each element of the array. If any given element is itself an array, the function calls itself recursively, passing the subarray and a new string representing the current index. Otherwise, the function simply prints the element’s name, index, and value. Here is a main program to demonstrate: BEGIN { a[1] = 1 a[2][1] = 21 Traversing Arrays of Arrays | 273 a[2][2] = 22 a[3] = 3 a[4][1][1] = 411 a[4][2] = 42 walk_array(a, "a") } When run, the program produces the following output: $ gawk -f walk_array.awk a[1] = 1 a[2][1] = 21 a[2][2] = 22 a[3] = 3 a[4][1][1] = 411 a[4][2] = 42 The function just presented simply prints the name and value of each scalar array ele‐ ment. However, it is easy to generalize it, by passing in the name of a function to call when walking an array. The modified function looks like this: function process_array(arr, name, process, do_arrays, i, new_name) { for (i in arr) { new_name = (name "[" i "]") if (isarray(arr[i])) { if (do_arrays) @process(new_name, arr[i]) process_array(arr[i], new_name, process, do_arrays) } else @process(new_name, arr[i]) } } The arguments are as follows: arr The array. name The name of the array (a string). process The name of the function to call. do_arrays If this is true, the function can handle elements that are subarrays. If subarrays are to be processed, that is done before walking them further. When run with the following scaffolding, the function produces the same results as does the earlier version of walk_array(): 274 | Chapter 10: A Library of awk Functions BEGIN { a[1] = 1 a[2][1] = 21 a[2][2] = 22 a[3] = 3 a[4][1][1] = 411 a[4][2] = 42 process_array(a, "a", "do_print", 0) } function do_print(name, element) { printf "%s = %s\n", name, element } Summary • Reading programs is an excellent way to learn Good Programming. The functions and programs provided in this chapter and the next are intended to serve that purpose. • When writing general-purpose library functions, put some thought into how to name any global variables so that they won’t conflict with variables from a user’s program. • The functions presented here fit into the following categories: General problems Number-to-string conversion, testing assertions, rounding, random number generation, converting characters to numbers, joining strings, getting easily usable time-of-day information, and reading a whole file in one shot Managing datafiles Noting datafile boundaries, rereading the current file, checking for readable files, checking for zero-length files, and treating assignments as filenames Processing command-line options An awk version of the standard C getopt() function Reading the user and group databases Two sets of routines that parallel the C library versions Traversing arrays of arrays Tow function that traverse an array of arrays to any depth Summary | 275 CHAPTER 11 Practical awk Programs Chapter 10, A Library of awk Functions, presents the idea that reading programs in a language contributes to learning that language. This chapter continues that theme, pre‐ senting a potpourri of awk programs for your reading enjoyment. There are three sec‐ tions. The first describes how to run the programs presented in this chapter. The second presents awk versions of several common POSIX utilities. These are pro‐ grams that you are hopefully already familiar with, and therefore whose problems are understood. By reimplementing these programs in awk, you can focus on the awk-related aspects of solving the programming problems. The third is a grab bag of interesting programs. These solve a number of different data- manipulation and management problems. Many of the programs are short, which em‐ phasizes awk’s ability to do a lot in just a few lines of code. Many of these programs use library functions presented in Chapter 10. Running the Example Programs To run a given program, you would typically do something like this: awk -f program -- options files Here, program is the name of the awk program (such as cut.awk), the options are any command-line options for the program that start with a ‘-’, and the files are the actual datafiles. If your system supports the ‘#!’ executable interpreter mechanism (see “Executable awk Programs” on page 6), you can instead run your program directly: cut.awk -c1-8 myfiles > results If your awk is not gawk, you may instead need to use this: 277 cut.awk -- -c1-8 myfiles > results Reinventing Wheels for Fun and Profit This section presents a number of POSIX utilities implemented in awk. Reinventing these programs in awk is often enjoyable, because the algorithms can be very clearly expressed, and the code is usually very concise and simple. This is true because awk does so much for you. It should be noted that these programs are not necessarily intended to replace the in‐ stalled versions on your system. Nor may all of these programs be fully compliant with the most recent POSIX standard. This is not a problem; their purpose is to illustrate awk language programming for “real-world” tasks. The programs are presented in alphabetical order. Cutting Out Fields and Columns The cut utility selects, or “cuts,” characters or fields from its standard input and sends them to its standard output. Fields are separated by TABs by default, but you may supply a command-line option to change the field delimiter (i.e., the field-separator character). cut’s definition of fields is less general than awk’s. A common use of cut might be to pull out just the login names of logged-on users from the output of who. For example, the following pipeline generates a sorted, unique list of the logged-on users: who | cut -c1-8 | sort | uniq The options for cut are: -c list Use list as the list of characters to cut out. Items within the list may be separated by commas, and ranges of characters can be separated with dashes. The list ‘1-8,15,22-35’ specifies characters 1 through 8, 15, and 22 through 35. -f list Use list as the list of fields to cut out. -d delim Use delim as the field-separator character instead of the TAB character. -s Suppress printing of lines that do not contain the field delimiter. 278 | Chapter 11: Practical awk Programs The awk implementation of cut uses the getopt() library function (see “Processing Command-Line Options” on page 259) and the join() library function (see “Merging an Array into a String” on page 250). The program begins with a comment describing the options, the library functions needed, and a usage() function that prints out a usage message and exits. usage() is called if invalid arguments are supplied: # cut.awk --- implement cut in awk # Options: # -f list Cut fields # -d c Field delimiter character # -c list Cut characters # # -s Suppress lines without the delimiter # # Requires getopt() and join() library functions function usage() { print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr" print("usage: cut [-c list] [files...]") > "/dev/stderr" exit 1 } Next comes a BEGIN rule that parses the command-line options. It sets FS to a single TAB character, because that is cut’s default field separator. The rule then sets the output field separator to be the same as the input field separator. A loop using getopt() steps through the command-line options. Exactly one of the variables by_fields or by_chars is set to true, to indicate that processing should be done by fields or by characters, respectively. When cutting by characters, the output field separator is set to the null string: BEGIN { FS = "\t" # default OFS = FS while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) { if (c == "f") { by_fields = 1 fieldlist = Optarg } else if (c == "c") { by_chars = 1 fieldlist = Optarg OFS = "" } else if (c == "d") { if (length(Optarg) > 1) { printf("cut: using first character of %s" \ " for delimiter\n", Optarg) > "/dev/stderr" Optarg = substr(Optarg, 1, 1) Reinventing Wheels for Fun and Profit | 279 } FS = Optarg OFS = FS if (FS == " ") # defeat awk semantics FS = "[ ]" } else if (c == "s") suppress = 1 else usage() } # Clear out options for (i = 1; i < Optind; i++) ARGV[i] = "" The code must take special care when the field delimiter is a space. Using a single space (" ") for the value of FS is incorrect—awk would separate fields with runs of spaces, TABs, and/or newlines, and we want them to be separated with individual spaces. Also remember that after getopt() is through (as described in “Processing Command-Line Options” on page 259), we have to clear out all the elements of ARGV from 1 to Optind, so that awk does not try to process the command-line options as filenames. After dealing with the command-line options, the program verifies that the options make sense. Only one or the other of -c and -f should be used, and both require a field list. Then the program calls either set_fieldlist() or set_charlist() to pull apart the list of fields or characters: if (by_fields && by_chars) usage() if (by_fields == 0 && by_chars == 0) by_fields = 1 # default if (fieldlist == "") { print "cut: needs list for -c or -f" > "/dev/stderr" exit 1 } if (by_fields) set_fieldlist() else set_charlist() } set_fieldlist() splits the field list apart at the commas into an array. Then, for each element of the array, it looks to see if the element is actually a range, and if so, splits it apart. The function checks the range to make sure that the first number is smaller than the second. Each number in the list is added to the flist array, which simply lists the fields that will be printed. Normal field splitting is used. The program lets awk handle the job of doing the field splitting: 280 | Chapter 11: Practical awk Programs function set_fieldlist( n, m, i, j, k, f, g) { n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) { if (index(f[i], "-") != 0) { # a range m = split(f[i], g, "-") if (m != 2 || g[1] >= g[2]) { printf("cut: bad field list: %s\n", f[i]) > "/dev/stderr" exit 1 } for (k = g[1]; k <= g[2]; k++) flist[j++] = k } else flist[j++] = f[i] } nfields = j - 1 } The set_charlist() function is more complicated than set_fieldlist(). The idea here is to use gawk’s FIELDWIDTHS variable (see “Reading Fixed-Width Data” on page 71), which describes constant-width input. When using a character list, that is exactly what we have. Setting up FIELDWIDTHS is more complicated than simply listing the fields that need to be printed. We have to keep track of the fields to print and also the intervening characters that have to be skipped. For example, suppose you wanted characters 1 through 8, 15, and 22 through 35. You would use ‘-c 1-8,15,22-35’. The necessary value for FIELD WIDTHS is "8 6 1 6 14". This yields five fields, and the fields to print are $1, $3, and $5. The intermediate fields are filler, which is stuff in between the desired data. flist lists the fields to print, and t tracks the complete field list, including filler fields: function set_charlist( field, i, j, f, g, n, m, t, filler, last, len) { field = 1 # count total fields n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) { if (index(f[i], "-") != 0) { # range m = split(f[i], g, "-") if (m != 2 || g[1] >= g[2]) { printf("cut: bad character list: %s\n", f[i]) > "/dev/stderr" exit 1 } len = g[2] - g[1] + 1 if (g[1] > 1) # compute length of filler filler = g[1] - last - 1 else Reinventing Wheels for Fun and Profit | 281 filler = 0 if (filler) t[field++] = filler t[field++] = len # length of field last = g[2] flist[j++] = field - 1 } else { if (f[i] > 1) filler = f[i] - last - 1 else filler = 0 if (filler) t[field++] = filler t[field++] = 1 last = f[i] flist[j++] = field - 1 } } FIELDWIDTHS = join(t, 1, field - 1) nfields = j - 1 } Next is the rule that processes the data. If the -s option is given, then suppress is true. The first if statement makes sure that the input record does have the field separator. If cut is processing fields, suppress is true, and the field separator character is not in the record, then the record is skipped. If the record is valid, then gawk has split the data into fields, either using the character in FS or using fixed-length fields and FIELDWIDTHS. The loop goes through the list of fields that should be printed. The corresponding field is printed if it contains data. If the next field also has data, then the separator character is written out between the fields: { if (by_fields && suppress && index($0, FS) == 0) next for (i = 1; i <= nfields; i++) { if ($flist[i] != "") { printf "%s", $flist[i] if (i < nfields && $flist[i+1] != "") printf "%s", OFS } } print "" } This version of cut relies on gawk’s FIELDWIDTHS variable to do the character-based cutting. It is possible in other awk implementations to use substr() (see “String- Manipulation Functions” on page 194), but it is also extremely painful. The 282 | Chapter 11: Practical awk Programs FIELDWIDTHS variable supplies an elegant solution to the problem of picking the input line apart by characters. Searching for Regular Expressions in Files The egrep utility searches files for patterns. It uses regular expressions that are almost identical to those available in awk (see Chapter 3). You invoke it as follows: egrep [options] 'pattern' files … The pattern is a regular expression. In typical usage, the regular expression is quoted to prevent the shell from expanding any of the special characters as filename wildcards. Normally, egrep prints the lines that matched. If multiple filenames are provided on the command line, each output line is preceded by the name of the file and a colon. The options to egrep are as follows: -c Print out a count of the lines that matched the pattern, instead of the lines them‐ selves. -s Be silent. No output is produced and the exit value indicates whether the pattern was matched. -v Invert the sense of the test. egrep prints the lines that do not match the pattern and exits successfully if the pattern is not matched. -i Ignore case distinctions in both the pattern and the input data. -l Only print (list) the names of the files that matched, not the lines that matched. -e pattern Use pattern as the regexp to match. The purpose of the -e option is to allow patterns that start with a ‘-’. This version uses the getopt() library function (see “Processing Command-Line Op‐ tions” on page 259) and the file transition library program (see “Noting Datafile Bound‐ aries” on page 254). The program begins with a descriptive comment and then a BEGIN rule that processes the command-line arguments with getopt(). The -i (ignore case) option is particularly easy with gawk; we just use the IGNORECASE predefined variable (see “Predefined Vari‐ ables” on page 160): Reinventing Wheels for Fun and Profit | 283 # egrep.awk --- simulate egrep in awk # # Options: # -c count of lines # -s silent - use exit value # -v invert test, success if no match # -i ignore case # -l print filenames only # -e argument is pattern # # Requires getopt and file transition library functions BEGIN { while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) { if (c == "c") count_only++ else if (c == "s") no_print++ else if (c == "v") invert++ else if (c == "i") IGNORECASE = 1 else if (c == "l") filenames_only++ else if (c == "e") pattern = Optarg else usage() } Next comes the code that handles the egrep-specific behavior. If no pattern is supplied with -e, the first nonoption on the command line is used. The awk command-line arguments up to ARGV[Optind] are cleared, so that awk won’t try to process them as files. If no files are specified, the standard input is used, and if multiple files are specified, we make sure to note this so that the filenames can precede the matched lines in the output: if (pattern == "") pattern = ARGV[Optind++] for (i = 1; i < Optind; i++) ARGV[i] = "" if (Optind >= ARGC) { ARGV[1] = "-" ARGC = 2 } else if (ARGC - Optind > 1) do_filenames++ # if (IGNORECASE) # pattern = tolower(pattern) } 284 | Chapter 11: Practical awk Programs 1. It also introduces a subtle bug; if a match happens, we output the translated line, not the original. The last two lines are commented out, as they are not needed in gawk. They should be uncommented if you have to use another version of awk. The next set of lines should be uncommented if you are not using gawk. This rule translates all the characters in the input line into lowercase if the -i option is specified.1 The rule is commented out as it is not necessary with gawk: #{ # if (IGNORECASE) # $0 = tolower($0) #} The beginfile() function is called by the rule in ftrans.awk when each new file is processed. In this case, it is very simple; all it does is initialize a variable fcount to zero. fcount tracks how many lines in the current file matched the pattern. Naming the parameter junk shows we know that beginfile() is called with a parameter, but that we’re not interested in its value: function beginfile(junk) { fcount = 0 } The endfile() function is called after each file has been processed. It affects the output only when the user wants a count of the number of lines that matched. no_print is true only if the exit status is desired. count_only is true if line counts are desired. egrep therefore only prints line counts if printing and counting are enabled. The output format must be adjusted depending upon the number of files to process. Finally, fcount is added to total, so that we know the total number of lines that matched the pattern: function endfile(file) { if (! no_print && count_only) { if (do_filenames) print file ":" fcount else print fcount } total += fcount } The BEGINFILE and ENDFILE special patterns (see “The BEGINFILE and ENDFILE Special Patterns” on page 147) could be used, but then the program would be gawk- specific. Additionally, this example was written before gawk acquired BEGINFILE and ENDFILE. Reinventing Wheels for Fun and Profit | 285 The following rule does most of the work of matching lines. The variable matches is true if the line matched the pattern. If the user wants lines that did not match, the sense of matches is inverted using the ‘!’ operator. fcount is incremented with the value of matches, which is either one or zero, depending upon a successful or unsuccessful match. If the line does not match, the next statement just moves on to the next record. A number of additional tests are made, but they are only done if we are not counting lines. First, if the user only wants the exit status (no_print is true), then it is enough to know that one line in this file matched, and we can skip on to the next file with next file. Similarly, if we are only printing filenames, we can print the filename, and then skip to the next file with nextfile. Finally, each line is printed, with a leading filename and colon if necessary: { matches = ($0 ~ pattern) if (invert) matches = ! matches fcount += matches # 1 or 0 if (! matches) next if (! count_only) { if (no_print) nextfile if (filenames_only) { print FILENAME nextfile } if (do_filenames) print FILENAME ":" $0 else print } } The END rule takes care of producing the correct exit status. If there are no matches, the exit status is one; otherwise, it is zero: END { exit (total == 0) } The usage() function prints a usage message in case of invalid options, and then exits: function usage() { print("Usage: egrep [-csvil] [-e pat] [files ...]") > "/dev/stderr" 286 | Chapter 11: Practical awk Programs print("\n\tegrep [-csvil] pat [files ...]") > "/dev/stderr" exit 1 } Printing Out User Information The id utility lists a user’s real and effective user ID numbers, real and effective group ID numbers, and the user’s group set, if any. id only prints the effective user ID and group ID if they are different from the real ones. If possible, id also supplies the cor‐ responding user and group names. The output might look like this: $ id uid=1000(arnold) gid=1000(arnold) groups=1000(arnold),4(adm),7(lp),27(sudo) This information is part of what is provided by gawk’s PROCINFO array (see “Predefined Variables” on page 160). However, the id utility provides a more palatable output than just individual numbers. Here is a simple version of id written in awk. It uses the user database library functions (see “Reading the User Database” on page 264) and the group database library functions (see “Reading the Group Database” on page 269) from Chapter 10. The program is fairly straightforward. All the work is done in the BEGIN rule. The user and group ID numbers are obtained from PROCINFO. The code is repetitive. The entry in the user database for the real user ID number is split into parts at the ‘:’. The name is the first field. Similar code is used for the effective user ID number and the group numbers: # id.awk --- implement id in awk # # Requires user and group library functions # output is: # uid=12(foo) euid=34(bar) gid=3(baz) \ # egid=5(blat) groups=9(nine),2(two),1(one) BEGIN { uid = PROCINFO["uid"] euid = PROCINFO["euid"] gid = PROCINFO["gid"] egid = PROCINFO["egid"] printf("uid=%d", uid) pw = getpwuid(uid) pr_first_field(pw) if (euid != uid) { printf(" euid=%d", euid) pw = getpwuid(euid) pr_first_field(pw) } Reinventing Wheels for Fun and Profit | 287 printf(" gid=%d", gid) pw = getgrgid(gid) pr_first_field(pw) if (egid != gid) { printf(" egid=%d", egid) pw = getgrgid(egid) pr_first_field(pw) } for (i = 1; ("group" i) in PROCINFO; i++) { if (i == 1) printf(" groups=") group = PROCINFO["group" i] printf("%d", group) pw = getgrgid(group) pr_first_field(pw) if (("group" (i+1)) in PROCINFO) printf(",") } print "" } function pr_first_field(str, a) { if (str != "") { split(str, a, ":") printf("(%s)", a[1]) } } The test in the for loop is worth noting. Any supplementary groups in the PROCINFO array have the indices "group1" through "groupN" for some N (i.e., the total number of supplementary groups). However, we don’t know in advance how many of these groups there are. This loop works by starting at one, concatenating the value with "group", and then using in to see if that value is in the array (see “Referring to an Array Element” on page 175). Eventually, i is incremented past the last group in the array and the loop exits. The loop is also correct if there are no supplementary groups; then the condition is false the first time it’s tested, and the loop body never executes. The pr_first_field() function simply isolates out some code that is used repeatedly, making the whole program shorter and cleaner. In particular, moving the check for the empty string into this function saves several lines of code. 288 | Chapter 11: Practical awk Programs 2. This is the traditional usage. The POSIX usage is different, but not relevant for what the program aims to demonstrate. Splitting a Large File into Pieces The split program splits large text files into smaller pieces. Usage is as follows:2 split [-count] [file] [prefix] By default, the output files are named xaa, xab, and so on. Each file has 1,000 lines in it, with the likely exception of the last file. To change the number of lines in each file, supply a number on the command line preceded with a minus sign (e.g., ‘-500’ for files with 500 lines in them instead of 1,000). To change the names of the output files to something like myfileaa, myfileab, and so on, supply an additional argument that specifies the filename prefix. Here is a version of split in awk. It uses the ord() and chr() functions presented in “Translating Between Characters and Numbers” on page 248. The program first sets its defaults, and then tests to make sure there are not too many arguments. It then looks at each argument in turn. The first argument could be a minus sign followed by a number. If it is, this happens to look like a negative number, so it is made positive, and that is the count of lines. The datafile name is skipped over and the final argument is used as the prefix for the output filenames: # split.awk --- do split in awk # # Requires ord() and chr() library functions # usage: split [-count] [file] [outname] BEGIN { outfile = "x" # default count = 1000 if (ARGC > 4) usage() i = 1 if (i in ARGV && ARGV[i] ~ /^-[[:digit:]]+$/) { count = -ARGV[i] ARGV[i] = "" i++ } # test argv in case reading from stdin instead of file if (i in ARGV) i++ # skip datafile name if (i in ARGV) { outfile = ARGV[i] ARGV[i] = "" } Reinventing Wheels for Fun and Profit | 289 s1 = s2 = "a" out = (outfile s1 s2) } The next rule does most of the work. tcount (temporary count) tracks how many lines have been printed to the output file so far. If it is greater than count, it is time to close the current file and start a new one. s1 and s2 track the current suffixes for the filename. If they are both ‘z’, the file is just too big. Otherwise, s1 moves to the next letter in the alphabet and s2 starts over again at ‘a’: { if (++tcount > count) { close(out) if (s2 == "z") { if (s1 == "z") { printf("split: %s is too large to split\n", FILENAME) > "/dev/stderr" exit 1 } s1 = chr(ord(s1) + 1) s2 = "a" } else s2 = chr(ord(s2) + 1) out = (outfile s1 s2) tcount = 1 } print > out } The usage() function simply prints an error message and exits: function usage() { print("usage: split [-num] [file] [outname]") > "/dev/stderr" exit 1 } This program is a bit sloppy; it relies on awk to automatically close the last file instead of doing it in an END rule. It also assumes that letters are contiguous in the character set, which isn’t true for EBCDIC systems. You might want to consider how to eliminate the use of ord() and chr(); this can be done in such a way as to solve the EBCDIC issue as well. Duplicating Output into Multiple Files The tee program is known as a “pipe fitting.” tee copies its standard input to its standard output and also duplicates it to the files named on the command line. Its usage is as follows: 290 | Chapter 11: Practical awk Programs tee [-a] file … The -a option tells tee to append to the named files, instead of truncating them and starting over. The BEGIN rule first makes a copy of all the command-line arguments into an array named copy. ARGV[0] is not needed, so it is not copied. tee cannot use ARGV directly, because awk attempts to process each filename in ARGV as input data. If the first argument is -a, then the flag variable append is set to true, and both ARGV[1] and copy[1] are deleted. If ARGC is less than two, then no filenames were supplied and tee prints a usage message and exits. Finally, awk is forced to read the standard input by setting ARGV[1] to "-" and ARGC to two: # tee.awk --- tee in awk # # Copy standard input to all named output files. # Append content if -a option is supplied. # BEGIN { for (i = 1; i < ARGC; i++) copy[i] = ARGV[i] if (ARGV[1] == "-a") { append = 1 delete ARGV[1] delete copy[1] ARGC-- } if (ARGC < 2) { print "usage: tee [-a] file ..." > "/dev/stderr" exit 1 } ARGV[1] = "-" ARGC = 2 } The following single rule does all the work. Because there is no pattern, it is executed for each line of input. The body of the rule simply prints the line into each file on the command line, and then to the standard output: { # moving the if outside the loop makes it run faster if (append) for (i in copy) print >> copy[i] else for (i in copy) print > copy[i] print } Reinventing Wheels for Fun and Profit | 291 It is also possible to write the loop this way: for (i in copy) if (append) print >> copy[i] else print > copy[i] This is more concise, but it is also less efficient. The ‘if’ is tested for each record and for each output file. By duplicating the loop body, the ‘if’ is only tested once for each input record. If there are N input records and M output files, the first method only executes N ‘if’ statements, while the second executes N*M ‘if’ statements. Finally, the END rule cleans up by closing all the output files: END { for (i in copy) close(copy[i]) } Printing Nonduplicated Lines of Text The uniq utility reads sorted lines of data on its standard input, and by default removes duplicate lines. In other words, it only prints unique lines—hence the name. uniq has a number of options. The usage is as follows: uniq [-udc [-n]] [+n] [inputfile [outputfile]] The options for uniq are: -d Print only repeated (duplicated) lines. -u Print only nonrepeated (unique) lines. -c Count lines. This option overrides -d and -u. Both repeated and nonrepeated lines are counted. -n Skip n fields before comparing lines. The definition of fields is similar to awk’s default: nonwhitespace characters separated by runs of spaces and/or TABs. +n Skip n characters before comparing lines. Any fields specified with ‘-n’ are skipped first. 292 | Chapter 11: Practical awk Programs inputfile Data is read from the input file named on the command line, instead of from the standard input. outputfile The generated output is sent to the named output file, instead of to the standard output. Normally uniq behaves as if both the -d and -u options are provided. uniq uses the getopt() library function (see “Processing Command-Line Options” on page 259) and the join() library function (see “Merging an Array into a String” on page 250). The program begins with a usage() function and then a brief outline of the options and their meanings in comments. The BEGIN rule deals with the command-line arguments and options. It uses a trick to get getopt() to handle options of the form ‘-25’, treating such an option as the option letter ‘2’ with an argument of ‘5’. If indeed two or more digits are supplied (Optarg looks like a number), Optarg is concatenated with the option digit and then the result is added to zero to make it into a number. If there is only one digit in the option, then Optarg is not needed. In this case, Optind must be decremented so that getopt() processes it next time. This code is admittedly a bit tricky. If no options are supplied, then the default is taken, to print both repeated and non‐ repeated lines. The output file, if provided, is assigned to outputfile. Early on, outputfile is initialized to the standard output, /dev/stdout: # uniq.awk --- do uniq in awk # # Requires getopt() and join() library functions function usage() { print("Usage: uniq [-udc [-n]] [+n] [ in [ out ]]") > "/dev/stderr" exit 1 } # -c count lines. overrides -d and -u # -d only repeated lines # -u only nonrepeated lines # -n skip n fields # +n skip n characters, skip fields first BEGIN { count = 1 outputfile = "/dev/stdout" opts = "udc0:1:2:3:4:5:6:7:8:9:" while ((c = getopt(ARGC, ARGV, opts)) != -1) { if (c == "u") Reinventing Wheels for Fun and Profit | 293 non_repeated_only++ else if (c == "d") repeated_only++ else if (c == "c") do_count++ else if (index("0123456789", c) != 0) { # getopt() requires args to options # this messes us up for things like -5 if (Optarg ~ /^[[:digit:]]+$/) fcount = (c Optarg) + 0 else { fcount = c + 0 Optind-- } } else usage() } if (ARGV[Optind] ~ /^\+[[:digit:]]+$/) { charcount = substr(ARGV[Optind], 2) + 0 Optind++ } for (i = 1; i < Optind; i++) ARGV[i] = "" if (repeated_only == 0 && non_repeated_only == 0) repeated_only = non_repeated_only = 1 if (ARGC - Optind == 2) { outputfile = ARGV[ARGC - 1] ARGV[ARGC - 1] = "" } } The following function, are_equal(), compares the current line, $0, to the previous line, last. It handles skipping fields and characters. If no field count and no character count are specified, are_equal() returns one or zero depending upon the result of a simple string comparison of last and $0. Otherwise, things get more complicated. If fields have to be skipped, each line is broken into an array using split() (see “String-Manipulation Functions” on page 194); the desired fields are then joined back into a line using join(). The joined lines are stored in clast and cline. If no fields are skipped, clast and cline are set to last and $0, respectively. Finally, if characters are skipped, substr() is used to strip off the leading charcount characters in clast and cline. The two strings are then compared and are_equal() returns the result: function are_equal( n, m, clast, cline, alast, aline) { 294 | Chapter 11: Practical awk Programs if (fcount == 0 && charcount == 0) return (last == $0) if (fcount > 0) { n = split(last, alast) m = split($0, aline) clast = join(alast, fcount+1, n) cline = join(aline, fcount+1, m) } else { clast = last cline = $0 } if (charcount) { clast = substr(clast, charcount + 1) cline = substr(cline, charcount + 1) } return (clast == cline) } The following two rules are the body of the program. The first one is executed only for the very first line of data. It sets last equal to $0, so that subsequent lines of text have something to be compared to. The second rule does the work. The variable equal is one or zero, depending upon the results of are_equal()’s comparison. If uniq is counting repeated lines, and the lines are equal, then it increments the count variable. Otherwise, it prints the line and resets count, because the two lines are not equal. If uniq is not counting, and if the lines are equal, count is incremented. Nothing is printed, as the point is to remove duplicates. Otherwise, if uniq is counting repeated lines and more than one line is seen, or if uniq is counting nonrepeated lines and only one line is seen, then the line is printed, and count is reset. Finally, similar logic is used in the END rule to print the final line of input data: NR == 1 { last = $0 next } { equal = are_equal() if (do_count) { # overrides -d and -u if (equal) count++ else { printf("%4d %s\n", count, last) > outputfile last = $0 count = 1 # reset Reinventing Wheels for Fun and Profit | 295 3. This is the definition returned from entering define: state machine into Google. } next } if (equal) count++ else { if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile last = $0 count = 1 } } END { if (do_count) printf("%4d %s\n", count, last) > outputfile else if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile close(outputfile) } The logic for choosing which lines to print represents a state machine, which is “a device that can be in one of a set number of stable conditions depending on its previous con‐ dition and on the present values of its inputs.”3 Brian Kernighan suggests that “an al‐ ternative approach to state machines is to just read the input into an array, then use indexing. It’s almost always easier code, and for most inputs where you would use this, just as fast.” Consider how to rewrite the logic to follow this suggestion. Counting Things The wc (word count) utility counts lines, words, and characters in one or more input files. Its usage is as follows: wc [-lwc] [files …] If no files are specified on the command line, wc reads its standard input. If there are multiple files, it also prints total counts for all the files. The options and their meanings are as follows: -l Count only lines. 296 | Chapter 11: Practical awk Programs -w Count only words. A “word” is a contiguous sequence of nonwhitespace characters, separated by spaces and/or TABs. Luckily, this is the normal way awk separates fields in its input data. -c Count only characters. Implementing wc in awk is particularly elegant, because awk does a lot of the work for us; it splits lines into words (i.e., fields) and counts them, it counts lines (i.e., records), and it can easily tell us how long a line is. This program uses the getopt() library function (see “Processing Command-Line Op‐ tions” on page 259) and the file-transition functions (see “Noting Datafile Bound‐ aries” on page 254). This version has one notable difference from traditional versions of wc: it always prints the counts in the order lines, words, and characters. Traditional versions note the order of the -l, -w, and -c options on the command line, and print the counts in that order. The BEGIN rule does the argument processing. The variable print_total is true if more than one file is named on the command line: # wc.awk --- count lines, words, characters # Options: # -l only count lines # -w only count words # -c only count characters # # Default is to count lines, words, characters # # Requires getopt() and file transition library functions BEGIN { # let getopt() print a message about # invalid options. we ignore them while ((c = getopt(ARGC, ARGV, "lwc")) != -1) { if (c == "l") do_lines = 1 else if (c == "w") do_words = 1 else if (c == "c") do_chars = 1 } for (i = 1; i < Optind; i++) ARGV[i] = "" # if no options, do all if (! do_lines && ! do_words && ! do_chars) Reinventing Wheels for Fun and Profit | 297 4. Because gawk understands multibyte locales, this code counts characters, not bytes. do_lines = do_words = do_chars = 1 print_total = (ARGC - i > 2) } The beginfile() function is simple; it just resets the counts of lines, words, and char‐ acters to zero, and saves the current filename in fname: function beginfile(file) { lines = words = chars = 0 fname = FILENAME } The endfile() function adds the current file’s numbers to the running totals of lines, words, and characters. It then prints out those numbers for the file that was just read. It relies on beginfile() to reset the numbers for the following datafile: function endfile(file) { tlines += lines twords += words tchars += chars if (do_lines) printf "\t%d", lines if (do_words) printf "\t%d", words if (do_chars) printf "\t%d", chars printf "\t%s\n", fname } There is one rule that is executed for each line. It adds the length of the record, plus one, to chars.4 Adding one plus the record length is needed because the newline character separating records (the value of RS) is not part of the record itself, and thus not included in its length. Next, lines is incremented for each line read, and words is incremented by the value of NF, which is the number of “words” on this line: # do per line { chars += length($0) + 1 # get newline lines++ words += NF } Finally, the END rule simply prints the totals for all the files: END { if (print_total) { 298 | Chapter 11: Practical awk Programs if (do_lines) printf "\t%d", tlines if (do_words) printf "\t%d", twords if (do_chars) printf "\t%d", tchars print "\ttotal" } } A Grab Bag of awk Programs This section is a large “grab bag” of miscellaneous programs. We hope you find them both interesting and enjoyable. Finding Duplicated Words in a Document A common error when writing large amounts of prose is to accidentally duplicate words. Typically you will see this in text as something like “the the program does the following…” When the text is online, often the duplicated words occur at the end of one line and the beginning of another, making them very difficult to spot. This program, dupword.awk, scans through a file one line at a time and looks for adjacent occurrences of the same word. It also saves the last word on a line (in the variable prev) for comparison with the first word on the next line. The first two statements make sure that the line is all lowercase, so that, for example, “The” and “the” compare equal to each other. The next statement replaces nonalpha‐ numeric and nonwhitespace characters with spaces, so that punctuation does not affect the comparison either. The characters are replaced with spaces so that formatting con‐ trols don’t create nonsense words (e.g., the Texinfo ‘@code{NF}’ becomes ‘codeNF’ if punctuation is simply deleted). The record is then resplit into fields, yielding just the actual words on the line, and ensuring that there are no empty fields. If there are no fields left after removing all the punctuation, the current record is skipped. Otherwise, the program loops through each word, comparing it to the previous one: # dupword.awk --- find duplicate words in text { $0 = tolower($0) gsub(/[^[:alnum:][:blank:]]/, " "); $0 = $0 # re-split if (NF == 0) next if ($1 == prev) printf("%s:%d: duplicate %s\n", FILENAME, FNR, $1) for (i = 2; i <= NF; i++) if ($i == $(i-1)) A Grab Bag of awk Programs | 299 printf("%s:%d: duplicate %s\n", FILENAME, FNR, $i) prev = $NF } An Alarm Clock Program Nothing cures insomnia like a ringing alarm clock. —Arnold Robbins Sleep is for web developers. —Erik Quanstrom The following program is a simple “alarm clock” program. You give it a time of day and an optional message. At the specified time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message as well as a delay between repetitions. This program uses the getlocaltime() function from “Managing the Time of Day” on page 250. All the work is done in the BEGIN rule. The first part is argument checking and setting of defaults: the delay, the count, and the message to print. If the user supplied a message without the ASCII BEL character (known as the “alert” character, "\a"), then it is added to the message. (On many systems, printing the ASCII BEL generates an audible alert. Thus, when the alarm goes off, the system calls attention to itself in case the user is not looking at the computer.) Just for a change, this program uses a switch statement (see “The switch Statement” on page 154), but the processing could be done with a series of if-else statements instead. Here is the program: # alarm.awk --- set an alarm # # Requires getlocaltime() library function # usage: alarm time [ "message" [ count [ delay ] ] ] BEGIN { # Initial argument sanity checking usage1 = "usage: alarm time ['message' [count [delay]]]" usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1]) if (ARGC < 2) { print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 } switch (ARGC) { case 5: delay = ARGV[4] + 0 # fall through case 4: 300 | Chapter 11: Practical awk Programs count = ARGV[3] + 0 # fall through case 3: message = ARGV[2] break default: if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]{2}/) { print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 } break } # set defaults for once we reach the desired time if (delay == 0) delay = 180 # 3 minutes if (count == 0) count = 5 if (message == "") message = sprintf("\aIt is now %s!\a", ARGV[1]) else if (index(message, "\a") == 0) message = "\a" message "\a" The next section of code turns the alarm time into hours and minutes, converts it (if necessary) to a 24-hour clock, and then turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm: # split up alarm time split(ARGV[1], atime, ":") hour = atime[1] + 0 # force numeric minute = atime[2] + 0 # force numeric # get current broken down time getlocaltime(now) # if time given is 12-hour hours and it's after that # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m., # then add 12 to real hour if (hour < 12 && now["hour"] > hour) hour += 12 # set target time in seconds since midnight target = (hour * 60 * 60) + (minute * 60) # get current time in seconds since midnight current = (now["hour"] * 60 * 60) + \ (now["minute"] * 60) + now["second"] # how long to sleep for naptime = target - current A Grab Bag of awk Programs | 301 5. On some older systems, including Solaris, the system version of tr may require that the lists be written as range expressions enclosed in square brackets (‘[a-z]’) and quoted, to prevent the shell from attempting a filename expansion. This is not a feature. if (naptime <= 0) { print "alarm: time is in the past!" > "/dev/stderr" exit 1 } Finally, the program uses the system() function (see “Input/Output Functions” on page 208) to call the sleep utility. The sleep utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that sleep was interrupted and exits. If sleep exited with an OK status (zero), then the program prints the message in a loop, again using sleep to delay for however many seconds are necessary: # zzzzzz..... go away if interrupted if (system(sprintf("sleep %d", naptime)) != 0) exit 1 # time to notify! command = sprintf("sleep %d", delay) for (i = 1; i <= count; i++) { print message # if sleep command interrupted, go away if (system(command) != 0) break } exit 0 } Transliterating Characters The system tr utility transliterates characters. For example, it is often used to map uppercase letters into lowercase for further processing: generate data | tr 'A-Z' 'a-z' | process data … tr requires two lists of characters.5 When processing the input, the first character in the first list is replaced with the first character in the second list, the second character in the first list is replaced with the second character in the second list, and so on. If there are more characters in the “from” list than in the “to” list, the last character of the “to” list is used for the remaining characters in the “from” list. Once upon a time, a user proposed adding a transliteration function to gawk. The fol‐ lowing program was written to prove that character transliteration could be done with a user-level function. This program is not as complete as the system tr utility, but it does most of the job. 302 | Chapter 11: Practical awk Programs The translate program was written long before gawk acquired the ability to split each character in a string into separate array elements. Thus, it makes repeated use of the substr(), index(), and gsub() built-in functions (see “String-Manipulation Func‐ tions” on page 194). There are two functions. The first, stranslate(), takes three arguments: from A list of characters from which to translate to A list of characters to which to translate target The string on which to do the translation Associative arrays make the translation part fairly easy. t_ar holds the “to” characters, indexed by the “from” characters. Then a simple loop goes through from, one character at a time. For each character in from, if the character appears in target, it is replaced with the corresponding to character. The translate() function calls stranslate(), using $0 as the target. The main pro‐ gram sets two global variables, FROM and TO, from the command line, and then changes ARGV so that awk reads from the standard input. Finally, the processing rule simply calls translate() for each record: # translate.awk --- do tr-like stuff # Bugs: does not handle things like tr A-Z a-z; it has # to be spelled out. However, if `to' is shorter than `from', # the last character in `to' is used for the rest of `from'. function stranslate(from, to, target, lf, lt, ltarget, t_ar, i, c, result) { lf = length(from) lt = length(to) ltarget = length(target) for (i = 1; i <= lt; i++) t_ar[substr(from, i, 1)] = substr(to, i, 1) if (lt < lf) for (; i <= lf; i++) t_ar[substr(from, i, 1)] = substr(to, lt, 1) for (i = 1; i <= ltarget; i++) { c = substr(target, i, 1) if (c in t_ar) c = t_ar[c] result = result c } return result } A Grab Bag of awk Programs | 303 6. “Real world” is defined as “a program actually used to get something done.” function translate(from, to) { return $0 = stranslate(from, to, $0) } # main program BEGIN { if (ARGC < 3) { print "usage: translate from to" > "/dev/stderr" exit } FROM = ARGV[1] TO = ARGV[2] ARGC = 2 ARGV[1] = "-" } { translate(FROM, TO) print } It is possible to do character transliteration in a user-level function, but it is not neces‐ sarily efficient, and we (the gawk developers) started to consider adding a built-in func‐ tion. However, shortly after writing this program, we learned that Brian Kernighan had added the toupper() and tolower() functions to his awk (see “String-Manipulation Functions” on page 194). These functions handle the vast majority of the cases where character transliteration is necessary, and so we chose to simply add those functions to gawk as well and then leave well enough alone. An obvious improvement to this program would be to set up the t_ar array only once, in a BEGIN rule. However, this assumes that the “from” and “to” lists will never change throughout the lifetime of the program. Another obvious improvement is to enable the use of ranges, such as ‘a-z’, as allowed by the tr utility. Look at the code for cut.awk (see “Cutting Out Fields and Columns” on page 278) for inspiration. Printing Mailing Labels Here is a “real-world”6 program. This script reads lists of names and addresses and generates mailing labels. Each page of labels has 20 labels on it, two across and 10 down. The addresses are guaranteed to be no more than five lines of data. Each address is separated from the next by a blank line. 304 | Chapter 11: Practical awk Programs The basic idea is to read 20 labels’ worth of data. Each line of each label is stored in the line array. The single rule takes care of filling the line array and printing the page when 20 labels have been read. The BEGIN rule simply sets RS to the empty string, so that awk splits records at blank lines (see “How Input Is Split into Records” on page 55). It sets MAXLINES to 100, because 100 is the maximum number of lines on the page (20 ⋅ 5 = 100). Most of the work is done in the printpage() function. The label lines are stored se‐ quentially in the line array. But they have to print horizontally: line[1] next to line[6], line[2] next to line[7], and so on. Two loops accomplish this. The outer loop, controlled by i, steps through every 10 lines of data; this is each row of labels. The inner loop, controlled by j, goes through the lines within the row. As j goes from 0 to 4, ‘i+j’ is the jth line in the row, and ‘i+j+5’ is the entry next to it. The output ends up looking something like this: line 1 line 6 line 2 line 7 line 3 line 8 line 4 line 9 line 5 line 10 … The printf format string ‘%-41s’ left-aligns the data and prints it within a fixed- width field. As a final note, an extra blank line is printed at lines 21 and 61, to keep the output lined up on the labels. This is dependent on the particular brand of labels in use when the program was written. You will also note that there are two blank lines at the top and two blank lines at the bottom. The END rule arranges to flush the final page of labels; there may not have been an even multiple of 20 labels in the data: # labels.awk --- print mailing labels # Each label is 5 lines of data that may have blank lines. # The label sheets have 2 blank lines at the top and 2 at # the bottom. BEGIN { RS = "" ; MAXLINES = 100 } function printpage( i, j) { if (Nlines <= 0) return printf "\n\n" # header for (i = 1; i <= Nlines; i += 10) { A Grab Bag of awk Programs | 305 if (i == 21 || i == 61) print "" for (j = 0; j < 5; j++) { if (i + j > MAXLINES) break printf " %-41s %s\n", line[i+j], line[i+j+5] } print "" } printf "\n\n" # footer delete line } # main rule { if (Count >= 20) { printpage() Count = 0 Nlines = 0 } n = split($0, a, "\n") for (i = 1; i <= n; i++) line[++Nlines] = a[i] for (; i <= 5; i++) line[++Nlines] = "" Count++ } END { printpage() } Generating Word-Usage Counts When working with large amounts of text, it can be interesting to know how often different words appear. For example, an author may overuse certain words, in which case he or she might wish to find synonyms to substitute for words that appear too often. This subsection develops a program for counting words and presenting the frequency information in a useful format. At first glance, a program like this would seem to do the job: # wordfreq-first-try.awk --- print list of word frequencies { for (i = 1; i <= NF; i++) freq[$i]++ } 306 | Chapter 11: Practical awk Programs END { for (word in freq) printf "%s\t%d\n", word, freq[word] } The program relies on awk’s default field-splitting mechanism to break each line up into “words” and uses an associative array named freq, indexed by each word, to count the number of times the word occurs. In the END rule, it prints the counts. This program has several problems that prevent it from being useful on real text files: • The awk language considers upper- and lowercase characters to be distinct. There‐ fore, “bartender” and “Bartender” are not treated as the same word. This is unde‐ sirable, because words are capitalized if they begin sentences in normal text, and a frequency analyzer should not be sensitive to capitalization. • Words are detected using the awk convention that fields are separated just by white‐ space. Other characters in the input (except newlines) don’t have any special mean‐ ing to awk. This means that punctuation characters count as part of words. • The output does not come out in any useful order. You’re more likely to be interested in which words occur most frequently or in having an alphabetized table of how frequently each word occurs. The first problem can be solved by using tolower() to remove case distinctions. The second problem can be solved by using gsub() to remove punctuation characters. Fi‐ nally, we solve the third problem by using the system sort utility to process the output of the awk script. Here is the new version of the program: # wordfreq.awk --- print list of word frequencies { $0 = tolower($0) # remove case distinctions # remove punctuation gsub(/[^[:alnum:]_[:blank:]]/, "", $0) for (i = 1; i <= NF; i++) freq[$i]++ } END { for (word in freq) printf "%s\t%d\n", word, freq[word] } The regexp /[^[:alnum:]_[:blank:]]/ might have been written /[[:punct:]]/, but then underscores would also be removed, and we want to keep them. Assuming we have saved this program in a file named wordfreq.awk, and that the data is in file1, the following pipeline: A Grab Bag of awk Programs | 307 awk -f wordfreq.awk file1 | sort -k 2nr produces a table of the words appearing in file1 in order of decreasing frequency. The awk program suitably massages the data and produces a word frequency table, which is not ordered. The awk script’s output is then sorted by the sort utility and printed on the screen. The options given to sort specify a sort that uses the second field of each input line (skipping one field), that the sort keys should be treated as numeric quantities (otherwise ‘15’ would come before ‘5’), and that the sorting should be done in descend‐ ing (reverse) order. The sort could even be done from within the program, by changing the END action to: END { sort = "sort -k 2nr" for (word in freq) printf "%s\t%d\n", word, freq[word] | sort close(sort) } This way of sorting must be used on systems that do not have true pipes at the command- line (or batch-file) level. See the general operating system documentation for more information on how to use the sort program. Removing Duplicates from Unsorted Text The uniq program (see “Printing Nonduplicated Lines of Text” on page 292) removes duplicate lines from sorted data. Suppose, however, you need to remove duplicate lines from a datafile but that you want to preserve the order the lines are in. A good example of this might be a shell history file. The history file keeps a copy of all the commands you have entered, and it is not unusual to repeat a command several times in a row. Occasionally you might want to compact the history by removing duplicate entries. Yet it is desirable to maintain the order of the original commands. This simple program does the job. It uses two arrays. The data array is indexed by the text of each line. For each line, data[$0] is incremented. If a particular line has not been seen before, then data[$0] is zero. In this case, the text of the line is stored in lines[count]. Each element of lines is a unique command, and the indices of lines indicate the order in which those lines are encountered. The END rule simply prints out the lines, in order: # histsort.awk --- compact a shell history file # Thanks to Byron Rakitzis for the general idea { 308 | Chapter 11: Practical awk Programs if (data[$0]++ == 0) lines[++count] = $0 } END { for (i = 1; i <= count; i++) print lines[i] } This program also provides a foundation for generating other useful information. For example, using the following print statement in the END rule indicates how often a particular command is used: print data[lines[i]], lines[i] This works because data[$0] is incremented each time a line is seen. Extracting Programs from Texinfo Source Files Both this chapter and the previous chapter (Chapter 10) present a large number of awk programs. If you want to experiment with these programs, it is tedious to type them in by hand. Here we present a program that can extract parts of a Texinfo input file into separate files. This book is written in Texinfo, the GNU Project’s document formatting language. A single Texinfo source file can be used to produce both printed documentation, with TeX, and online documentation. (Texinfo is fully documented in the book Texinfo—The GNU Documentation Format, available from the Free Software Foundation, and also available online.) For our purposes, it is enough to know three things about Texinfo input files: • The “at” symbol (‘@’) is special in Texinfo, much as the backslash (‘\’) is in C or awk. Literal ‘@’ symbols are represented in Texinfo source files as ‘@@’. • Comments start with either ‘@c’ or ‘@comment’. The file-extraction program works by using special comments that start at the beginning of a line. • Lines containing ‘@group’ and ‘@end group’ commands bracket example text that should not be split across a page boundary. (Unfortunately, TeX isn’t always smart enough to do things exactly right, so we have to give it some help.) The following program, extract.awk, reads through a Texinfo source file and does two things, based on the special comments. Upon seeing ‘@c system …’, it runs a command, by extracting the command text from the control line and passing it on to the system() function (see “Input/Output Functions” on page 208). Upon seeing ‘@c file file name’, each subsequent line is sent to the file filename, until ‘@c endfile’ is encountered. The rules in extract.awk match either ‘@c’ or ‘@comment’ by letting the ‘omment’ part be A Grab Bag of awk Programs | 309 optional. Lines containing ‘@group’ and ‘@end group’ are simply removed. extract.awk uses the join() library function (see “Merging an Array into a String” on page 250). The example programs in the online Texinfo source for Effective awk Programming (gawktexi.in) have all been bracketed inside ‘file’ and ‘endfile’ lines. The gawk distribution uses a copy of extract.awk to extract the sample programs and install many of them in a standard directory where gawk can find them. The Texinfo file looks some‐ thing like this: … This program has a @code{BEGIN} rule that prints a nice message: @example @c file examples/messages.awk BEGIN @{ print "Don't panic!" @} @c end file @end example It also prints some final advice: @example @c file examples/messages.awk END @{ print "Always avoid bored archaeologists!" @} @c end file @end example … extract.awk begins by setting IGNORECASE to one, so that mixed upper- and lowercase letters in the directives won’t matter. The first rule handles calling system(), checking that a command is given (NF is at least three), and also checking that the command exits with a zero exit status, signifying OK: # extract.awk --- extract files and run programs from Texinfo files BEGIN { IGNORECASE = 1 } /^@c(omment)?[ \t]+system/ { if (NF < 3) { e = ("extract: " FILENAME ":" FNR) e = (e ": badly formed `system' line") print e > "/dev/stderr" next } $1 = "" $2 = "" stat = system($0) if (stat != 0) { e = ("extract: " FILENAME ":" FNR) e = (e ": warning: system returned " stat) 310 | Chapter 11: Practical awk Programs print e > "/dev/stderr" } } The variable e is used so that the rule fits nicely on the page. The second rule handles moving data into files. It verifies that a filename is given in the directive. If the file named is not the current file, then the current file is closed. Keeping the current file open until a new file is encountered allows the use of the ‘>’ redirection for printing the contents, keeping open-file management simple. The for loop does the work. It reads lines using getline (see “Explicit Input with getline” on page 77). For an unexpected end-of-file, it calls the unexpected_eof() function. If the line is an “endfile” line, then it breaks out of the loop. If the line is an ‘@group’ or ‘@end group’ line, then it ignores it and goes on to the next line. Similarly, comments within examples are also ignored. Most of the work is in the following few lines. If the line has no ‘@’ symbols, the program can print it directly. Otherwise, each leading ‘@’ must be stripped off. To remove the ‘@’ symbols, the line is split into separate elements of the array a, using the split() function (see “String-Manipulation Functions” on page 194). The ‘@’ symbol is used as the sep‐ arator character. Each element of a that is empty indicates two successive ‘@’ symbols in the original line. For each two empty elements (‘@@’ in the original file), we have to add a single ‘@’ symbol back in. When the processing of the array is finished, join() is called with the value of SUBSEP (see “Multidimensional Arrays” on page 185), to rejoin the pieces back into a single line. That line is then printed to the output file: /^@c(omment)?[ \t]+file/ { if (NF != 3) { e = ("extract: " FILENAME ":" FNR ": badly formed `file' line") print e > "/dev/stderr" next } if ($3 != curfile) { if (curfile != "") close(curfile) curfile = $3 } for (;;) { if ((getline line) <= 0) unexpected_eof() if (line ~ /^@c(omment)?[ \t]+endfile/) break else if (line ~ /^@(end[ \t]+)?group/) continue else if (line ~ /^@c(omment+)?[ \t]+/) A Grab Bag of awk Programs | 311 continue if (index(line, "@") == 0) { print line > curfile continue } n = split(line, a, "@") # if a[1] == "", means leading @, # don't add one back in. for (i = 2; i <= n; i++) { if (a[i] == "") { # was an @@ a[i] = "@" if (a[i+1] == "") i++ } } print join(a, 1, n, SUBSEP) > curfile } } An important thing to note is the use of the ‘>’ redirection. Output done with ‘>’ only opens the file once; it stays open and subsequent output is appended to the file (see “Redirecting Output of print and printf” on page 100). This makes it easy to mix pro‐ gram text and explanatory prose for the same sample source file (as has been done here!) without any hassle. The file is only closed when a new datafile name is encountered or at the end of the input file. Finally, the function unexpected_eof() prints an appropriate error message and then exits. The END rule handles the final cleanup, closing the open file: function unexpected_eof() { printf("extract: %s:%d: unexpected EOF or error\n", FILENAME, FNR) > "/dev/stderr" exit 1 } END { if (curfile) close(curfile) } A Simple Stream Editor The sed utility is a stream editor, a program that reads a stream of data, makes changes to it, and passes it on. It is often used to make global changes to a large file or to a stream of data generated by a pipeline of commands. Although sed is a complicated program in its own right, its most common use is to perform global substitutions in the middle of a pipeline: command1 < orig.data | sed 's/old/new/g' | command2 > result 312 | Chapter 11: Practical awk Programs Here, ‘s/old/new/g’ tells sed to look for the regexp ‘old’ on each input line and globally replace it with the text ‘new’ (i.e., all the occurrences on a line). This is similar to awk’s gsub() function (see “String-Manipulation Functions” on page 194). The following program, awksed.awk, accepts at least two command-line arguments: the pattern to look for and the text to replace it with. Any additional arguments are treated as datafile names to process. If none are provided, the standard input is used: # awksed.awk --- do s/foo/bar/g using just print # Thanks to Michael Brennan for the idea function usage() { print "usage: awksed pat repl [files...]" > "/dev/stderr" exit 1 } BEGIN { # validate arguments if (ARGC < 3) usage() RS = ARGV[1] ORS = ARGV[2] # don't use arguments as files ARGV[1] = ARGV[2] = "" } # look ma, no hands! { if (RT == "") printf "%s", $0 else print } The program relies on gawk’s ability to have RS be a regexp, as well as on the setting of RT to the actual text that terminates the record (see “How Input Is Split into Records” on page 55). The idea is to have RS be the pattern to look for. gawk automatically sets $0 to the text between matches of the pattern. This is text that we want to keep, unmodified. Then, by setting ORS to the replacement text, a simple print statement outputs the text we want to keep, followed by the replacement text. There is one wrinkle to this scheme, which is what to do if the last record doesn’t end with text that matches RS. Using a print statement unconditionally prints the replace‐ ment text, which is not correct. However, if the file did not end in text that matches RS, A Grab Bag of awk Programs | 313 RT is set to the null string. In this case, we can print $0 using printf (see “Using printf Statements for Fancier Printing” on page 93). The BEGIN rule handles the setup, checking for the right number of arguments and calling usage() if there is a problem. Then it sets RS and ORS from the command-line arguments and sets ARGV[1] and ARGV[2] to the null string, so that they are not treated as filenames (see “Using ARGC and ARGV” on page 170). The usage() function prints an error message and exits. Finally, the single rule handles the printing scheme outlined earlier, using print or printf as appropriate, depending upon the value of RT. An Easy Way to Use Library Functions In “Including Other Files into Your Program” on page 35, we saw how gawk provides a built-in file-inclusion capability. However, this is a gawk extension. This section provides the motivation for making file inclusion available for standard awk, and shows how to do it using a combination of shell and awk programming. Using library functions in awk can be very beneficial. It encourages code reuse and the writing of general functions. Programs are smaller and therefore clearer. However, using library functions is only easy when writing awk programs; it is painful when running them, requiring multiple -f options. If gawk is unavailable, then so too is the AWKPATH environment variable and the ability to put awk functions into a library directory (see “Command-Line Options” on page 22). It would be nice to be able to write programs in the following manner: # library functions @include getopt.awk @include join.awk … # main program BEGIN { while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1) … … } The following program, igawk.sh, provides this service. It simulates gawk’s searching of the AWKPATH variable and also allows nested includes (i.e., a file that is included with @include can contain further @include statements). igawk makes an effort to only include files once, so that nested includes don’t accidentally include a library func‐ tion twice. 314 | Chapter 11: Practical awk Programs 7. Fully explaining the sh language is beyond the scope of this book. We provide some minimal explanations, but see a good shell programming book if you wish to understand things in more depth. igawk should behave just like gawk externally. This means it should accept all of gawk’s command-line arguments, including the ability to have multiple source files specified via -f and the ability to mix command-line and library source files. The program is written using the POSIX Shell (sh) command language.7 It works as follows: 1. Loop through the arguments, saving anything that doesn’t represent awk source code for later, when the expanded program is run. 2. For any arguments that do represent awk text, put the arguments into a shell variable that will be expanded. There are two cases: a. Literal text, provided with -e or --source. This text is just appended directly. b. Source filenames, provided with -f. We use a neat trick and append ‘@include filename’ to the shell variable’s contents. Because the file-inclusion program works the way gawk does, this gets the text of the file included in the program at the correct point. 3. Run an awk program (naturally) over the shell variable’s contents to expand @include statements. The expanded program is placed in a second shell variable. 4. Run the expanded program with gawk and any other original command-line ar‐ guments that the user supplied (such as the datafile names). This program uses shell variables extensively: for storing command-line arguments and the text of the awk program that will expand the user’s program, for the user’s original program, and for the expanded program. Doing so removes some potential problems that might arise were we to use temporary files instead, at the cost of making the script somewhat more complicated. The initial part of the program turns on shell tracing if the first argument is ‘debug’. The next part loops through all the command-line arguments. There are several cases of interest: -- This ends the arguments to igawk. Anything else should be passed on to the user’s awk program without being evaluated. -W This indicates that the next option is specific to gawk. To make argument processing easier, the -W is appended to the front of the remaining arguments and the loop A Grab Bag of awk Programs | 315 continues. (This is an sh programming trick. Don’t worry about it if you are not familiar with sh.) -v, -F These are saved and passed on to gawk. -f, --file, --file=, -Wfile= The filename is appended to the shell variable program with an @include statement. The expr utility is used to remove the leading option part of the argument (e.g., ‘--file=’). (Typical sh usage would be to use the echo and sed utilities to do this work. Unfortunately, some versions of echo evaluate escape sequences in their ar‐ guments, possibly mangling the program text. Using expr avoids this problem.) --source, --source=, -Wsource= The source text is appended to program. --version, -Wversion igawk prints its version number, runs ‘gawk --version’ to get the gawk version information, and then exits. If none of the -f, --file, -Wfile, --source, or -Wsource arguments are supplied, then the first nonoption argument should be the awk program. If there are no command- line arguments left, igawk prints an error message and exits. Otherwise, the first argu‐ ment is appended to program. In any case, after the arguments have been processed, the shell variable program contains the complete text of the original awk program. The program is as follows: #! /bin/sh # igawk --- like gawk but do @include processing if [ "$1" = debug ] then set -x shift fi # A literal newline, so that program text is formatted correctly n=' ' # Initialize variables to empty program= opts= while [ $# -ne 0 ] # loop over arguments do case $1 in --) shift 316 | Chapter 11: Practical awk Programs break ;; -W) shift # The ${x?'message here'} construct prints a # diagnostic if $x is the null string set -- -W"${@?'missing operand'}" continue ;; -[vF]) opts="$opts $1 '${2?'missing operand'}'" shift ;; -[vF]*) opts="$opts '$1'" ;; -f) program="$program$n@include ${2?'missing operand'}" shift ;; -f*) f=$(expr "$1" : '-f\(.*\)') program="$program$n@include $f" ;; -[W-]file=*) f=$(expr "$1" : '-.file=\(.*\)') program="$program$n@include $f" ;; -[W-]file) program="$program$n@include ${2?'missing operand'}" shift ;; -[W-]source=*) t=$(expr "$1" : '-.source=\(.*\)') program="$program$n$t" ;; -[W-]source) program="$program$n${2?'missing operand'}" shift ;; -[W-]version) echo igawk: version 3.0 1>&2 gawk --version exit 0 ;; -[W-]*) opts="$opts '$1'" ;; *) break ;; esac shift done if [ -z "$program" ] then program=${1?'missing program'} shift fi A Grab Bag of awk Programs | 317 8. On some very old versions of awk, the test ‘getline junk < t’ can loop forever if the file exists but is empty. # At this point, `program' has the program. The awk program to process @include directives is stored in the shell variable expand_prog. Doing this keeps the shell script readable. The awk program reads through the user’s program, one line at a time, using getline (see “Explicit Input with getline” on page 77). The input filenames and @include statements are managed using a stack. As each @include is encountered, the current filename is “pushed” onto the stack and the file named in the @include directive becomes the current filename. As each file is finished, the stack is “popped,” and the previous input file becomes the current input file again. The process is started by making the original file the first one on the stack. The pathto() function does the work of finding the full path to a file. It simulates gawk’s behavior when searching the AWKPATH environment variable (see “The AWK‐ PATH Environment Variable” on page 31). If a filename has a ‘/’ in it, no path search is done. Similarly, if the filename is "-", then that string is used as-is. Otherwise, the filename is concatenated with the name of each directory in the path, and an attempt is made to open the generated filename. The only way to test if a file can be read in awk is to go ahead and try to read it with getline; this is what pathto() does.8 If the file can be read, it is closed and the filename is returned: expand_prog=' function pathto(file, i, t, junk) { if (index(file, "/") != 0) return file if (file == "-") return file for (i = 1; i <= ndirs; i++) { t = (pathlist[i] "/" file) if ((getline junk < t) > 0) { # found it close(t) return t } } return "" } 318 | Chapter 11: Practical awk Programs The main program is contained inside one BEGIN rule. The first thing it does is set up the pathlist array that pathto() uses. After splitting the path on ‘:’, null elements are replaced with ".", which represents the current directory: BEGIN { path = ENVIRON["AWKPATH"] ndirs = split(path, pathlist, ":") for (i = 1; i <= ndirs; i++) { if (pathlist[i] == "") pathlist[i] = "." } The stack is initialized with ARGV[1], which will be "/dev/stdin". The main loop comes next. Input lines are read in succession. Lines that do not start with @include are printed verbatim. If the line does start with @include, the filename is in $2. pathto() is called to generate the full path. If it cannot, then the program prints an error message and continues. The next thing to check is if the file is included already. The processed array is indexed by the full filename of each included file and it tracks this information for us. If the file is seen again, a warning message is printed. Otherwise, the new filename is pushed onto the stack and processing continues. Finally, when getline encounters the end of the input file, the file is closed and the stack is popped. When stackptr is less than zero, the program is done: stackptr = 0 input[stackptr] = ARGV[1] # ARGV[1] is first file for (; stackptr >= 0; stackptr--) { while ((getline < input[stackptr]) > 0) { if (tolower($1) != "@include") { print continue } fpath = pathto($2) if (fpath == "") { printf("igawk: %s:%d: cannot find %s\n", input[stackptr], FNR, $2) > "/dev/stderr" continue } if (! (fpath in processed)) { processed[fpath] = input[stackptr] input[++stackptr] = fpath # push onto stack } else print $2, "included in", input[stackptr], "already included in", processed[fpath] > "/dev/stderr" } close(input[stackptr]) } A Grab Bag of awk Programs | 319 }' # close quote ends `expand_prog' variable processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF $program EOF ) The shell construct ‘command << marker’ is called a here document. Everything in the shell script up to the marker is fed to command as input. The shell processes the contents of the here document for variable and command substitution (and possibly other things as well, depending upon the shell). The shell construct ‘$(…)’ is called command substitution. The output of the command inside the parentheses is substituted into the command line. Because the result is used in a variable assignment, it is saved as a single string, even if the results contain whitespace. The expanded program is saved in the variable processed_program. It’s done in these steps: 1. Run gawk with the @include-processing program (the value of the expand_prog shell variable) reading standard input. 2. Standard input is the contents of the user’s program, from the shell variable program. Feed its contents to gawk via a here document. 3. Save the results of this processing in the shell variable processed_program by using command substitution. The last step is to call gawk with the expanded program, along with the original options and command-line arguments that the user supplied: eval gawk $opts -- '"$processed_program"' '"$@"' The eval command is a shell construct that reruns the shell’s parsing process. This keeps things properly quoted. This version of igawk represents the fifth version of this program. There are four key simplifications that make the program work better: • Using @include even for the files named with -f makes building the initial collected awk program much simpler; all the @include processing can be done once. • Not trying to save the line read with getline in the pathto() function when testing for the file’s accessibility for use with the main program simplifies things considerably. • Using a getline loop in the BEGIN rule does it all in one place. It is not necessary to call out to a separate loop for processing nested @include statements. 320 | Chapter 11: Practical awk Programs 9. gawk does @include processing itself in order to support the use of awk programs as Web CGI scripts. • Instead of saving the expanded program in a temporary file, putting it in a shell variable avoids some potential security problems. This has the disadvantage that the script relies upon more features of the sh language, making it harder to follow for those who aren’t familiar with sh. Also, this program illustrates that it is often worthwhile to combine sh and awk pro‐ gramming together. You can usually accomplish quite a lot, without having to resort to low-level programming in C or C++, and it is frequently easier to do certain kinds of string and argument manipulation using the shell than it is in awk. Finally, igawk shows that it is not always necessary to add new features to a program; they can often be layered on top.9 Finding Anagrams from a Dictionary An interesting programming challenge is to search for anagrams in a word list (such as /usr/share/dict/words on many GNU/Linux systems). One word is an anagram of another if both words contain the same letters (e.g., “babbling” and “blabbing”). Column 2, Problem C, of Jon Bentley’s Programming Pearls, Second Edition, presents an elegant algorithm. The idea is to give words that are anagrams a common signature, sort all the words together by their signatures, and then print them. Dr. Bentley observes that taking the letters in each word and sorting them produces those common signatures. The following program uses arrays of arrays to bring together words with the same signature and array sorting to print the words in sorted order: # anagram.awk --- An implementation of the anagram-finding algorithm # from Jon Bentley's "Programming Pearls," 2nd edition. # Addison Wesley, 2000, ISBN 0-201-65788-0. # Column 2, Problem C, section 2.8, pp 18-20. /'s$/ { next } # Skip possessives The program starts with a header, and then a rule to skip possessives in the dictionary file. The next rule builds up the data structure. The first dimension of the array is indexed by the signature; the second dimension is the word itself: { key = word2key($1) # Build signature data[key][$1] = $1 # Store word with signature } A Grab Bag of awk Programs | 321 The word2key() function creates the signature. It splits the word apart into individual letters, sorts the letters, and then joins them back together: # word2key --- split word apart into letters, sort, and join back together function word2key(word, a, i, n, result) { n = split(word, a, "") asort(a) for (i = 1; i <= n; i++) result = result a[i] return result } Finally, the END rule traverses the array and prints out the anagram lists. It sends the output to the system sort command because otherwise the anagrams would appear in arbitrary order: END { sort = "sort" for (key in data) { # Sort words with same key nwords = asorti(data[key], words) if (nwords == 1) continue # And print. Minor glitch: trailing space at end of each line for (j = 1; j <= nwords; j++) printf("%s ", words[j]) | sort print "" | sort } close(sort) } Here is some partial output when the program is run: $ gawk -f anagram.awk /usr/share/dict/words | grep '^b' … babbled blabbed babbler blabber brabble babblers blabbers brabbles babbling blabbing babbly blabby babel bable babels beslab babery yabber … 322 | Chapter 11: Practical awk Programs And Now for Something Completely Different The following program was written by Davide Brini and is published on his website. It serves as his signature in the Usenet group comp.lang.awk. He supplies the following copyright terms: Copyright © 2008 Davide Brini Copying and distribution of the code published in this page, with or without modifica‐ tion, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. Here is the program: awk 'BEGIN{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c"; printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O, X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O, O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O}' We leave it to you to determine what the program does. (If you are truly desperate to understand it, see Chris Johansen’s explanation, which is embedded in the Texinfo source file for this book.) Summary • The programs provided in this chapter continue on the theme that reading pro‐ grams is an excellent way to learn Good Programming. • Using ‘#!’ to make awk programs directly runnable makes them easier to use. Otherwise, invoke the program using ‘awk -f …’. • Reimplementing standard POSIX programs in awk is a pleasant exercise; awk’s ex‐ pressive power lets you write such programs in relatively few lines of code, yet they are functionally complete and usable. • One of standard awk’s weaknesses is working with individual characters. The ability to use split() with the empty string as the separator can considerably simplify such tasks. • The examples here demonstrate the usefulness of the library functions from Chap‐ ter 10 for a number of real (if small) programs. • Besides reinventing POSIX wheels, other programs solved a selection of interesting problems, such as finding duplicate words in text, printing mailing labels, and finding anagrams. Summary | 323 PART III Moving Beyond Standard awk with gawk Part III focuses on features specific to gawk. It contains the following chapters: • Chapter 12, Advanced Features of gawk • Chapter 13, Internationalization with gawk • Chapter 14, Debugging awk Programs • Chapter 15, Arithmetic and Arbitrary-Precision Arithmetic with gawk • Chapter 16, Writing Extensions for gawk CHAPTER 12 Advanced Features of gawk Write documentation as if whoever reads it is a violent psychopath who knows where you live. —Steve English, as quoted by Peter Langston This chapter discusses advanced features in gawk. It’s a bit of a “grab bag” of items that are otherwise unrelated to each other. First, we look at a command-line option that allows gawk to recognize nondecimal numbers in input data, not just in awk programs. Then, gawk’s special features for sorting arrays are presented. Next, two-way I/O, dis‐ cussed briefly in earlier parts of this book, is described in full detail, along with the basics of TCP/IP networking. Finally, we see how gawk can profile an awk program, making it possible to tune it for performance. Additional advanced features are discussed in separate chapters of their own: • Chapter 13, Internationalization with gawk, discusses how to internationalize your awk programs, so that they can speak multiple national languages. • Chapter 14, Debugging awk Programs, describes gawk’s built-in command-line de‐ bugger for debugging awk programs. • Chapter 15, Arithmetic and Arbitrary-Precision Arithmetic with gawk, describes how you can use gawk to perform arbitrary-precision arithmetic. • Chapter 16, Writing Extensions for gawk, discusses the ability to dynamically add new built-in functions to gawk. Allowing Nondecimal Input Data If you run gawk with the --non-decimal-data option, you can have nondecimal val‐ ues in your input data: 327 $ echo 0123 123 0x123 | > gawk --non-decimal-data '{ printf "%d, %d, %d\n", $1, $2, $3 }' 83, 123, 291 For this feature to work, write your program so that gawk treats your data as numeric: $ echo 0123 123 0x123 | gawk '{ print $1, $2, $3 }' 0123 123 0x123 The print statement treats its expressions as strings. Although the fields can act as numbers when necessary, they are still strings, so print does not try to treat them numerically. You need to add zero to a field to force it to be treated as a number. For example: $ echo 0123 123 0x123 | gawk --non-decimal-data ' > { print $1, $2, $3 > print $1 + 0, $2 + 0, $3 + 0 }' 0123 123 0x123 83 123 291 Because it is common to have decimal data with leading zeros, and because using this facility could lead to surprising results, the default is to leave it disabled. If you want it, you must explicitly request it. Use of this option is not recommended. It can break old programs very badly. Instead, use the strtonum() function to convert your data (see “String-Manipulation Functions” on page 194). This makes your programs easier to write and easier to read, and leads to less sur‐ prising results. This option may disappear in a future version of gawk. Controlling Array Traversal and Array Sorting gawk lets you control the order in which a ‘for (indx in array)’ loop traverses an array. In addition, two built-in functions, asort() and asorti(), let you sort arrays based on the array values and indices, respectively. These two functions also provide control over the sorting criteria used to order the elements during sorting. Controlling Array Traversal By default, the order in which a ‘for (indx in array)’ loop scans an array is not defined; it is generally based upon the internal implementation of arrays inside awk. Often, though, it is desirable to be able to loop over the elements in a particular order that you, the programmer, choose. gawk lets you do this. 328 | Chapter 12: Advanced Features of gawk 1. This is why the predefined sorting orders start with an ‘@’ character, which cannot be part of an identifier. “Using Predefined Array Scanning Orders with gawk” on page 179 describes how you can assign special, predefined values to PROCINFO["sorted_in"] in order to control the order in which gawk traverses an array during a for loop. In addition, the value of PROCINFO["sorted_in"] can be a function name.1 This lets you traverse an array based on any custom criterion. The array elements are ordered according to the return value of this function. The comparison function should be defined with at least four arguments: function comp_func(i1, v1, i2, v2) { compare elements 1 and 2 in some fashion return < 0; 0; or > 0 } Here, i1 and i2 are the indices, and v1 and v2 are the corresponding values of the two elements being compared. Either v1 or v2, or both, can be arrays if the array being traversed contains subarrays as values. (See “Arrays of Arrays” on page 187 for more information about subarrays.) The three possible return values are interpreted as follows: comp_func(i1, v1, i2, v2) < 0 Index i1 comes before index i2 during loop traversal. comp_func(i1, v1, i2, v2) == 0 Indices i1 and i2 come together, but the relative order with respect to each other is undefined. comp_func(i1, v1, i2, v2) > 0 Index i1 comes after index i2 during loop traversal. Our first comparison function can be used to scan an array in numerical order of the indices: function cmp_num_idx(i1, v1, i2, v2) { # numerical index comparison, ascending order return (i1 - i2) } Our second function traverses an array based on the string order of the element values rather than by indices: function cmp_str_val(i1, v1, i2, v2) { # string value comparison, ascending order v1 = v1 "" Controlling Array Traversal and Array Sorting | 329 v2 = v2 "" if (v1 < v2) return -1 return (v1 != v2) } The third comparison function makes all numbers, and numeric strings without any leading or trailing spaces, come out first during loop traversal: function cmp_num_str_val(i1, v1, i2, v2, n1, n2) { # numbers before string value comparison, ascending order n1 = v1 + 0 n2 = v2 + 0 if (n1 == v1) return (n2 == v2) ? (n1 - n2) : -1 else if (n2 == v2) return 1 return (v1 < v2) ? -1 : (v1 != v2) } Here is a main program to demonstrate how gawk behaves using each of the previous functions: BEGIN { data["one"] = 10 data["two"] = 20 data[10] = "one" data[100] = 100 data[20] = "two" f[1] = "cmp_num_idx" f[2] = "cmp_str_val" f[3] = "cmp_num_str_val" for (i = 1; i <= 3; i++) { printf("Sort function: %s\n", f[i]) PROCINFO["sorted_in"] = f[i] for (j in data) printf("\tdata[%s] = %s\n", j, data[j]) print "" } } Here are the results when the program is run: $ gawk -f compdemo.awk Sort function: cmp_num_idx Sort by numeric index data[two] = 20 data[one] = 10 Both strings are numerically zero data[10] = one data[20] = two data[100] = 100 Sort function: cmp_str_val Sort by element values as strings data[one] = 10 330 | Chapter 12: Advanced Features of gawk data[100] = 100 String 100 is less than string 20 data[two] = 20 data[10] = one data[20] = two Sort function: cmp_num_str_val Sort all numeric values before all strings data[one] = 10 data[two] = 20 data[100] = 100 data[10] = one data[20] = two Consider sorting the entries of a GNU/Linux system password file according to login name. The following program sorts records by a specific field position and can be used for this purpose: # passwd-sort.awk --- simple program to sort by field position # field position is specified by the global variable POS function cmp_field(i1, v1, i2, v2) { # comparison by value, as string, and ascending order return v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS]) } { for (i = 1; i <= NF; i++) a[NR][i] = $i } END { PROCINFO["sorted_in"] = "cmp_field" if (POS < 1 || POS > NF) POS = 1 for (i in a) { for (j = 1; j <= NF; j++) printf("%s%c", a[i][j], j < NF ? ":" : "") print "" } } The first field in each entry of the password file is the user’s login name, and the fields are separated by colons. Each record defines a subarray, with each field as an element in the subarray. Running the program produces the following output: $ gawk -v POS=1 -F: -f sort.awk /etc/passwd adm:x:3:4:adm:/var/adm:/sbin/nologin apache:x:48:48:Apache:/var/www:/sbin/nologin avahi:x:70:70:Avahi daemon:/:/sbin/nologin … The comparison should normally always return the same value when given a specific pair of array elements as its arguments. If inconsistent results are returned, then the Controlling Array Traversal and Array Sorting | 331 order is undefined. This behavior can be exploited to introduce random order into otherwise seemingly ordered data: function cmp_randomize(i1, v1, i2, v2) { # random order (caution: this may never terminate!) return (2 - 4 * rand()) } As already mentioned, the order of the indices is arbitrary if two elements compare equal. This is usually not a problem, but letting the tied elements come out in arbitrary order can be an issue, especially when comparing item values. The partial ordering of the equal elements may change the next time the array is traversed, if other elements are added to or removed from the array. One way to resolve ties when comparing ele‐ ments with otherwise equal values is to include the indices in the comparison rules. Note that doing this may make the loop traversal less efficient, so consider it only if necessary. The following comparison functions force a deterministic order, and are based on the fact that the (string) indices of two elements are never equal: function cmp_numeric(i1, v1, i2, v2) { # numerical value (and index) comparison, descending order return (v1 != v2) ? (v2 - v1) : (i2 - i1) } function cmp_string(i1, v1, i2, v2) { # string value (and index) comparison, descending order v1 = v1 i1 v2 = v2 i2 return (v1 > v2) ? -1 : (v1 != v2) } A custom comparison function can often simplify ordered loop traversal, and the sky is really the limit when it comes to designing such a function. When string comparisons are made during a sort, either for element values where one or both aren’t numbers, or for element indices handled as strings, the value of IGNORE CASE (see “Predefined Variables” on page 160) controls whether the comparisons treat corresponding upper- and lowercase letters as equivalent or distinct. Another point to keep in mind is that in the case of subarrays, the element values can themselves be arrays; a production comparison function should use the isarray() function (see “Getting Type Information” on page 219) to check for this, and choose a defined sorting order for subarrays. All sorting based on PROCINFO["sorted_in"] is disabled in POSIX mode, because the PROCINFO array is not special in that case. 332 | Chapter 12: Advanced Features of gawk As a side note, sorting the array indices before traversing the array has been reported to add a 15% to 20% overhead to the execution time of awk programs. For this reason, sorted array traversal is not the default. Sorting Array Values and Indices with gawk In most awk implementations, sorting an array requires writing a sort() function. This can be educational for exploring different sorting algorithms, but usually that’s not the point of the program. gawk provides the built-in asort() and asorti() functions (see “String-Manipulation Functions” on page 194) for sorting arrays. For example: populate the array data n = asort(data) for (i = 1; i <= n; i++) do something with data[i] After the call to asort(), the array data is indexed from 1 to some number n, the total number of elements in data. (This count is asort()’s return value.) data[1] ≤ data[2] ≤ data[3], and so on. The default comparison is based on the type of the elements (see “Variable Typing and Comparison Expressions” on page 128). All numeric values come before all string values, which in turn come before all subarrays. An important side effect of calling asort() is that the array’s original indices are irrev‐ ocably lost. As this isn’t always desirable, asort() accepts a second argument: populate the array source n = asort(source, dest) for (i = 1; i <= n; i++) do something with dest[i] In this case, gawk copies the source array into the dest array and then sorts dest, destroying its indices. However, the source array is not affected. Often, what’s needed is to sort on the values of the indices instead of the values of the elements. To do that, use the asorti() function. The interface and behavior are identical to that of asort(), except that the index values are used for sorting and become the values of the result array: { source[$0] = some_func($0) } END { n = asorti(source, dest) for (i = 1; i <= n; i++) { Work with sorted indices directly: do something with dest[i] … Access original array via sorted indices: do something with source[dest[i]] Controlling Array Traversal and Array Sorting | 333 2. This is true because locale-based comparison occurs only when in POSIX-compatibility mode, and because asort() and asorti() are gawk extensions, they are not available in that case. } } So far, so good. Now it starts to get interesting. Both asort() and asorti() accept a third string argument to control comparison of array elements. When we introduced asort() and asorti() in “String-Manipulation Functions” on page 194, we ignored this third argument; however, now is the time to describe how this argument affects these two functions. Basically, the third argument specifies how the array is to be sorted. There are two possibilities. As with PROCINFO["sorted_in"], this argument may be one of the pre‐ defined names that gawk provides (see “Using Predefined Array Scanning Orders with gawk” on page 179), or it may be the name of a user-defined function (see “Controlling Array Traversal” on page 328). In the latter case, the function can compare elements in any way it chooses, taking into account just the indices, just the values, or both. This is extremely powerful. Once the array is sorted, asort() takes the values in their final order and uses them to fill in the result array, whereas asorti() takes the indices in their final order and uses them to fill in the result array. Copying array indices and elements isn’t expensive in terms of mem‐ ory. Internally, gawk maintains reference counts to data. For exam‐ ple, when asort() copies the first array to the second one, there is only one copy of the original array elements’ data, even though both arrays use the values. Because IGNORECASE affects string comparisons, the value of IGNORECASE also affects sorting for both asort() and asorti(). Note also that the locale’s sorting order does not come into play; comparisons are based on character values only.2 Two-Way Communications with Another Process It is often useful to be able to send data to a separate program for processing and then read the result. This can always be done with temporary files: # Write the data for processing tempfile = ("mydata." PROCINFO["pid"]) while (not done with data) print data | ("subprogram > " tempfile) close("subprogram > " tempfile) 334 | Chapter 12: Advanced Features of gawk 3. Michael Brennan suggests the use of rand() to generate unique filenames. This is a valid point; nevertheless, temporary files remain more difficult to use than two-way pipes. 4. This is very different from the same operator in the C shell and in Bash. # Read the results, remove tempfile when done while ((getline newdata < tempfile) > 0) process newdata appropriately close(tempfile) system("rm " tempfile) This works, but not elegantly. Among other things, it requires that the program be run in a directory that cannot be shared among users; for example, /tmp will not do, as another user might happen to be using a temporary file with the same name.3 However, with gawk, it is possible to open a two-way pipe to another process. The second process is termed a coprocess, as it runs in parallel with gawk. The two-way connection is created using the ‘|&’ operator (borrowed from the Korn shell, ksh):4 do { print data |& "subprogram" "subprogram" |& getline results } while (data left to process) close("subprogram") The first time an I/O operation is executed using the ‘|&’ operator, gawk creates a two- way pipeline to a child process that runs the other program. Output created with print or printf is written to the program’s standard input, and output from the program’s standard output can be read by the gawk program using getline. As is the case with processes started by ‘|’, the subprogram can be any program, or pipeline of programs, that can be started by the shell. There are some cautionary items to be aware of: • As the code inside gawk currently stands, the coprocess’s standard error goes to the same place that the parent gawk’s standard error goes. It is not possible to read the child’s standard error separately. • I/O buffering may be a problem. gawk automatically flushes all output down the pipe to the coprocess. However, if the coprocess does not flush its output, gawk may hang when doing a getline in order to read the coprocess’s results. This could lead to a situation known as deadlock, where each process is waiting for the other one to do something. It is possible to close just one end of the two-way pipe to a coprocess, by supplying a second argument to the close() function of either "to" or "from" (see “Closing Input Two-Way Communications with Another Process | 335 and Output Redirections” on page 105). These strings tell gawk to close the end of the pipe that sends data to the coprocess or the end that reads from it, respectively. This is particularly necessary in order to use the system sort utility as part of a copro‐ cess; sort must read all of its input data before it can produce any output. The sort program does not receive an end-of-file indication until gawk closes the write end of the pipe. When you have finished writing data to the sort utility, you can close the "to" end of the pipe, and then start reading sorted data via getline. For example: BEGIN { command = "LC_ALL=C sort" n = split("abcdefghijklmnopqrstuvwxyz", a, "") for (i = n; i > 0; i--) print a[i] |& command close(command, "to") while ((command |& getline line) > 0) print "got", line close(command) } This program writes the letters of the alphabet in reverse order, one per line, down the two-way pipe to sort. It then closes the write end of the pipe, so that sort receives an end-of-file indication. This causes sort to sort the data and write the sorted data back to the gawk program. Once all of the data has been read, gawk terminates the coprocess and exits. As a side note, the assignment ‘LC_ALL=C’ in the sort command ensures traditional Unix (ASCII) sorting from sort. This is not strictly necessary here, but it’s good to know how to do this. You may also use pseudo-ttys (ptys) for two-way communication instead of pipes, if your system supports them. This is done on a per-command basis, by setting a special element in the PROCINFO array (see “Built-in Variables That Convey Information” on page 163), like so: command = "sort -nr" # command, save in convenience variable PROCINFO[command, "pty"] = 1 # update PROCINFO print … |& command # start two-way pipe … Using ptys usually avoids the buffer deadlock issues described earlier, at some loss in performance. If your system does not have ptys, or if all the system’s ptys are in use, gawk automatically falls back to using regular pipes. 336 | Chapter 12: Advanced Features of gawk Using gawk for Network Programming EMISTERED: A host is a host from coast to coast, and nobody talks to a host that’s close, unless the host that isn’t close is busy, hung, or dead. —Mike O’Brien (aka Mr. Protocol) In addition to being able to open a two-way pipeline to a coprocess on the same system (see “Two-Way Communications with Another Process” on page 334), it is possible to make a two-way connection to another process on another system across an IP net‐ work connection. You can think of this as just a very long two-way pipeline to a coprocess. The way gawk decides that you want to use TCP/IP networking is by recognizing special filenames that begin with one of ‘/inet/’, ‘/inet4/’, or ‘/inet6/’. The full syntax of the special filename is /net-type/protocol/local-port/remote- host/remote-port. The components are: net-type Specifies the kind of Internet connection to make. Use ‘/inet4/’ to force IPv4, and ‘/inet6/’ to force IPv6. Plain ‘/inet/’ (which used to be the only option) uses the system default, most likely IPv4. protocol The protocol to use over IP. This must be either ‘tcp’, or ‘udp’, for a TCP or UDP IP connection, respectively. TCP should be used for most applications. local-port The local TCP or UDP port number to use. Use a port number of ‘0’ when you want the system to pick a port. This is what you should do when writing a TCP or UDP client. You may also use a well-known service name, such as ‘smtp’ or ‘http’, in which case gawk attempts to determine the predefined port number using the C getaddrinfo() function. remote-host The IP address or fully qualified domain name of the Internet host to which you want to connect. remote-port The TCP or UDP port number to use on the given remote-host. Again, use ‘0’ if you don’t care, or else a well-known service name. Using gawk for Network Programming | 337 Failure in opening a two-way socket will result in a nonfatal error being returned to the calling code. The value of ERRNO indicates the error (see “Built-in Variables That Convey Information” on page 163). Consider the following very simple example: BEGIN { Service = "/inet/tcp/0/localhost/daytime" Service |& getline print $0 close(Service) } This program reads the current date and time from the local system’s TCP daytime server. It then prints the results and closes the connection. Because this topic is extensive, the use of gawk for TCP/IP programming is documented separately. See TCP/IP Internetworking with gawk, which comes as part of the gawk distribution, for a much more complete introduction and discussion, as well as extensive examples. Profiling Your awk Programs You may produce execution traces of your awk programs. This is done by passing the option --profile to gawk. When gawk has finished running, it creates a profile of your program in a file named awkprof.out. Because it is profiling, it also executes up to 45% slower than gawk normally does. As shown in the following example, the --profile option can be used to change the name of the file where gawk will write the profile: gawk --profile=myprog.prof -f myprog.awk data1 data2 In the preceding example, gawk places the profile in myprog.prof instead of in awk prof.out. Here is a sample session showing a simple awk program, its input data, and the results from running gawk with the --profile option. First, the awk program: BEGIN { print "First BEGIN rule" } END { print "First END rule" } /foo/ { print "matched /foo/, gosh" for (i = 1; i <= 3; i++) sing() 338 | Chapter 12: Advanced Features of gawk } { if (/foo/) print "if is true" else print "else is true" } BEGIN { print "Second BEGIN rule" } END { print "Second END rule" } function sing( dummy) { print "I gotta be me!" } Following is the input data: foo bar baz foo junk Here is the awkprof.out that results from running the gawk profiler on this program and data (this example also illustrates that awk programmers sometimes get up very early in the morning to work): # gawk profile, created Mon Sep 29 05:16:21 2014 # BEGIN rule(s) BEGIN { 1 print "First BEGIN rule" } BEGIN { 1 print "Second BEGIN rule" } # Rule(s) 5 /foo/ { # 2 2 print "matched /foo/, gosh" 6 for (i = 1; i <= 3; i++) { 6 sing() } } 5 { 5 if (/foo/) { # 2 Profiling Your awk Programs | 339 2 print "if is true" 3 } else { 3 print "else is true" } } # END rule(s) END { 1 print "First END rule" } END { 1 print "Second END rule" } # Functions, listed alphabetically 6 function sing(dummy) { 6 print "I gotta be me!" } This example illustrates many of the basic features of profiling output. They are as follows: • The program is printed in the order BEGIN rules, BEGINFILE rules, pattern–action rules, ENDFILE rules, END rules, and functions, listed alphabetically. Multiple BEGIN and END rules retain their separate identities, as do multiple BEGINFILE and END FILE rules. • Pattern–action rules have two counts. The first count, to the left of the rule, shows how many times the rule’s pattern was tested. The second count, to the right of the rule’s opening left brace in a comment, shows how many times the rule’s action was executed. The difference between the two indicates how many times the rule’s pat‐ tern evaluated to false. • Similarly, the count for an if-else statement shows how many times the condition was tested. To the right of the opening left brace for the if’s body is a count showing how many times the condition was true. The count for the else indicates how many times the test failed. • The count for a loop header (such as for or while) shows how many times the loop test was executed. (Because of this, you can’t just look at the count on the first statement in a rule to determine how many times the rule was executed. If the first statement is a loop, the count is misleading.) 340 | Chapter 12: Advanced Features of gawk • For user-defined functions, the count next to the function keyword indicates how many times the function was called. The counts next to the statements in the body show how many times those statements were executed. • The layout uses “K&R” style with TABs. Braces are used everywhere, even when the body of an if, else, or loop is only a single statement. • Parentheses are used only where needed, as indicated by the structure of the pro‐ gram and the precedence rules. For example, ‘(3 + 5) * 4’ means add three and five, then multiply the total by four. However, ‘3 + 5 * 4’ has no parentheses, and means ‘3 + (5 * 4)’. • Parentheses are used around the arguments to print and printf only when the print or printf statement is followed by a redirection. Similarly, if the target of a redirection isn’t a scalar, it gets parenthesized. • gawk supplies leading comments in front of the BEGIN and END rules, the BEGIN FILE and ENDFILE rules, the pattern–action rules, and the functions. The profiled version of your program may not look exactly like what you typed when you wrote it. This is because gawk creates the profiled version by “pretty-printing” its internal representation of the program. The advantage to this is that gawk can produce a standard representation. The disadvantage is that all source code comments are lost. Also, things such as: /foo/ come out as: /foo/ { print $0 } which is correct, but possibly unexpected. Besides creating profiles when a program has completed, gawk can produce a profile while it is running. This is useful if your awk program goes into an infinite loop and you want to see what has been executed. To use this feature, run gawk with the --profile option in the background: $ gawk --profile -f myprog & [1] 13992 The shell prints a job number and process ID number; in this case, 13992. Use the kill command to send the USR1 signal to gawk: $ kill -USR1 13992 As usual, the profiled version of the program is written to awkprof.out, or to a different file if one was specified with the --profile option. Profiling Your awk Programs | 341 Along with the regular profile, as shown earlier, the profile file includes a trace of any active functions: # Function Call Stack: # 3. baz # 2. bar # 1. foo # -- main -- You may send gawk the USR1 signal as many times as you like. Each time, the profile and function call trace are appended to the output profile file. If you use the HUP signal instead of the USR1 signal, gawk produces the profile and the function call trace and then exits. When gawk runs on MS-Windows systems, it uses the INT and QUIT signals for pro‐ ducing the profile, and in the case of the INT signal, gawk exits. This is because these systems don’t support the kill command, so the only signals you can deliver to a pro‐ gram are those generated by the keyboard. The INT signal is generated by the Ctrl-c or Ctrl-BREAK key, while the QUIT signal is generated by the Ctrl-\ key. Finally, gawk also accepts another option, --pretty-print. When called this way, gawk “pretty-prints” the program into awkprof.out, without any execution counts. The --pretty-print option still runs your program. This will change in the next major release. Summary • The --non-decimal-data option causes gawk to treat octal- and hexadecimal- looking input data as octal and hexadecimal. This option should be used with cau‐ tion or not at all; use of strtonum() is preferable. Note that this option may disap‐ pear in a future version of gawk. • You can take over complete control of sorting in ‘for (indx in array)’ array traversal by setting PROCINFO["sorted_in"] to the name of a user-defined function that does the comparison of array elements based on index and value. • Similarly, you can supply the name of a user-defined comparison function as the third argument to either asort() or asorti() to control how those functions sort arrays. Or you may provide one of the predefined control strings that work for PROCINFO["sorted_in"]. 342 | Chapter 12: Advanced Features of gawk • You can use the ‘|&’ operator to create a two-way pipe to a coprocess. You read from the coprocess with getline and write to it with print or printf. Use close() to close off the coprocess completely, or optionally close off one side of the two-way communications. • By using special filenames with the ‘|&’ operator, you can open a TCP/IP (or UDP/ IP) connection to remote hosts on the Internet. gawk supports both IPv4 and IPv6. • You can generate statement count profiles of your program. This can help you determine which parts of your program may be taking the most time and let you tune them more easily. Sending the USR1 signal while profiling causes gawk to dump the profile and keep going, including a function call stack. • You can also just “pretty-print” the program. This currently also runs the program, but that will change in the next major release. Summary | 343 CHAPTER 13 Internationalization with gawk Once upon a time, computer makers wrote software that worked only in English. Even‐ tually, hardware and software vendors noticed that if their systems worked in the native languages of non-English-speaking countries, they were able to sell more systems. As a result, internationalization and localization of programs and software systems became a common practice. For many years, the ability to provide internationalization was largely restricted to pro‐ grams written in C and C++. This chapter describes the underlying library gawk uses for internationalization, as well as how gawk makes internationalization features avail‐ able at the awk program level. Having internationalization available at the awk level gives software developers additional flexibility—they are no longer forced to write in C or C++ when internationalization is a requirement. Internationalization and Localization Internationalization means writing (or modifying) a program once, in such a way that it can use multiple languages without requiring further source code changes. Localiza‐ tion means providing the data necessary for an internationalized program to work in a particular language. Most typically, these terms refer to features such as the language used for printing error messages, the language used to read responses, and information related to how numerical and monetary values are printed and read. 345 1. For some operating systems, the gawk port doesn’t support GNU gettext. Therefore, these features are not available if you are using one of those operating systems. Sorry. GNU gettext gawk uses GNU gettext to provide its internationalization features. The facilities in GNU gettext focus on messages: strings printed by a program, either directly or via formatting with printf or sprintf().1 When using GNU gettext, each application has its own text domain. This is a unique name, such as ‘kpilot’ or ‘gawk’, that identifies the application. A complete application may have multiple components—programs written in C or C++, as well as scripts writ‐ ten in sh or awk. All of the components use the same text domain. To make the discussion concrete, assume we’re writing an application named guide. Internationalization consists of the following steps, in this order: 1. The programmer reviews the source for all of guide’s components and marks each string that is a candidate for translation. For example, "`-F': option required" is a good candidate for translation. A table with strings of option names is not (e.g., gawk’s --profile option should remain the same, no matter what the local language). 2. The programmer indicates the application’s text domain ("guide") to the get- text library, by calling the textdomain() function. 3. Messages from the application are extracted from the source code and collected into a portable object template file (guide.pot), which lists the strings and their translations. The translations are initially empty. The original (usually English) messages serve as the key for lookup of the translations. 4. For each language with a translator, guide.pot is copied to a portable object file (.po) and translations are created and shipped with the application. For example, there might be a fr.po for a French translation. 5. Each language’s .po file is converted into a binary message object (.gmo) file. A message object file contains the original messages and their translations in a binary format that allows fast lookup of translations at runtime. 6. When guide is built and installed, the binary translation files are installed in a standard place. 7. For testing and development, it is possible to tell gettext to use .gmo files in a different directory than the standard one by using the bindtextdomain() function. 346 | Chapter 13: Internationalization with gawk 8. At runtime, guide looks up each string via a call to gettext(). The returned string is the translated string if available, or the original string if not. 9. If necessary, it is possible to access messages from a different text domain than the one belonging to the application, without having to switch the application’s default text domain back and forth. In C (or C++), the string marking and dynamic translation lookup are accomplished by wrapping each string in a call to gettext(): printf("%s", gettext("Don't Panic!\n")); The tools that extract messages from source code pull out all strings enclosed in calls to gettext(). The GNU gettext developers, recognizing that typing ‘gettext(…)’ over and over again is both painful and ugly to look at, use the macro ‘_’ (an underscore) to make things easier: /* In the standard header file: */ #define _(str) gettext(str) /* In the program text: */ printf("%s", _("Don't Panic!\n")); This reduces the typing overhead to just three extra characters per string and is con‐ siderably easier to read as well. There are locale categories for different types of locale-related information. The defined locale categories that gettext knows about are: LC_MESSAGES Text messages. This is the default category for gettext operations, but it is possible to supply a different one explicitly, if necessary. (It is almost never necessary to supply a different category.) LC_COLLATE Text-collation information (i.e., how different characters and/or groups of charac‐ ters sort in a given language). LC_CTYPE Character-type information (alphabetic, digit, upper- or lowercase, and so on) as well as character encoding. This information is accessed via the POSIX character classes in regular expressions, such as /[[:alnum:]]/ (see “Using Bracket Expres‐ sions” on page 46). LC_MONETARY Monetary information, such as the currency symbol, and whether the symbol goes before or after a number. GNU gettext | 347 2. Americans use a comma every three decimal places and a period for the decimal point, while many Europeans do exactly the opposite: 1,234.56 versus 1.234,56. LC_NUMERIC Numeric information, such as which characters to use for the decimal point and the thousands separator.2 LC_TIME Time- and date-related information, such as 12- or 24-hour clock, month printed before or after the day in a date, local month abbreviations, and so on. LC_ALL All of the above. (Not too useful in the context of gettext.) Internationalizing awk Programs gawk provides the following variables for internationalization: TEXTDOMAIN This variable indicates the application’s text domain. For compatibility with GNU gettext, the default value is "messages". _"your message here" String constants marked with a leading underscore are candidates for translation at runtime. String constants without a leading underscore are not translated. gawk provides the following functions for internationalization: dcgettext(string [, domain [, category]]) Return the translation of string in text domain domain for locale category catego ry. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES". If you supply a value for category, it must be a string equal to one of the known locale categories described in the previous section. You must also supply a text domain. Use TEXTDOMAIN if you want to use the current domain. The order of arguments to the awk version of the dcget text() function is purposely different from the order for the C version. The awk version’s order was chosen to be simple and to allow for reasonable awk-style default arguments. 348 | Chapter 13: Internationalization with gawk 3. Thanks to Bruno Haible for this example. dcngettext(string1, string2, number [, domain [, category]]) Return the plural form used for number of the translation of string1 and string2 in text domain domain for locale category category. string1 is the English singular variant of a message, and string2 is the English plural variant of the same message. The default value for domain is the current value of TEXTDOMAIN. The default value for category is "LC_MESSAGES". The same remarks about argument order as for the dcgettext() function apply. bindtextdomain(directory [, domain ]) Change the directory in which gettext looks for .gmo files, in case they will not or cannot be placed in the standard locations (e.g., during testing). Return the di‐ rectory in which domain is “bound.” The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then bindtextdomain() returns the current binding for the given domain. To use these facilities in your awk program, follow these steps: 1. Set the variable TEXTDOMAIN to the text domain of your program. This is best done in a BEGIN rule (see “The BEGIN and END Special Patterns” on page 145), or it can also be done via the -v command-line option (see “Command-Line Options” on page 22): BEGIN { TEXTDOMAIN = "guide" … } 2. Mark all translatable strings with a leading underscore (‘_’) character. It must be adjacent to the opening quote of the string. For example: print _"hello, world" x = _"you goofed" printf(_"Number of users is %d\n", nusers) 3. If you are creating strings dynamically, you can still translate them, using the dcget text() built-in function:3 if (groggy) message = dcgettext("%d customers disturbing me\n", "adminprog") else message = dcgettext("enjoying %d customers\n", "adminprog") printf(message, ncustomers) Here, the call to dcgettext() supplies a different text domain ("adminprog") in which to find the message, but it uses the default "LC_MESSAGES" category. Internationalizing awk Programs | 349 The previous example only works if ncustomers is greater than one. This example would be better done with dcngettext(): if (groggy) message = dcngettext("%d customer disturbing me\n", "%d customers disturbing me\n", "adminprog") else message = dcngettext("enjoying %d customer\n", "enjoying %d customers\n", "adminprog") printf(message, ncustomers) 4. During development, you might want to put the .gmo file in a private directory for testing. This is done with the bindtextdomain() built-in function: BEGIN { TEXTDOMAIN = "guide" # our text domain if (Testing) { # where to find our files bindtextdomain("testdir") # joe is in charge of adminprog bindtextdomain("../joe/testdir", "adminprog") } … } See “A Simple Internationalization Example” on page 353 for an example program showing the steps to create and use translations from awk. Translating awk Programs Once a program’s translatable strings have been marked, they must be extracted to create the initial .pot file. As part of translation, it is often helpful to rearrange the order in which arguments to printf are output. gawk’s --gen-pot command-line option extracts the messages and is discussed next. After that, printf’s ability to rearrange the order for printf arguments at runtime is covered. Extracting Marked Strings Once your awk program is working, and all the strings have been marked and you’ve set (and perhaps bound) the text domain, it is time to produce translations. First, use the --gen-pot command-line option to create the initial .pot file: gawk --gen-pot -f guide.awk > guide.pot When run with --gen-pot, gawk does not execute your program. Instead, it parses it as usual and prints all marked strings to standard output in the format of a GNU gettext Portable Object file. Also included in the output are any constant strings that 350 | Chapter 13: Internationalization with gawk 4. The xgettext utility that comes with GNU gettext can handle .awk files. 5. This example is borrowed from the GNU gettext manual. appear as the first argument to dcgettext() or as the first and second argument to dcngettext().4 You should distribute the generated .pot file with your awk program; translators will eventually use it to provide you translations that you can also then dis‐ tribute. See “A Simple Internationalization Example” on page 353 for the full list of steps to go through to create and test translations for guide. Rearranging printf Arguments Format strings for printf and sprintf() (see “Using printf Statements for Fancier Printing” on page 93) present a special problem for translation. Consider the following:5 printf(_"String `%s' has %d characters\n", string, length(string))) A possible German translation for this might be: "%d Zeichen lang ist die Zeichenkette `%s'\n" The problem should be obvious: the order of the format specifications is different from the original! Even though gettext() can return the translated string at runtime, it cannot change the argument order in the call to printf. To solve this problem, printf format specifiers may have an additional optional ele‐ ment, which we call a positional specifier. For example: "%2$d Zeichen lang ist die Zeichenkette `%1$s'\n" Here, the positional specifier consists of an integer count, which indicates which argu‐ ment to use, and a ‘$’. Counts are one-based, and the format string itself is not included. Thus, in the following example, ‘string’ is the first argument and ‘length(string)’ is the second: $ gawk 'BEGIN { > string = "Don\47t Panic" > printf "%2$d characters live in \"%1$s\"\n", > string, length(string) > }' 11 characters live in "Don't Panic" If present, positional specifiers come first in the format specification, before the flags, the field width, and/or the precision. Positional specifiers can be used with the dynamic field width and precision capability: Translating awk Programs | 351 $ gawk 'BEGIN { > printf("%*.*s\n", 10, 20, "hello") > printf("%3$*2$.*1$s\n", 20, 10, "hello") > }' hello hello When using ‘*’ with a positional specifier, the ‘*’ comes first, then the integer position, and then the ‘$’. This is somewhat counterintuitive. gawk does not allow you to mix regular format specifiers and those with positional specifiers in the same string: $ gawk 'BEGIN { printf "%d %3$s\n", 1, 2, "hi" }' error→ gawk: cmd. line:1: fatal: must use `count$' on all formats or none There are some pathological cases that gawk may fail to diagnose. In such cases, the output may not be what you expect. It’s still a bad idea to try mixing them, even if gawk doesn’t detect it. Although positional specifiers can be used directly in awk programs, their primary pur‐ pose is to help in producing correct translations of format strings into languages dif‐ ferent from the one in which the program is first written. awk Portability Issues gawk’s internationalization features were purposely chosen to have as little impact as possible on the portability of awk programs that use them to other versions of awk. Consider this program: BEGIN { TEXTDOMAIN = "guide" if (Test_Guide) # set with -v bindtextdomain("/test/guide/messages") print _"don't panic!" } As written, it won’t work on other versions of awk. However, it is actually almost portable, requiring very little change: • Assignments to TEXTDOMAIN won’t have any effect, because TEXTDOMAIN is not special in other awk implementations. 352 | Chapter 13: Internationalization with gawk 6. This is good fodder for an “Obfuscated awk” contest. • Non-GNU versions of awk treat marked strings as the concatenation of a variable named _ with the string following it.6 Typically, the variable _ has the null string ("") as its value, leaving the original string constant as the result. • By defining “dummy” functions to replace dcgettext(), dcngettext(), and bind textdomain(), the awk program can be made to run, but all the messages are output in the original language. For example: function bindtextdomain(dir, domain) { return dir } function dcgettext(string, domain, category) { return string } function dcngettext(string1, string2, number, domain, category) { return (number == 1 ? string1 : string2) } • The use of positional specifications in printf or sprintf() is not portable. To support gettext() at the C level, many systems’ C versions of sprintf() do support positional specifiers. But it works only if enough arguments are supplied in the function call. Many versions of awk pass printf formats and arguments unchanged to the underlying C library version of sprintf(), but only one format and argument at a time. What happens if a positional specification is used is anybody’s guess. However, because the positional specifications are primarily for use in translated format strings, and because non-GNU awks never retrieve the translated string, this should not be a problem in practice. A Simple Internationalization Example Now let’s look at a step-by-step example of how to internationalize and localize a simple awk program, using guide.awk as our original source: BEGIN { TEXTDOMAIN = "guide" bindtextdomain(".") # for testing print _"Don't Panic" print _"The Answer Is", 42 print "Pardon me, Zaphod who?" } A Simple Internationalization Example | 353 7. Perhaps it would be better if it were called “Hippy.” Ah, well. Run ‘gawk --gen-pot’ to create the .pot file: $ gawk --gen-pot -f guide.awk > guide.pot This produces: #: guide.awk:4 msgid "Don't Panic" msgstr "" #: guide.awk:5 msgid "The Answer Is" msgstr "" This original portable object template file is saved and reused for each language into which the application is translated. The msgid is the original string and the msgstr is the translation. Strings not marked with a leading underscore do not appear in the guide.pot file. Next, the messages must be translated. Here is a translation to a hypothetical dialect of English, called “Mellow”:7 $ cp guide.pot guide-mellow.po Add translations to guide-mellow.po … Following are the translations: #: guide.awk:4 msgid "Don't Panic" msgstr "Hey man, relax!" #: guide.awk:5 msgid "The Answer Is" msgstr "Like, the scoop is" The next step is to make the directory to hold the binary message object file and then to create the guide.mo file. We pretend that our file is to be used in the en_US.UTF-8 locale, because we have to use a locale name known to the C gettext routines. The directory layout shown here is standard for GNU gettext on GNU/Linux systems. Other versions of gettext may use a different layout: $ mkdir en_US.UTF-8 en_US.UTF-8/LC_MESSAGES 354 | Chapter 13: Internationalization with gawk The msgfmt utility does the conversion from human-readable .po file to machine- readable .mo file. By default, msgfmt creates a file named messages. This file must be renamed and placed in the proper directory (using the -o option) so that gawk can find it: $ msgfmt guide-mellow.po -o en_US.UTF-8/LC_MESSAGES/guide.mo Finally, we run the program to test it: $ gawk -f guide.awk Hey man, relax! Like, the scoop is 42 Pardon me, Zaphod who? If the three replacement functions for dcgettext(), dcngettext(), and bindtextdo main() (see “awk Portability Issues” on page 352) are in a file named libintl.awk, then we can run guide.awk unchanged as follows: $ gawk --posix -f guide.awk -f libintl.awk Don't Panic The Answer Is 42 Pardon me, Zaphod who? gawk Can Speak Your Language gawk itself has been internationalized using the GNU gettext package. (GNU get- text is described in complete detail in GNU gettext utilities.) As of this writing, the latest version of GNU gettext is version 0.19.4. If a translation of gawk’s messages exists, then gawk produces usage messages, warnings, and fatal errors in the local language. Summary • Internationalization means writing a program such that it can use multiple lan‐ guages without requiring source code changes. Localization means providing the data necessary for an internationalized program to work in a particular language. • gawk uses GNU gettext to let you internationalize and localize awk programs. A program’s text domain identifies the program for grouping all messages and other data together. • You mark a program’s strings for translation by preceding them with an underscore. Once that is done, the strings are extracted into a .pot file. This file is copied for each language into a .po file, and the .po files are compiled into .gmo files for use at runtime. gawk Can Speak Your Language | 355 • You can use positional specifications with sprintf() and printf to rearrange the placement of argument values in formatted strings and output. This is useful for the translation of format control strings. • The internationalization features have been designed so that they can be easily worked around in a standard awk. • gawk itself has been internationalized and ships with a number of translations for its messages. 356 | Chapter 13: Internationalization with gawk CHAPTER 14 Debugging awk Programs It would be nice if computer programs worked perfectly the first time they were run, but in real life, this rarely happens for programs of any complexity. Thus, most pro‐ gramming languages have facilities available for “debugging” programs, and now awk is no exception. The gawk debugger is purposely modeled after the GNU Debugger (GDB) command- line debugger. If you are familiar with GDB, learning how to use gawk for debugging your program is easy. Introduction to the gawk Debugger This section introduces debugging in general and begins the discussion of debugging in gawk. Debugging in General (If you have used debuggers in other languages, you may want to skip ahead to “awk Debugging” on page 359.) Of course, a debugging program cannot remove bugs for you, because it has no way of knowing what you or your users consider a “bug” versus a “feature.” (Sometimes, we humans have a hard time with this ourselves.) In that case, what can you expect from such a tool? The answer to that depends on the language being debugged, but in general, you can expect at least the following: • The ability to watch a program execute its instructions one by one, giving you, the programmer, the opportunity to think about what is happening on a time scale of seconds, minutes, or hours, rather than the nanosecond time scale at which the code usually runs. 357 • The opportunity to not only passively observe the operation of your program, but to control it and try different paths of execution, without having to change your source files. • The chance to see the values of data in the program at any point in execution, and also to change that data on the fly, to see how that affects what happens afterward. (This often includes the ability to look at internal data structures besides the vari‐ ables you actually defined in your code.) • The ability to obtain additional information about your program’s state or even its internal structure. All of these tools provide a great amount of help in using your own skills and under‐ standing of the goals of your program to find where it is going wrong (or, for that matter, to better comprehend a perfectly functional program that you or someone else has written). Debugging Concepts Before diving into the details, we need to introduce several important concepts that apply to just about all debuggers. The following list defines terms used throughout the rest of this chapter: Stack frame Programs generally call functions during the course of their execution. One func‐ tion can call another, or a function can call itself (recursion). You can view the chain of called functions (main program calls A, which calls B, which calls C) as a stack of executing functions: the currently running function is the topmost one on the stack, and when it finishes (returns), the next one down then becomes the active function. Such a stack is termed a call stack. For each function on the call stack, the system maintains a data area that contains the function’s parameters, local variables, and return value, as well as any other “bookkeeping” information needed to manage the call stack. This data area is termed a stack frame. gawk also follows this model, and gives you access to the call stack and to each stack frame. You can see the call stack, as well as where in the program each function on the stack was invoked. Commands that print the call stack print information about each stack frame (as detailed later on). Breakpoint During debugging, you often wish to let the program run until it reaches a certain point, and then continue execution from there one statement (or instruction) at a time. The way to do this is to set a breakpoint within the program. A breakpoint is where the execution of the program should break off (stop), so that you can take 358 | Chapter 14: Debugging awk Programs over control of the program’s execution. You can add and remove as many break‐ points as you like. Watchpoint A watchpoint is similar to a breakpoint. The difference is that breakpoints are ori‐ ented around the code: stop when a certain point in the code is reached. A watch‐ point, however, specifies that program execution should stop when a data value is changed. This is useful, as sometimes it happens that a variable receives an erro‐ neous value, and it’s hard to track down where this happens just by looking at the code. By using a watchpoint, you can stop whenever a variable is assigned to, and usually find the errant code quite quickly. awk Debugging Debugging an awk program has some specific aspects that are not shared with programs written in other languages. First of all, the fact that awk programs usually take input line by line from a file or files and operate on those lines using specific rules makes it especially useful to organize viewing the execution of the program in terms of these rules. As we will see, each awk rule is treated almost like a function call, with its own specific block of instructions. In addition, because awk is by design a very concise language, it is easy to lose sight of everything that is going on “inside” each line of awk code. The debugger provides the opportunity to look at the individual primitive instructions carried out by the higher- level awk commands. Sample gawk Debugging Session In order to illustrate the use of gawk as a debugger, let’s look at a sample debugging session. We will use the awk implementation of the POSIX uniq command described earlier (see “Printing Nonduplicated Lines of Text” on page 292) as our example. How to Start the Debugger Starting the debugger is almost exactly like running gawk normally, except you have to pass an additional option, --debug, or the corresponding short option, -D. The file(s) containing the program and any supporting code are given on the command line as arguments to one or more -f options. (gawk is not designed to debug command-line programs, only programs contained in files.) In our case, we invoke the debugger like this: $ gawk -D -f getopt.awk -f join.awk -f uniq.awk -1 inputfile Sample gawk Debugging Session | 359 where both getopt.awk and uniq.awk are in $AWKPATH. (Experienced users of GDB or similar debuggers should note that this syntax is slightly different from what you are used to. With the gawk debugger, you give the arguments for running the program in the command line to the debugger rather than as part of the run command at the debugger prompt.) The -1 is an option to uniq.awk. Instead of immediately running the program on inputfile, as gawk would ordinarily do, the debugger merely loads all the program source files, compiles them internally, and then gives us a prompt: gawk> from which we can issue commands to the debugger. At this point, no code has been executed. Finding the Bug Let’s say that we are having a problem using (a faulty version of) uniq.awk in the “field- skipping” mode, and it doesn’t seem to be catching lines that should be identical when skipping the first field, such as: awk is a wonderful program! gawk is a wonderful program! This could happen if we were thinking (C-like) of the fields in a record as being num‐ bered in a zero-based fashion, so instead of the lines: clast = join(alast, fcount+1, n) cline = join(aline, fcount+1, m) we wrote: clast = join(alast, fcount, n) cline = join(aline, fcount, m) The first thing we usually want to do when trying to investigate a problem like this is to put a breakpoint in the program so that we can watch it at work and catch what it is doing wrong. A reasonable spot for a breakpoint in uniq.awk is at the beginning of the function are_equal(), which compares the current line with the previous one. To set the breakpoint, use the b (breakpoint) command: gawk> b are_equal Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 63 The debugger tells us the file and line number where the breakpoint is. Now type ‘r’ or ‘run’ and the program runs until it hits the breakpoint for the first time: gawk> r Starting program: Stopping in Rule ... Breakpoint 1, are_equal(n, m, clast, cline, alast, aline) 360 | Chapter 14: Debugging awk Programs at `awklib/eg/prog/uniq.awk':63 63 if (fcount == 0 && charcount == 0) gawk> Now we can look at what’s going on inside our program. First of all, let’s see how we got to where we are. At the prompt, we type ‘bt’ (short for “backtrace”), and the debugger responds with a listing of the current stack frames: gawk> bt #0 are_equal(n, m, clast, cline, alast, aline) at `awklib/eg/prog/uniq.awk':68 #1 in main() at `awklib/eg/prog/uniq.awk':88 This tells us that are_equal() was called by the main program at line 88 of uniq.awk. (This is not a big surprise, because this is the only call to are_equal() in the program, but in more complex programs, knowing who called a function and with what param‐ eters can be the key to finding the source of the problem.) Now that we’re in are_equal(), we can start looking at the values of some variables. Let’s say we type ‘p n’ (p is short for “print”). We would expect to see the value of n, a parameter to are_equal(). Actually, the debugger gives us: gawk> p n n = untyped variable In this case, n is an uninitialized local variable, because the function was called without arguments (see “Function Calls” on page 134). A more useful variable to display might be the current record: gawk> p $0 $0 = "gawk is a wonderful program!" This might be a bit puzzling at first, as this is the second line of our test input. Let’s look at NR: gawk> p NR NR = 2 So we can see that are_equal() was only called for the second record of the file. Of course, this is because our program contains a rule for ‘NR == 1’: NR == 1 { last = $0 next } OK, let’s just check that that rule worked correctly: gawk> p last last = "awk is a wonderful program!" Sample gawk Debugging Session | 361 Everything we have done so far has verified that the program has worked as planned, up to and including the call to are_equal(), so the problem must be inside this function. To investigate further, we must begin “stepping through” the lines of are_equal(). We start by typing ‘n’ (for “next”): gawk> n 66 if (fcount > 0) { This tells us that gawk is now ready to execute line 66, which decides whether to give the lines the special “field-skipping” treatment indicated by the -1 command-line op‐ tion. (Notice that we skipped from where we were before, at line 63, to here, because the condition in line 63, ‘if (fcount == 0 && charcount == 0)’, was false.) Continuing to step, we now get to the splitting of the current and last records: gawk> n 67 n = split(last, alast) gawk> n 68 m = split($0, aline) At this point, we should be curious to see what our records were split into, so we try to look: gawk> p n m alast aline n = 5 m = untyped variable alast = array, 5 elements aline = untyped variable (The p command can take more than one argument, similar to awk’s print statement.) This is kind of disappointing, though. All we found out is that there are five elements in alast; m and aline don’t have values because we are at line 68 but haven’t executed it yet. This information is useful enough (we now know that none of the words were accidentally left out), but what if we want to see inside the array? The first choice would be to use subscripts: gawk> p alast[0] "0" not in array `alast' Oops! gawk> p alast[1] alast["1"] = "awk" This would be kind of slow for a 100-member array, though, so gawk provides a shortcut (reminiscent of another language not to be mentioned): gawk> p @alast alast["1"] = "awk" alast["2"] = "is" alast["3"] = "a" 362 | Chapter 14: Debugging awk Programs alast["4"] = "wonderful" alast["5"] = "program!" It looks like we got this far OK. Let’s take another step or two: gawk> n 69 clast = join(alast, fcount, n) gawk> n 70 cline = join(aline, fcount, m) Well, here we are at our error (sorry to spoil the suspense). What we had in mind was to join the fields starting from the second one to make the virtual record to compare, and if the first field were numbered zero, this would work. Let’s look at what we’ve got: gawk> p cline clast cline = "gawk is a wonderful program!" clast = "awk is a wonderful program!" Hey, those look pretty familiar! They’re just our original, unaltered input records. A little thinking (the human brain is still the best debugging tool), and we realize that we were off by one! We get out of the debugger: gawk> q The program is running. Exit anyway (y/n)? y Then we get into an editor: clast = join(alast, fcount+1, n) cline = join(aline, fcount+1, m) and problem solved! Main Debugger Commands The gawk debugger command set can be divided into the following categories: • Breakpoint control • Execution control • Viewing and changing data • Working with the stack • Getting information • Miscellaneous Each of these are discussed in the following subsections. In the following descriptions, commands that may be abbreviated show the abbreviation on a second description line. A debugger command name may also be truncated if that partial name is unambiguous. The debugger has the built-in capability to automatically repeat the previous command Main Debugger Commands | 363 just by hitting Enter. This works for the commands list, next, nexti, step, stepi, and continue executed without any argument. Control of Breakpoints As we saw earlier, the first thing you probably want to do in a debugging session is to get your breakpoints set up, because your program will otherwise just run as if it was not under the debugger. The commands for controlling breakpoints are: break [[filename:]n | function] ["expression"] b [[filename:]n | function] ["expression"] Without any argument, set a breakpoint at the next instruction to be executed in the selected stack frame. Arguments can be one of the following: n Set a breakpoint at line number n in the current source file. filename:n Set a breakpoint at line number n in source file filename. function Set a breakpoint at entry to (the first instruction of) function function. Each breakpoint is assigned a number that can be used to delete it from the break‐ point list using the delete command. With a breakpoint, you may also supply a condition. This is an awk expression (enclosed in double quotes) that the debugger evaluates whenever the breakpoint is reached. If the condition is true, then the debugger stops execution and prompts for a command. Otherwise, it continues executing the program. clear [[filename:]n | function] Without any argument, delete any breakpoint at the next instruction to be executed in the selected stack frame. If the program stops at a breakpoint, this deletes that breakpoint so that the program does not stop at that location again. Arguments can be one of the following: n Delete breakpoint(s) set at line number n in the current source file. filename:n Delete breakpoint(s) set at line number n in source file filename. function Delete breakpoint(s) set at entry to function function. 364 | Chapter 14: Debugging awk Programs condition n "expression" Add a condition to existing breakpoint or watchpoint n. The condition is an awk expression enclosed in double quotes that the debugger evaluates whenever the breakpoint or watchpoint is reached. If the condition is true, then the debugger stops execution and prompts for a command. Otherwise, the debugger continues executing the program. If the condition expression is not specified, any existing condition is removed (i.e., the breakpoint or watchpoint is made unconditional). delete [n1 n2 …] [n–m] d [n1 n2 …] [n–m] Delete specified breakpoints or a range of breakpoints. Delete all defined break‐ points if no argument is supplied. disable [n1 n2 … | n–m] Disable specified breakpoints or a range of breakpoints. Without any argument, disable all breakpoints. enable [del | once] [n1 n2 …] [n–m] e [del | once] [n1 n2 …] [n–m] Enable specified breakpoints or a range of breakpoints. Without any argument, enable all breakpoints. Optionally, you can specify how to enable the breakpoints: del Enable the breakpoints temporarily, then delete each one when the program stops at it. once Enable the breakpoints temporarily, then disable each one when the program stops at it. ignore n count Ignore breakpoint number n the next count times it is hit. tbreak [[filename:]n | function] t [[filename:]n | function] Set a temporary breakpoint (enabled for only one stop). The arguments are the same as for break. Control of Execution Now that your breakpoints are ready, you can start running the program and observing its behavior. There are more commands for controlling execution of the program than we saw in our earlier example: Main Debugger Commands | 365 commands [n] silent … end Set a list of commands to be executed upon stopping at a breakpoint or watchpoint. n is the breakpoint or watchpoint number. Without a number, the last one set is used. The actual commands follow, starting on the next line, and terminated by the end command. If the command silent is in the list, the usual messages about stopping at a breakpoint and the source line are not printed. Any command in the list that resumes execution (e.g., continue) terminates the list (an implicit end), and subsequent commands are ignored. For example: gawk> commands > silent > printf "A silent breakpoint; i = %d\n", i > info locals > set i = 10 > continue > end gawk> continue [count] c [count] Resume program execution. If continued from a breakpoint and count is specified, ignore the breakpoint at that location the next count times before stopping. finish Execute until the selected stack frame returns. Print the returned value. next [count] n [count] Continue execution to the next source line, stepping over function calls. The ar‐ gument count controls how many times to repeat the action, as in step. nexti [count] ni [count] Execute one (or count) instruction(s), stepping over function calls. return [value] Cancel execution of a function call. If value (either a string or a number) is speci‐ fied, it is used as the function’s return value. If used in a frame other than the in‐ nermost one (the currently executing function; i.e., frame number 0), discard all inner frames in addition to the selected one, and the caller of that frame becomes the innermost frame. 366 | Chapter 14: Debugging awk Programs run r Start/restart execution of the program. When restarting, the debugger retains the current breakpoints, watchpoints, command history, automatic display variables, and debugger options. step [count] s [count] Continue execution until control reaches a different source line in the current stack frame, stepping inside any function called within the line. If the argument count is supplied, steps that many times before stopping, unless it encounters a breakpoint or watchpoint. stepi [count] si [count] Execute one (or count) instruction(s), stepping inside function calls. (For illustra‐ tion of what is meant by an “instruction” in gawk, see the output shown under dump in “Miscellaneous Commands” on page 372.) until [[filename:]n | function] u [[filename:]n | function] Without any argument, continue execution until a line past the current line in the current stack frame is reached. With an argument, continue execution until the specified location is reached, or the current stack frame returns. Viewing and Changing Data The commands for viewing and changing variables inside of gawk are: display [var | $n] Add variable var (or field $n) to the display list. The value of the variable or field is displayed each time the program stops. Each variable added to the list is identified by a unique number: gawk> display x 10: x = 1 This displays the assigned item number, the variable name, and its current value. If the display variable refers to a function parameter, it is silently deleted from the list as soon as the execution reaches a context where no such variable of the given name exists. Without argument, display displays the current values of items on the list. Main Debugger Commands | 367 eval "awk statements" Evaluate awk statements in the context of the running program. You can do any‐ thing that an awk program would do: assign values to variables, call functions, and so on. eval param, … awk statements end This form of eval is similar, but it allows you to define “local variables” that exist in the context of the awk statements, instead of using variables or function pa‐ rameters defined by the program. print var1[, var2 …] p var1[, var2 …] Print the value of a gawk variable or field. Fields must be referenced by constants: gawk> print $3 This prints the third field in the input record (if the specified field does not exist, it prints ‘Null field’). A variable can be an array element, with the subscripts being constant string values. To print the contents of an array, prefix the name of the array with the ‘@’ symbol: gawk> print @a This prints the indices and the corresponding values for all elements in the array a. printf format [, arg …] Print formatted text. The format may include escape sequences, such as ‘\n’ (see “Escape Sequences” on page 40). No newline is printed unless one is specified. set var=value Assign a constant (number or string) value to an awk variable or field. String values must be enclosed between double quotes ("…"). You can also set special awk variables, such as FS, NF, NR, and so on. watch var | $n ["expression"] w var | $n ["expression"] Add variable var (or field $n) to the watch list. The debugger then stops whenever the value of the variable or field changes. Each watched item is assigned a number that can be used to delete it from the watch list using the unwatch command. With a watchpoint, you may also supply a condition. This is an awk expression (enclosed in double quotes) that the debugger evaluates whenever the watchpoint is reached. If the condition is true, then the debugger stops execution and prompts for a command. Otherwise, gawk continues executing the program. 368 | Chapter 14: Debugging awk Programs undisplay [n] Remove item number n (or all items, if no argument) from the automatic display list. unwatch [n] Remove item number n (or all items, if no argument) from the watch list. Working with the Stack Whenever you run a program that contains any function calls, gawk maintains a stack of all of the function calls leading up to where the program is right now. You can see how you got to where you are, and also move around in the stack to see what the state of things was in the functions that called the one you are in. The commands for doing this are: backtrace [count] bt [count] where [count] Print a backtrace of all function calls (stack frames), or innermost count frames if count > 0. Print the outermost count frames if count < 0. The backtrace displays the name and arguments to each function, the source filename, and the line num‐ ber. The alias where for backtrace is provided for longtime GDB users who may be used to that command. down [count] Move count (default 1) frames down the stack toward the innermost frame. Then select and print the frame. frame [n] f [n] Select and print stack frame n. Frame 0 is the currently executing, or innermost, frame (function call); frame 1 is the frame that called the innermost one. The highest-numbered frame is the one for the main program. The printed information consists of the frame number, function and argument names, source file, and the source line. up [count] Move count (default 1) frames up the stack toward the outermost frame. Then select and print the frame. Obtaining Information About the Program and the Debugger State Besides looking at the values of variables, there is often a need to get other sorts of information about the state of your program and of the debugging environment itself. The gawk debugger has one command that provides this information, appropriately Main Debugger Commands | 369 called info. info is used with one of a number of arguments that tell it exactly what you want to know: info what i what The value for what should be one of the following: args List arguments of the selected frame. break List all currently set breakpoints. display List all items in the automatic display list. frame Give a description of the selected stack frame. functions List all function definitions including source filenames and line numbers. locals List local variables of the selected frame. source Print the name of the current source file. Each time the program stops, the current source file is the file containing the current instruction. When the de‐ bugger first starts, the current source file is the first file included via the -f option. The ‘list filename:lineno’ command can be used at any time to change the current source. sources List all program sources. variables List all global variables. watch List all items in the watch list. Additional commands give you control over the debugger, the ability to save the de‐ bugger’s state, and the ability to run debugger commands from a file. The commands are: 370 | Chapter 14: Debugging awk Programs option [name[=value]] o [name[=value]] Without an argument, display the available debugger options and their current values. ‘option name’ shows the current value of the named option. ‘option name=value’ assigns a new value to the named option. The available options are: history_size Set the maximum number of lines to keep in the history file ./.gawk_history. The default is 100. listsize Specify the number of lines that list prints. The default is 15. outfile Send gawk output to a file; debugger output still goes to standard output. An empty string ("") resets output to standard output. prompt Change the debugger prompt. The default is ‘gawk> ’. save_history [on | off] Save command history to file ./.gawk_history. The default is on. save_options [on | off] Save current options to file ./.gawkrc upon exit. The default is on. Options are read back into the next session upon startup. trace [on | off] Turn instruction tracing on or off. The default is off. save filename Save the commands from the current session to the given filename, so that they can be replayed using the source command. source filename Run command(s) from a file; an error in any command does not terminate execu‐ tion of subsequent commands. Comments (lines starting with ‘#’) are allowed in a command file. Empty lines are ignored; they do not repeat the last command. You can’t restart the program by having more than one run command in the file. Also, the list of commands may include additional source commands; however, the gawk debugger will not source the same file more than once in order to avoid infinite recursion. In addition to, or instead of, the source command, you can use the -D file or --debug=file command-line options to execute commands from a file non- interactively (see “Command-Line Options” on page 22). Main Debugger Commands | 371 Miscellaneous Commands There are a few more commands that do not fit into the previous categories, as follows: dump [filename] Dump byte code of the program to standard output or to the file named in file name. This prints a representation of the internal instructions that gawk executes to implement the awk commands in a program. This can be very enlightening, as the following partial dump of Davide Brini’s obfuscated code (see “And Now for Some‐ thing Completely Different” on page 323) demonstrates: gawk> dump # BEGIN [ 1:0xfcd340] Op_rule : [in_rule = BEGIN] [source_file = brini.awk] [ 1:0xfcc240] Op_push_i : "~" [MALLOC|STRING|STRCUR] [ 1:0xfcc2a0] Op_push_i : "~" [MALLOC|STRING|STRCUR] [ 1:0xfcc280] Op_match : [ 1:0xfcc1e0] Op_store_var : O [ 1:0xfcc2e0] Op_push_i : "==" [MALLOC|STRING|STRCUR] [ 1:0xfcc340] Op_push_i : "==" [MALLOC|STRING|STRCUR] [ 1:0xfcc320] Op_equal : [ 1:0xfcc200] Op_store_var : o [ 1:0xfcc380] Op_push : o [ 1:0xfcc360] Op_plus_i : 0 [MALLOC|NUMCUR|NUMBER] [ 1:0xfcc220] Op_push_lhs : o [do_reference = true] [ 1:0xfcc300] Op_assign_plus : [ :0xfcc2c0] Op_pop : [ 1:0xfcc400] Op_push : O [ 1:0xfcc420] Op_push_i : "" [MALLOC|STRING|STRCUR] [ :0xfcc4a0] Op_no_op : [ 1:0xfcc480] Op_push : O [ :0xfcc4c0] Op_concat : [expr_count = 3] [concat_flag = 0] [ 1:0xfcc3c0] Op_store_var : x [ 1:0xfcc440] Op_push_lhs : X [do_reference = true] [ 1:0xfcc3a0] Op_postincrement : [ 1:0xfcc4e0] Op_push : x [ 1:0xfcc540] Op_push : o [ 1:0xfcc500] Op_plus : [ 1:0xfcc580] Op_push : o [ 1:0xfcc560] Op_plus : [ 1:0xfcc460] Op_leq : [ :0xfcc5c0] Op_jmp_false : [target_jmp = 0xfcc5e0] [ 1:0xfcc600] Op_push_i : "%c" [MALLOC|STRING|STRCUR] [ :0xfcc660] Op_no_op : [ 1:0xfcc520] Op_assign_concat : c [ :0xfcc620] Op_jmp : [target_jmp = 0xfcc440] … [ 2:0xfcc5a0] Op_K_printf : [expr_count = 17] [redir_type = ""] [ :0xfcc140] Op_no_op : [ :0xfcc1c0] Op_atexit : [ :0xfcc640] Op_stop : 372 | Chapter 14: Debugging awk Programs [ :0xfcc180] Op_no_op : [ :0xfcd150] Op_after_beginfile : [ :0xfcc160] Op_no_op : [ :0xfcc1a0] Op_after_endfile : gawk> help h Print a list of all of the gawk debugger commands with a short summary of their usage. ‘help command ’ prints the information about the command command. list [- | + | n | filename:n | n-m | function] l [– | + | n | filename:n | n–m | function] Print the specified lines (default 15) from the current source file or the file named filename. The possible arguments to list are as follows: - (Minus) Print lines before the lines last printed. + Print lines after the lines last printed. list without any argument does the same thing. n Print lines centered around line number n. n–m Print lines from n to m. filename:n Print lines centered around line number n in source file filename. This com‐ mand may change the current source file. function Print lines centered around the beginning of the function function. This com‐ mand may change the current source file. quit q Exit the debugger. Debugging is great fun, but sometimes we all have to tend to other obligations in life, and sometimes we find the bug and are free to go on to the next one! As we saw earlier, if you are running a program, the debugger warns you when you type ‘q’ or ‘quit’, to make sure you really want to quit. trace [on | off] Turn on or off continuous printing of the instructions that are about to be executed, along with the awk lines they implement. The default is off. Main Debugger Commands | 373 It is to be hoped that most of the “opcodes” in these instructions are fairly self- explanatory, and using stepi and nexti while trace is on will make them into familiar friends. Readline Support If gawk is compiled with the GNU Readline library, you can take advantage of that library’s command completion and history expansion features. The following types of completion are available: Command completion Command names. Source filename completion Source filenames. Relevant commands are break, clear, list, tbreak, and until. Argument completion Non-numeric arguments to a command. Relevant commands are enable and info. Variable name completion Global variable names, and function arguments in the current context if the pro‐ gram is running. Relevant commands are display, print, set, and watch. Limitations We hope you find the gawk debugger useful and enjoyable to work with, but as with any program, especially in its early releases, it still has some limitations. A few that it’s worth being aware of are: • At this point, the debugger does not give a detailed explanation of what you did wrong when you type in something it doesn’t like. Rather, it just responds ‘syntax error’. When you do figure out what your mistake was, though, you’ll feel like a real guru. • If you perused the dump of opcodes in “Miscellaneous Commands” on page 372 (or if you are already familiar with gawk internals), you will realize that much of the internal manipulation of data in gawk, as in many interpreters, is done on a stack. Op_push, Op_pop, and the like are the “bread and butter” of most gawk code. Unfortunately, as of now, the gawk debugger does not allow you to examine the stack’s contents. That is, the intermediate results of expression evaluation are on the stack, but cannot be printed. Rather, only variables that are defined in the pro‐ gram can be printed. Of course, a workaround for this is to use more explicit vari‐ ables at the debugging stage and then change back to obscure, perhaps more optimal code later. 374 | Chapter 14: Debugging awk Programs • There is no way to look “inside” the process of compiling regular expressions to see if you got it right. As an awk programmer, you are expected to know the meaning of /[^[:alnum:][:blank:]]/. • The gawk debugger is designed to be used by running a program (with all its pa‐ rameters) on the command line, as described in “How to Start the Debugger” on page 359. There is no way (as of now) to attach or “break into” a running program. This seems reasonable for a language that is used mainly for quickly executing, short programs. • The gawk debugger only accepts source code supplied with the -f option. Summary • Programs rarely work correctly the first time. Finding bugs is called debugging, and a program that helps you find bugs is a debugger. gawk has a built-in debugger that works very similarly to the GNU Debugger, GDB. • Debuggers let you step through your program one statement at a time, examine and change variable and array values, and do a number of other things that let you understand what your program is actually doing (as opposed to what it is supposed to do). • Like most debuggers, the gawk debugger works in terms of stack frames, and lets you set both breakpoints (stop at a point in the code) and watchpoints (stop when a data value changes). • The debugger command set is fairly complete, providing control over breakpoints, execution, viewing and changing data, working with the stack, getting information, and other tasks. • If the GNU Readline library is available when gawk is compiled, it is used by the debugger to provide command-line history and editing. Summary | 375 CHAPTER 15 Arithmetic and Arbitrary-Precision Arithmetic with gawk This chapter introduces some basic concepts relating to how computers do arithmetic and defines some important terms. It then proceeds to describe floating-point arith‐ metic, which is what awk uses for all its computations, including a discussion of arbitrary-precision floating-point arithmetic, which is a feature available only in gawk. It continues on to present arbitrary-precision integers, and concludes with a description of some points where gawk and the POSIX standard are not quite in agreement. Most users of gawk can safely skip this chapter. But if you want to do scientific calculations with gawk, this is the place to be. A General Description of Computer Arithmetic Until now, we have worked with data as either numbers or strings. Ultimately, however, computers represent everything in terms of binary digits, or bits. A decimal digit can take on any of 10 values: zero through nine. A binary digit can take on any of two values, zero or one. Using binary, computers (and computer software) can represent and ma‐ nipulate numerical and character data. In general, the more bits you can use to represent a particular thing, the greater the range of possible values it can take on. Modern computers support at least two, and often more, ways to do arithmetic. Each kind of arithmetic uses a different representation (organization of the bits) for the numbers. The kinds of arithmetic that interest us are: 377 1. We don’t know why they expect this, but they do. Decimal arithmetic This is the kind of arithmetic you learned in elementary school, using paper and pencil (and/or a calculator). In theory, numbers can have an arbitrary number of digits on either side (or both sides) of the decimal point, and the results of a com‐ putation are always exact. Some modern systems can do decimal arithmetic in hardware, but usually you need a special software library to provide access to these instructions. There are also libraries that do decimal arithmetic entirely in software. Despite the fact that some users expect gawk to be performing decimal arithmetic,1 it does not do so. Integer arithmetic In school, integer values were referred to as “whole” numbers—that is, numbers without any fractional part, such as 1, 42, or −17. The advantage to integer numbers is that they represent values exactly. The disadvantage is that their range is limited. In computers, integer values come in two flavors: signed and unsigned. Signed values may be negative or positive, whereas unsigned values are always positive (i.e., greater than or equal to zero). In computer systems, integer arithmetic is exact, but the possible range of values is limited. Integer arithmetic is generally faster than floating-point arithmetic. Floating-point arithmetic Floating-point numbers represent what were called in school “real” numbers (i.e., those that have a fractional part, such as 3.1415927). The advantage to floating- point numbers is that they can represent a much larger range of values than can integers. The disadvantage is that there are numbers that they cannot represent exactly. Modern systems support floating-point arithmetic in hardware, with a limited range of values. There are software libraries that allow the use of arbitrary-precision floating-point calculations. POSIX awk uses double-precision floating-point numbers, which can hold more digits than single-precision floating-point numbers. gawk has facilities for performing arbitrary-precision floating-point arithmetic, which we describe in more detail shortly. Computers work with integer and floating-point values of different ranges. Integer val‐ ues are usually either 32 or 64 bits in size. Single-precision floating-point values occupy 378 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk 2. Thanks to Michael Brennan for this description, which we have paraphrased, and for the examples. 32 bits, whereas double-precision floating-point values occupy 64 bits. Floating-point values are always signed. The possible ranges of values are shown in Table 15-1. Table 15-1. Value ranges for different numeric representations Numeric representation Minimum value Maximum value 32-bit signed integer −2,147,483,648 2,147,483,647 32-bit unsigned integer 0 4,294,967,295 64-bit signed integer −9,223,372,036,854,775,808 9,223,372,036,854,775,807 64-bit unsigned integer 0 18,446,744,073,709,551,615 Single-precision floating point (approximate) 1.175494-38 3.40282338 Double-precision floating point (approximate) 2.225074-308 1.797693308 Other Stuff to Know The rest of this chapter uses a number of terms. Here are some informal definitions that should help you work your way through the material here: Accuracy A floating-point calculation’s accuracy is how close it comes to the real (paper and pencil) value. Error The difference between what the result of a computation “should be” and what it actually is. It is best to minimize error as much as possible. Exponent The order of magnitude of a value; some number of bits in a floating-point value store the exponent. Inf A special value representing infinity. Operations involving another number and infinity produce infinity. NaN “Not a number.”2 A special value that results from attempting a calculation that has no answer as a real number. In such a case, programs can either receive a floating- point exception, or get NaN back as the result. The IEEE 754 standard recommends that systems return NaN. Some examples: sqrt(-1) This makes sense in the range of complex numbers, but not in the range of real numbers, so the result is NaN. Other Stuff to Know | 379 log(-8) −8 is out of the domain of log(), so the result is NaN. Normalized How the significand (see later in this list) is usually stored. The value is adjusted so that the first bit is one, and then that leading one is assumed instead of physically stored. This provides one extra bit of precision. Precision The number of bits used to represent a floating-point number. The more bits, the more digits you can represent. Binary and decimal precisions are related approxi‐ mately, according to the formula: prec = 3.322 ⋅ dps Here, prec denotes the binary precision (measured in bits) and dps (short for dec‐ imal places) is the decimal digits. Rounding mode How numbers are rounded up or down when necessary. More details are provided later. Significand A floating-point value consists of the significand multiplied by 10 to the power of the exponent. For example, in 1.2345e67, the significand is 1.2345. Stability From the Wikipedia article on numerical stability: “Calculations that can be proven not to magnify approximation errors are called numerically stable.” See the Wikipedia article on accuracy and precision for more information on some of those terms. On modern systems, floating-point hardware uses the representation and operations defined by the IEEE 754 standard. Three of the standard IEEE 754 types are 32-bit single precision, 64-bit double precision, and 128-bit quadruple precision. The standard also specifies extended precision formats to allow greater precisions and larger exponent ranges. (awk uses only the 64-bit double-precision format.) Table 15-2 lists the precision and exponent field values for the basic IEEE 754 binary formats. 380 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk Table 15-2. Basic IEEE format values Name Total bits Precision Minimum exponent Maximum exponent Single 32 24 −126 +127 Double 64 53 −1022 +1023 Quadruple 128 113 −16382 +16383 The precision numbers include the implied leading one that gives them one extra bit of significand. Arbitrary-Precision Arithmetic Features in gawk By default, gawk uses the double-precision floating-point values supplied by the hard‐ ware of the system it runs on. However, if it was compiled to do so, gawk uses the GNU MPFR and GNU MP (GMP) libraries for arbitrary-precision arithmetic on numbers. You can see if MPFR support is available like so: $ gawk --version GNU Awk 4.1.2, API: 1.1 (GNU MPFR 3.1.0-p3, GNU MP 5.0.2) Copyright (C) 1989, 1991-2015 Free Software Foundation. … (You may see different version numbers than what’s shown here. That’s OK; what’s im‐ portant is to see that GNU MPFR and GNU MP are listed in the output.) Additionally, there are a few elements available in the PROCINFO array to provide infor‐ mation about the MPFR and GMP libraries (see “Built-in Variables That Convey In‐ formation” on page 163). The MPFR library provides precise control over precisions and rounding modes, and gives correctly rounded, reproducible, platform-independent results. With the -M command-line option, all floating-point arithmetic operators and numeric functions can yield results to any desired precision level supported by MPFR. Two predefined variables, PREC and ROUNDMODE, provide control over the working pre‐ cision and the rounding mode. The precision and the rounding mode are set globally for every operation to follow. See “Setting the Precision” on page 385 and “Setting the Rounding Mode” on page 387 for more information. Arbitrary-Precision Arithmetic Features in gawk | 381 3. There is a very nice paper on floating-point arithmetic by David Goldberg, “What Every Computer Scientist Should Know About Floating-Point Arithmetic,” ACM Computing Surveys 23, 1 (1991-03): 5-48. This is worth reading if you are interested in the details, but it does require a background in computer science. Floating-Point Arithmetic: Caveat Emptor! Math class is tough! —Teen Talk Barbie, July 1992 This section provides a high-level overview of the issues involved when doing lots of floating-point arithmetic.3 The discussion applies to both hardware and arbitrary- precision floating-point arithmetic. The material here is purposely general. If you need to do se‐ rious computer arithmetic, you should do some research first, and not rely just on what we tell you. Floating-Point Arithmetic Is Not Exact Binary floating-point representations and arithmetic are inexact. Simple values like 0.1 cannot be precisely represented using binary floating-point numbers, and the limited precision of floating-point numbers means that slight changes in the order of operations or the precision of intermediate storage can change the result. To make matters worse, with arbitrary-precision floating-point arithmetic, you can set the precision before starting a computation, but then you cannot be sure of the number of significant decimal places in the final result. Many numbers cannot be represented exactly So, before you start to write any code, you should think about what you really want and what’s really happening. Consider the two numbers in the following example: x = 0.875 # 1/2 + 1/4 + 1/8 y = 0.425 Unlike the number in y, the number stored in x is exactly representable in binary because it can be written as a finite sum of one or more fractions whose denominators are all powers of two. When gawk reads a floating-point number from program source, it automatically rounds that number to whatever precision your machine supports. If you try to print the numeric content of a variable using an output format string of "%. 17g", it may not produce the same number as you assigned to it: 382 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk $ gawk 'BEGIN { x = 0.875; y = 0.425 > printf("%0.17g, %0.17g\n", x, y) }' 0.875, 0.42499999999999999 Often the error is so small you do not even notice it, and if you do, you can always specify how much precision you would like in your output. Usually this is a format string like "%.15g", which, when used in the previous example, produces an output identical to the input. Be careful comparing values Because the underlying representation can be a little bit off from the exact value, com‐ paring floating-point values to see if they are exactly equal is generally a bad idea. Here is an example where it does not work like you might expect: $ gawk 'BEGIN { print (0.1 + 12.2 == 12.3) }' 0 The general wisdom when comparing floating-point values is to see if they are within some small range of each other (called a delta, or tolerance). You have to decide how small a delta is important to you. Code to do this looks something like the following: delta = 0.00001 # for example difference = abs(a) - abs(b) # subtract the two values if (difference < delta) # all ok else # not ok (We assume that you have a simple absolute value function named abs() defined else‐ where in your program.) Errors accumulate The loss of accuracy during a single computation with floating-point numbers usually isn’t enough to worry about. However, if you compute a value that is the result of a sequence of floating-point operations, the error can accumulate and greatly affect the computation itself. Here is an attempt to compute the value of π using one of its many series representations: BEGIN { x = 1.0 / sqrt(3.0) n = 6 for (i = 1; i < 30; i++) { n = n * 2.0 x = (sqrt(x * x + 1) - 1) / x printf("%.15f\n", n * x) } } Floating-Point Arithmetic: Caveat Emptor! | 383 When run, the early errors propagate through later computations, causing the loop to terminate prematurely after attempting to divide by zero: $ gawk -f pi.awk 3.215390309173475 3.159659942097510 3.146086215131467 3.142714599645573 … 3.224515243534819 2.791117213058638 0.000000000000000 error→ gawk: pi.awk:6: fatal: division by zero attempted Here is an additional example where the inaccuracies in internal representations yield an unexpected result: $ gawk 'BEGIN { > for (d = 1.1; d <= 1.5; d += 0.1) # loop five times (?) > i++ > print i > }' 4 Getting the Accuracy You Need Can arbitrary-precision arithmetic give exact results? There are no easy answers. The standard rules of algebra often do not apply when using floating-point arithmetic. Among other things, the distributive and associative laws do not hold completely, and order of operation may be important for your computation. Rounding error, cumulative precision loss, and underflow are often troublesome. When gawk tests the expressions ‘0.1 + 12.2’ and ‘12.3’ for equality using the machine double-precision arithmetic, it decides that they are not equal! (See “Be careful com‐ paring values” on page 383.) You can get the result you want by increasing the precision; 56 bits in this case does the job: $ gawk -M -v PREC=56 'BEGIN { print (0.1 + 12.2 == 12.3) }' 1 If adding more bits is good, perhaps adding even more bits of precision is better? Here is what happens if we use an even larger value of PREC: $ gawk -M -v PREC=201 'BEGIN { print (0.1 + 12.2 == 12.3) }' 0 This is not a bug in gawk or in the MPFR library. It is easy to forget that the finite number of bits used to store the value is often just an approximation after proper rounding. The test for equality succeeds if and only if all bits in the two operands are exactly the same. Because this is not necessarily true after floating-point computations with a particular 384 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk precision and effective rounding mode, a straight test for equality may not work. Instead, compare the two numbers to see if they are within the desirable delta of each other. In applications where 15 or fewer decimal places suffice, hardware double-precision arithmetic can be adequate, and is usually much faster. But you need to keep in mind that every floating-point operation can suffer a new rounding error with catastrophic consequences, as illustrated by our earlier attempt to compute the value of π. Extra precision can greatly enhance the stability and the accuracy of your computation in such cases. Additionally, you should understand that repeated addition is not necessarily equivalent to multiplication in floating-point arithmetic. In the example in “Errors accumulate” on page 383: $ gawk 'BEGIN { > for (d = 1.1; d <= 1.5; d += 0.1) # loop five times (?) > i++ > print i > }' 4 you may or may not succeed in getting the correct result by choosing an arbitrarily large value for PREC. Reformulation of the problem at hand is often the correct approach in such situations. Try a Few Extra Bits of Precision and Rounding Instead of arbitrary-precision floating-point arithmetic, often all you need is an adjust‐ ment of your logic or a different order for the operations in your calculation. The stability and the accuracy of the computation of π in the earlier example can be enhanced by using the following simple algebraic transformation: (sqrt(x * x + 1) - 1) / x ≡ x / (sqrt(x * x + 1) + 1) After making this change, the program converges to π in under 30 iterations: $ gawk -f pi2.awk 3.215390309173473 3.159659942097501 3.146086215131436 3.142714599645370 3.141873049979825 … 3.141592653589797 3.141592653589797 Setting the Precision gawk uses a global working precision; it does not keep track of the precision or accuracy of individual numbers. Performing an arithmetic operation or calling a built-in function Floating-Point Arithmetic: Caveat Emptor! | 385 rounds the result to the current working precision. The default working precision is 53 bits, which you can modify using the predefined variable PREC. You can also set the value to one of the predefined case-insensitive strings shown in Table 15-3, to emulate an IEEE 754 binary format. Table 15-3. Predefined precision strings for PREC PREC IEEE 754 binary format "half" 16-bit half-precision "single" Basic 32-bit single precision "double" Basic 64-bit double precision "quad" Basic 128-bit quadruple precision "oct" 256-bit octuple precision The following example illustrates the effects of changing precision on arithmetic operations: $ gawk -M -v PREC=100 'BEGIN { x = 1.0e-400; print x + 0 > PREC = "double"; print x + 0 }' 1e-400 0 Be wary of floating-point constants! When reading a floating-point constant from program source code, gawk uses the default preci‐ sion (that of a C double), unless overridden by an assignment to the special variable PREC on the command line, to store it internally as an MPFR number. Changing the precision using PREC in the pro‐ gram text does not change the precision of a constant. If you need to represent a floating-point constant at a higher preci‐ sion than the default and cannot use a command-line assignment to PREC, you should either specify the constant as a string or as a ra‐ tional number, whenever possible. The following example illus‐ trates the differences among various ways to print a floating-point constant: $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 0.1) }' 0.1000000000000000055511151 $ gawk -M -v PREC=113 'BEGIN { printf("%0.25f\n", 0.1) }' 0.1000000000000000000000000 $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", "0.1") }' 0.1000000000000000000000000 $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 1/10) }' 0.1000000000000000000000000 386 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk 4. It is possible for the output to be completely different if the C library in your system does not use the IEEE 754 even-rounding rule to round halfway cases for printf. Setting the Rounding Mode The ROUNDMODE variable provides program-level control over the rounding mode. The correspondence between ROUNDMODE and the IEEE rounding modes is shown in Table 15-4. Table 15-4. gawk rounding modes Rounding mode IEEE name ROUNDMODE Round to nearest, ties to even roundTiesToEven "N" or "n" Round toward positive infinity roundTowardPositive "U" or "u" Round toward negative infinity roundTowardNegative "D" or "d" Round toward zero roundTowardZero "Z" or "z" Round to nearest, ties away from zero roundTiesToAway "A" or "a" ROUNDMODE has the default value "N", which selects the IEEE 754 rounding mode round TiesToEven. In Table 15-4, the value "A" selects roundTiesToAway. This is only available if your version of the MPFR library supports it; otherwise, setting ROUNDMODE to "A" has no effect. The default mode roundTiesToEven is the most preferred, but the least intuitive. This method does the obvious thing for most values, by rounding them up or down to the nearest digit. For example, rounding 1.132 to two digits yields 1.13, and rounding 1.157 yields 1.16. However, when it comes to rounding a value that is exactly halfway between, things do not work the way you probably learned in school. In this case, the number is rounded to the nearest even digit. So rounding 0.125 to two digits rounds down to 0.12, but rounding 0.6875 to three digits rounds up to 0.688. You probably have already encoun‐ tered this rounding mode when using printf to format floating-point numbers. For example: BEGIN { x = -4.5 for (i = 1; i < 10; i++) { x += 1.0 printf("%4.1f => %2.0f\n", x, x) } } produces the following output when run on the author’s system:4 -3.5 => -4 -2.5 => -2 Floating-Point Arithmetic: Caveat Emptor! | 387 -1.5 => -2 -0.5 => 0 0.5 => 0 1.5 => 2 2.5 => 2 3.5 => 4 4.5 => 4 The theory behind roundTiesToEven is that it more or less evenly distributes upward and downward rounds of exact halves, which might cause any accumulating round-off error to cancel itself out. This is the default rounding mode for IEEE 754 computing functions and operators. The other rounding modes are rarely used. Rounding toward positive infinity (round TowardPositive) and toward negative infinity (roundTowardNegative) are often used to implement interval arithmetic, where you adjust the rounding mode to calculate upper and lower bounds for the range of output. The roundTowardZero mode can be used for converting floating-point numbers to integers. The rounding mode round TiesToAway rounds the result to the nearest number and selects the number with the larger magnitude if a tie occurs. Some numerical analysts will tell you that your choice of rounding style has tremendous impact on the final outcome, and advise you to wait until final output for any rounding. Instead, you can often avoid round-off error problems by setting the precision initially to some value sufficiently larger than the final desired precision, so that the accumula‐ tion of round-off error does not influence the outcome. If you suspect that results from your computation are sensitive to accumulation of round-off error, look for a significant difference in output when you change the rounding mode to be sure. Arbitrary-Precision Integer Arithmetic with gawk When given the -M option, gawk performs all integer arithmetic using GMP arbitrary- precision integers. Any number that looks like an integer in a source or datafile is stored as an arbitrary-precision integer. The size of the integer is limited only by the available memory. For example, the following computes 5432 , the result of which is beyond the limits of ordinary hardware double-precision floating-point values: $ gawk -M 'BEGIN { > x = 5^4^3^2 > print "number of digits =", length(x) > print substr(x, 1, 20), "...", substr(x, length(x) - 19, 20) > }' number of digits = 183231 62060698786608744707 ... 92256259918212890625 388 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk 5. Weisstein, Eric W. Sylvester’s Sequence. From MathWorld—A Wolfram Web Resource (http://mathworld.wolf ram.com/SylvestersSequence.html). If instead you were to compute the same value using arbitrary-precision floating-point values, the precision needed for correct output (using the formula prec = 3.322 ⋅ dps) would be prec = 3.322 ⋅ 183231, or 608693. The result from an arithmetic operation with an integer and a floating-point value is a floating-point value with a precision equal to the working precision. The following program calculates the eighth term in Sylvester’s sequence5 using a recurrence: $ gawk -M 'BEGIN { > s = 2.0 > for (i = 1; i <= 7; i++) > s = s * (s - 1) + 1 > print s > }' 113423713055421845118910464 The output differs from the actual number, 113,423,713,055,421,844,361,000,443, be‐ cause the default precision of 53 bits is not enough to represent the floating-point results exactly. You can either increase the precision (100 bits is enough in this case), or replace the floating-point constant ‘2.0’ with an integer, to perform all computations using integer arithmetic to get the correct output. Sometimes gawk must implicitly convert an arbitrary-precision integer into an arbitrary-precision floating-point value. This is primarily because the MPFR library does not always provide the relevant interface to process arbitrary-precision integers or mixed-mode numbers as needed by an operation or function. In such a case, the pre‐ cision is set to the minimum value necessary for exact conversion, and the working precision is not used for this purpose. If this is not what you need or want, you can employ a subterfuge and convert the integer to floating point first, like this: gawk -M 'BEGIN { n = 13; print (n + 0.0) % 2.0 }' You can avoid this issue altogether by specifying the number as a floating-point value to begin with: gawk -M 'BEGIN { n = 13.0; print n % 2.0 }' Note that for this particular example, it is likely best to just use the following: gawk -M 'BEGIN { n = 13; print n % 2 }' Standards Versus Existing Practice Historically, awk has converted any nonnumeric-looking string to the numeric value zero, when required. Furthermore, the original definition of the language and the orig‐ Standards Versus Existing Practice | 389 6. You asked for it, you got it. inal POSIX standards specified that awk only understands decimal numbers (base 10), and not octal (base 8) or hexadecimal numbers (base 16). Changes in the language of the 2001 and 2004 POSIX standards can be interpreted to imply that awk should support additional features. These features are: • Interpretation of floating-point data values specified in hexadecimal notation (e.g., 0xDEADBEEF). (Note: data values, not source code constants.) • Support for the special IEEE 754 floating-point values “not a number” (NaN), pos‐ itive infinity (“inf ”), and negative infinity (“−inf ”). In particular, the format for these values is as specified by the ISO 1999 C standard, which ignores case and can allow implementation-dependent additional characters after the ‘nan’ and allow either ‘inf’ or ‘infinity’. The first problem is that both of these are clear changes to historical practice: • The gawk maintainer feels that supporting hexadecimal floating-point values, in particular, is ugly, and was never intended by the original designers to be part of the language. • Allowing completely alphabetic strings to have valid numeric values is also a very severe departure from historical practice. The second problem is that the gawk maintainer feels that this interpretation of the standard, which required a certain amount of “language lawyering” to arrive at in the first place, was not even intended by the standard developers. In other words, “We see how you got where you are, but we don’t think that that’s where you want to be.” Recognizing these issues, but attempting to provide compatibility with the earlier ver‐ sions of the standard, the 2008 POSIX standard added explicit wording to allow, but not require, that awk support hexadecimal floating-point values and special values for “not a number” and infinity. Although the gawk maintainer continues to feel that providing those features is inad‐ visable, nevertheless, on systems that support IEEE floating point, it seems reasonable to provide some way to support NaN and infinity values. The solution implemented in gawk is as follows: • With the --posix command-line option, gawk becomes “hands off.” String values are passed directly to the system library’s strtod() function, and if it successfully returns a numeric value, that is what’s used.6 By definition, the results are not portable across different systems. They are also a little surprising: 390 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk $ echo nanny | gawk --posix '{ print $1 + 0 }' nan $ echo 0xDeadBeef | gawk --posix '{ print $1 + 0 }' 3735928559 • Without --posix, gawk interprets the four string values ‘+inf’, ‘-inf’, ‘+nan’, and ‘- nan’ specially, producing the corresponding special numeric values. The leading sign acts a signal to gawk (and the user) that the value is really numeric. Hexadecimal floating point is not supported (unless you also use --non-decimal-data, which is not recommended). For example: $ echo nanny | gawk '{ print $1 + 0 }' 0 $ echo +nan | gawk '{ print $1 + 0 }' nan $ echo 0xDeadBeef | gawk '{ print $1 + 0 }' 0 gawk ignores case in the four special values. Thus, ‘+nan’ and ‘+NaN’ are the same. Summary • Most computer arithmetic is done using either integers or floating-point values. Standard awk uses double-precision floating-point values. • In the early 1990s Barbie mistakenly said, “Math class is tough!” Although math isn’t tough, floating-point arithmetic isn’t the same as pencil-and-paper math, and care must be taken: — Not all numbers can be represented exactly. — Comparing values should use a delta, instead of being done directly with ‘==’ and ‘!=’. — Errors accumulate. — Operations are not always truly associative or distributive. • Increasing the accuracy can help, but it is not a panacea. • Often, increasing the accuracy and then rounding to the desired number of digits produces reasonable results. • Use -M (or --bignum) to enable MPFR arithmetic. Use PREC to set the precision in bits, and ROUNDMODE to set the IEEE 754 rounding mode. • With -M, gawk performs arbitrary-precision integer arithmetic using the GMP library. This is faster and more space-efficient than using MPFR for the same cal‐ culations. Summary | 391 • There are several areas with respect to floating-point numbers where gawk disagrees with the POSIX standard. It pays to be aware of them. • Overall, there is no need to be unduly suspicious about the results from floating- point arithmetic. The lesson to remember is that floating-point arithmetic is always more complex than arithmetic using pencil and paper. In order to take advantage of the power of floating-point arithmetic, you need to know its limitations and work within them. For most casual use of floating-point arithmetic, you will often get the expected result if you simply round the display of your final results to the correct number of significant decimal digits. • As general advice, avoid presenting numerical data in a manner that implies better precision than is actually the case. 392 | Chapter 15: Arithmetic and Arbitrary-Precision Arithmetic with gawk CHAPTER 16 Writing Extensions for gawk It is possible to add new functions written in C or C++ to gawk using dynamically loaded libraries. This facility is available on systems that support the C dlopen() and dlsym() functions. This chapter describes how to create extensions using code written in C or C++. If you don’t know anything about C programming, you can safely skip this chapter, although you may wish to review the documentation on the extensions that come with gawk (see “The Sample Extensions in the gawk Distribution” on page 442) and the infor‐ mation on the gawkextlib project (see “The gawkextlib Project” on page 452). The sample extensions are automatically built and installed when gawk is. When --sandbox is specified, extensions are disabled (see “Command-Line Options” on page 22). Introduction An extension (sometimes called a plug-in) is a piece of external compiled code that gawk can load at runtime to provide additional functionality, over and above the built-in capabilities described in the rest of this book. Extensions are useful because they allow you (of course) to extend gawk’s functionality. For example, they can provide access to system calls (such as chdir() to change direc‐ tory) and to other C library routines that could be of use. As with most software, “the sky is the limit”; if you can imagine something that you might want to do and can write in C or C++, you can write an extension to do it! 393 Extensions are written in C or C++, using the application programming interface (API) defined for this purpose by the gawk developers. The rest of this chapter explains the facilities that the API provides and how to use them, and presents a small example extension. In addition, it documents the sample extensions included in the gawk dis‐ tribution and describes the gawkextlib project. See http://www.gnu.org/software/gawk/ manual/html_node/Extension-Design.html for a discussion of the extension mechanism goals and design. Extension Licensing Every dynamic extension must be distributed under a license that is compatible with the GNU GPL (see Appendix C). In order for the extension to tell gawk that it is properly licensed, the extension must define the global symbol plugin_is_GPL_compatible. If this symbol does not exist, gawk emits a fatal error and exits when it tries to load your extension. The declared type of the symbol should be int. It does not need to be in any allocated section, though. The code merely asserts that the symbol exists in the global scope. Something like this is enough: int plugin_is_GPL_compatible; How It Works at a High Level Communication between gawk and an extension is two-way. First, when an extension is loaded, gawk passes it a pointer to a struct whose fields are function pointers. This is shown in Figure 16-1. 394 | Chapter 16: Writing Extensions for gawk Figure 16-1. Loading the extension The extension can call functions inside gawk through these function pointers, at run‐ time, without needing (link-time) access to gawk’s symbols. One of these function pointers is to a function for “registering” new functions. This is shown in Figure 16-2. Figure 16-2. Registering a new function In the other direction, the extension registers its new functions with gawk by passing function pointers to the functions that provide the new feature (do_chdir(), for ex‐ ample). gawk associates the function pointer with a name and can then call it, using a defined calling convention. This is shown in Figure 16-3. How It Works at a High Level | 395 Figure 16-3. Calling the new function The do_xxx() function, in turn, then uses the function pointers in the API struct to do its work, such as updating variables or arrays, printing messages, setting ERRNO, and so on. Convenience macros make calling through the function pointers look like regular func‐ tion calls so that extension code is quite readable and understandable. Although all of this sounds somewhat complicated, the result is that extension code is quite straightforward to write and to read. You can see this in the sample extension filefuncs.c (see “Example: Some File Functions” on page 431) and also in the testext.c code for testing the APIs. Some other bits and pieces: • The API provides access to gawk’s do_xxx values, reflecting command-line options, like do_lint, do_profiling, and so on (see “API Variables” on page 428). These are informational: an extension cannot affect their values inside gawk. In addition, attempting to assign to them produces a compile-time error. • The API also provides major and minor version numbers, so that an extension can check if the gawk it is loaded with supports the facilities it was compiled with. (Version mismatches “shouldn’t” happen, but we all know how that goes.) See “API version constants and variables” on page 428 for details. API Description C or C++ code for an extension must include the header file gawkapi.h, which declares the functions and defines the data types used to communicate with gawk. This (rather large) section describes the API in detail. 396 | Chapter 16: Writing Extensions for gawk Introduction Access to facilities within gawk is achieved by calling through function pointers passed into your extension. API function pointers are provided for the following kinds of operations: • Allocating, reallocating, and releasing memory. • Registration functions. You may register: — Extension functions — Exit callbacks — A version string — Input parsers — Output wrappers — Two-way processors All of these are discussed in detail later in this chapter. • Printing fatal, warning, and “lint” warning messages. • Updating ERRNO, or unsetting it. • Accessing parameters, including converting an undefined parameter into an array. • Symbol table access: retrieving a global variable, creating one, or changing one. • Creating and releasing cached values; this provides an efficient way to use values for multiple variables and can be a big performance win. • Manipulating arrays: — Retrieving, adding, deleting, and modifying elements — Getting the count of elements in an array — Creating a new array — Clearing an array — Flattening an array for easy C-style looping over all its indices and elements Some points about using the API: • The following types, macros, and/or functions are referenced in gawkapi.h. For correct use, you must therefore include the corresponding standard header file before including gawkapi.h: C entity Header file EOF Values for errno API Description | 397 C entity Header file FILE NULL memcpy() memset() size_t struct stat Due to portability concerns, especially to systems that are not fully standards- compliant, it is your responsibility to include the correct files in the correct way. This requirement is necessary in order to keep gawkapi.h clean, instead of becom‐ ing a portability hodge-podge as can be seen in some parts of the gawk source code. • The gawkapi.h file may be included more than once without ill effect. Doing so, however, is poor coding practice. • Although the API only uses ISO C 90 features, there is an exception; the “con‐ structor” functions use the inline keyword. If your compiler does not support this keyword, you should either place ‘-Dinline=''’ on your command line or use the GNU Autotools and include a config.h file in your extensions. • All pointers filled in by gawk point to memory managed by gawk and should be treated by the extension as read-only. Memory for all strings passed into gawk from the extension must come from calling one of gawk_malloc(), gawk_calloc(), or gawk_realloc(), and is managed by gawk from then on. • The API defines several simple structs that map values as seen from awk. A value can be a double, a string, or an array (as in multidimensional arrays, or when cre‐ ating a new array). String values maintain both pointer and length, because em‐ bedded NUL characters are allowed. By intent, strings are maintained using the current multibyte encoding (as defined by LC_xxx environment variables) and not using wide characters. This matches how gawk stores strings internally and also how characters are likely to be input into and output from files. • When retrieving a value (such as a parameter or that of a global variable or array element), the extension requests a specific type (number, string, scalar, value cookie, array, or “undefined”). When the request is “undefined,” the returned value will have the real underlying type. 398 | Chapter 16: Writing Extensions for gawk However, if the request and actual type don’t match, the access function returns “false” and fills in the type of the actual value that is there, so that the extension can, e.g., print an error message (such as “scalar passed where array expected”). You may call the API functions by using the function pointers directly, but the interface is not so pretty. To make extension code look more like regular code, the gawkapi.h header file defines several macros that you should use in your code. This section presents the macros as if they were functions. General-Purpose Data Types I have a true love/hate relationship with unions. —Arnold Robbins That’s the thing about unions: the compiler will arrange things so they can accommodate both love and hate. —Chet Ramey The extension API defines a number of simple types and structures for general-purpose use. Additional, more specialized data structures are introduced in subsequent sections, together with the functions that use them. The general-purpose types and structures are as follows: typedef void *awk_ext_id_t; A value of this type is received from gawk when an extension is loaded. That value must then be passed back to gawk as the first parameter of each API function. #define awk_const … This macro expands to ‘const’ when compiling an extension, and to nothing when compiling gawk itself. This makes certain fields in the API data structures unwrit‐ able from extension code, while allowing gawk to use them as it needs to. typedef enum awk_bool { awk_false = 0, awk_true } awk_bool_t; A simple Boolean type. typedef struct awk_string { char *str; /* data */ size_t len; /* length thereof, in chars */ } awk_string_t; This represents a mutable string. gawk owns the memory pointed to if it supplied the value. Otherwise, it takes ownership of the memory pointed to. Such memory API Description | 399 must come from calling one of the gawk_malloc(), gawk_calloc(), or gawk_realloc() functions! As mentioned earlier, strings are maintained using the current multibyte encoding. typedef enum { AWK_UNDEFINED, AWK_NUMBER, AWK_STRING, AWK_ARRAY, AWK_SCALAR, /* opaque access to a variable */ AWK_VALUE_COOKIE /* for updating a previously created value */ } awk_valtype_t; This enum indicates the type of a value. It is used in the following struct. typedef struct awk_value { awk_valtype_t val_type; union { awk_string_t s; double d; awk_array_t a; awk_scalar_t scl; awk_value_cookie_t vc; } u; } awk_value_t; An “awk value.” The val_type member indicates what kind of value the union holds, and each member is of the appropriate type. #define str_value u.s #define num_value u.d #define array_cookie u.a #define scalar_cookie u.scl #define value_cookie u.vc Usings these macros makes accessing the fields of the awk_value_t more readable. typedef void *awk_scalar_t; Scalars can be represented as an opaque type. These values are obtained from gawk and then passed back into it. This is discussed in a general fashion in the text fol‐ lowing this list, and in more detail in “Variable access and update by cookie” on page 415. 400 | Chapter 16: Writing Extensions for gawk 1. See the “cookie” entry in the Jargon file for a definition of cookie, and the “magic cookie” entry in the Jargon file for a nice example. typedef void *awk_value_cookie_t; A “value cookie” is an opaque type representing a cached value. This is also dis‐ cussed in a general fashion in the text following this list, and in more detail in “Creating and using cached values” on page 417. Scalar values in awk are either numbers or strings. The awk_value_t represents values. The val_type member indicates what is in the union. Representing numbers is easy—the API uses a C double. Strings require more work. Because gawk allows embedded NUL bytes in string values, a string must be represented as a pair containing a data pointer and length. This is the awk_string_t type. Identifiers (i.e., the names of global variables) can be associated with either scalar values or with arrays. In addition, gawk provides true arrays of arrays, where any given array element can itself be an array. Discussion of arrays is delayed until “Array Manipula‐ tion” on page 419. The various macros listed earlier make it easier to use the elements of the union as if they were fields in a struct; this is a common coding practice in C. Such code is easier to write and to read, but it remains your responsibility to make sure that the val_type member correctly reflects the type of the value in the awk_value_t struct. Conceptually, the first three members of the union (number, string, and array) are all that is needed for working with awk values. However, because the API provides routines for accessing and changing the value of a global scalar variable only by using the vari‐ able’s name, there is a performance penalty: gawk must find the variable each time it is accessed and changed. This turns out to be a real issue, not just a theoretical one. Thus, if you know that your extension will spend considerable time reading and/or changing the value of one or more scalar variables, you can obtain a scalar cookie1 object for that variable, and then use the cookie for getting the variable’s value or for changing the variable’s value. The awk_scalar_t type holds a scalar cookie, and the scalar_cookie macro provides access to the value of that type in the awk_value_t struct. Given a scalar cookie, gawk can directly retrieve or modify the value, as required, without having to find it first. The awk_value_cookie_t type and value_cookie macro are similar. If you know that you wish to use the same numeric or string value for one or more variables, you can create the value once, retaining a value cookie for it, and then pass in that value cookie whenever you wish to set the value of a variable. This saves storage space within the running gawk process and reduces the time needed to create the value. API Description | 401 2. This is more common on MS-Windows systems, but it can happen on Unix-like systems as well. Memory Allocation Functions and Convenience Macros The API provides a number of memory allocation functions for allocating memory that can be passed to gawk, as well as a number of convenience macros. This subsection presents them all as function prototypes, in the way that extension code would use them: void *gawk_malloc(size_t size); Call the correct version of malloc() to allocate storage that may be passed to gawk. void *gawk_calloc(size_t nmemb, size_t size); Call the correct version of calloc() to allocate storage that may be passed to gawk. void *gawk_realloc(void *ptr, size_t size); Call the correct version of realloc() to allocate storage that may be passed to gawk. void gawk_free(void *ptr); Call the correct version of free() to release storage that was allocated with gawk_malloc(), gawk_calloc(), or gawk_realloc(). The API has to provide these functions because it is possible for an extension to be compiled and linked against a different version of the C library than was used for the gawk executable.2 If gawk were to use its version of free() when the memory came from an unrelated version of malloc(), unexpected behavior would likely result. Two convenience macros may be used for allocating storage from gawk_malloc() and gawk_realloc(). If the allocation fails, they cause gawk to exit with a fatal error message. They should be used as if they were procedure calls that do not return a value: #define emalloc(pointer, type, size, message) … The arguments to this macro are as follows: pointer The pointer variable to point at the allocated storage. type The type of the pointer variable. This is used to create a cast for the call to gawk_malloc(). size The total number of bytes to be allocated. message A message to be prefixed to the fatal error message. Typically this is the name of the function using the macro. 402 | Chapter 16: Writing Extensions for gawk For example, you might allocate a string value like so: awk_value_t result; char *message; const char greet[] = "Don't Panic!"; emalloc(message, char *, sizeof(greet), "myfunc"); strcpy(message, greet); make_malloced_string(message, strlen(message), & result); #define erealloc(pointer, type, size, message) … This is like emalloc(), but it calls gawk_realloc() instead of gawk_malloc(). The arguments are the same as for the emalloc() macro. Constructor Functions The API provides a number of constructor functions for creating string and numeric values, as well as a number of convenience macros. This subsection presents them all as function prototypes, in the way that extension code would use them: static inline awk_value_t * make_const_string(const char *string, size_t length, awk_value_t *result); This function creates a string value in the awk_value_t variable pointed to by result. It expects string to be a C string constant (or other string data), and au‐ tomatically creates a copy of the data for storage in result. It returns result. static inline awk_value_t * make_malloced_string(const char *string, size_t length, awk_value_t *result); This function creates a string value in the awk_value_t variable pointed to by result. It expects string to be a ‘char *’ value pointing to data previously obtained from gawk_malloc(), gawk_calloc(), or gawk_realloc(). The idea here is that the data is passed directly to gawk, which assumes responsibility for it. It returns result. static inline awk_value_t * make_null_string(awk_value_t *result); This specialized function creates a null string (the “undefined” value) in the awk_value_t variable pointed to by result. It returns result. static inline awk_value_t * make_number(double num, awk_value_t *result); This function simply creates a numeric value in the awk_value_t variable pointed to by result. API Description | 403 Registration Functions This section describes the API functions for registering parts of your extension with gawk. Registering an extension function Extension functions are described by the following record: typedef struct awk_ext_func { const char *name; awk_value_t *(*function)(int num_actual_args, awk_value_t *result); size_t num_expected_args; } awk_ext_func_t; The fields are: const char *name; The name of the new function. awk-level code calls the function by this name. This is a regular C string. Function names must obey the rules for awk identifiers. That is, they must begin with either an English letter or an underscore, which may be followed by any num‐ ber of letters, digits, and underscores. Letter case in function names is significant. awk_value_t *(*function)(int num_actual_args, awk_value_t *result); This is a pointer to the C function that provides the extension’s functionality. The function must fill in *result with either a number or a string. gawk takes ownership of any string memory. As mentioned earlier, string memory must come from one of gawk_malloc(), gawk_calloc(), or gawk_realloc(). The num_actual_args argument tells the C function how many actual parameters were passed from the calling awk code. The function must return the value of result. This is for the convenience of the calling code inside gawk. size_t num_expected_args; This is the number of arguments the function expects to receive. Each extension function may decide what to do if the number of arguments isn’t what it expected. As with real awk functions, it is likely OK to ignore extra arguments. Once you have a record representing your extension function, you register it with gawk using this API function: 404 | Chapter 16: Writing Extensions for gawk awk_bool_t add_ext_func(const char *namespace, const awk_ext_func_t *func); This function returns true upon success, false otherwise. The namespace parameter is currently not used; you should pass in an empty string (""). The func pointer is the address of a struct representing your function, as just described. Registering an exit callback function An exit callback function is a function that gawk calls before it exits. Such functions are useful if you have general “cleanup” tasks that should be performed in your extension (such as closing database connections or other resource deallocations). You can register such a function with gawk using the following function: void awk_atexit(void (*funcp)(void *data, int exit_status), void *arg0); The parameters are: funcp A pointer to the function to be called before gawk exits. The data parameter will be the original value of arg0. The exit_status parameter is the exit status value that gawk intends to pass to the exit() system call. arg0 A pointer to private data that gawk saves in order to pass to the function pointed to by funcp. Exit callback functions are called in last-in, first-out (LIFO) order—that is, in the reverse order in which they are registered with gawk. Registering an extension version string You can register a version string that indicates the name and version of your extension, with gawk, as follows: void register_ext_version(const char *version); Register the string pointed to by version with gawk. Note that gawk does not copy the version string, so it should not be changed. gawk prints all registered extension version strings when it is invoked with the --version option. Customized input parsers By default, gawk reads text files as its input. It uses the value of RS to find the end of the record, and then uses FS (or FIELDWIDTHS or FPAT) to split it into fields (see Chapter 4). Additionally, it sets the value of RT (see “Predefined Variables” on page 160). API Description | 405 If you want, you can provide your own custom input parser. An input parser’s job is to return a record to the gawk record-processing code, along with indicators for the value and length of the data to be used for RT, if any. To provide an input parser, you must first provide two functions (where XXX is a prefix name for your extension): awk_bool_t XXX_can_take_file(const awk_input_buf_t *iobuf); This function examines the information available in iobuf (which we discuss shortly). Based on the information there, it decides if the input parser should be used for this file. If so, it should return true. Otherwise, it should return false. It should not change any state (variable values, etc.) within gawk. awk_bool_t XXX_take_control_of(awk_input_buf_t *iobuf); When gawk decides to hand control of the file over to the input parser, it calls this function. This function in turn must fill in certain fields in the awk_input_buf_t structure and ensure that certain conditions are true. It should then return true. If an error of some kind occurs, it should not fill in any fields and should return false; then gawk will not use the input parser. The details are presented shortly. Your extension should package these functions inside an awk_input_parser_t, which looks like this: typedef struct awk_input_parser { const char *name; /* name of parser */ awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf); awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf); awk_const struct awk_input_parser *awk_const next; /* for gawk */ } awk_input_parser_t; The fields are: const char *name; The name of the input parser. This is a regular C string. awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf); A pointer to your XXX_can_take_file() function. awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf); A pointer to your XXX_take_control_of() function. awk_const struct input_parser *awk_const next; This is for use by gawk; therefore it is marked awk_const so that the extension cannot modify it. The steps are as follows: 1. Create a static awk_input_parser_t variable and initialize it appropriately. 406 | Chapter 16: Writing Extensions for gawk 2. When your extension is loaded, register your input parser with gawk using the register_input_parser() API function (described next). An awk_input_buf_t looks like this: typedef struct awk_input { const char *name; /* filename */ int fd; /* file descriptor */ #define INVALID_HANDLE (-1) void *opaque; /* private data for input parsers */ int (*get_record)(char **out, struct awk_input *iobuf, int *errcode, char **rt_start, size_t *rt_len); ssize_t (*read_func)(); void (*close_func)(struct awk_input *iobuf); struct stat sbuf; /* stat buf */ } awk_input_buf_t; The fields can be divided into two categories: those for use (initially, at least) by XXX_can_take_file(), and those for use by XXX_take_control_of(). The first group of fields and their uses are as follows: const char *name; The name of the file. int fd; A file descriptor for the file. If gawk was able to open the file, then fd will not be equal to INVALID_HANDLE. Otherwise, it will. struct stat sbuf; If the file descriptor is valid, then gawk will have filled in this structure via a call to the fstat() system call. The XXX_can_take_file() function should examine these fields and decide if the input parser should be used for the file. The decision can be made based upon gawk state (the value of a variable defined previously by the extension and set by awk code), the name of the file, whether or not the file descriptor is valid, the information in the struct stat, or any combination of these factors. Once XXX_can_take_file() has returned true, and gawk has decided to use your input parser, it calls XXX_take_control_of(). That function then fills either the get_record field or the read_func field in the awk_input_buf_t. It must also ensure that fd is not set to INVALID_HANDLE. The following list describes the fields that may be filled by XXX_take_control_of(): void *opaque; This is used to hold any state information needed by the input parser for this file. It is “opaque” to gawk. The input parser is not required to use this pointer. API Description | 407 int (*get_record)(char **out, struct awk_input *iobuf, int *errcode, char **rt_start, size_t *rt_len); This function pointer should point to a function that creates the input records. Said function is the core of the input parser. Its behavior is described in the text following this list. ssize_t (*read_func)(); This function pointer should point to a function that has the same behavior as the standard POSIX read() system call. It is an alternative to the get_record pointer. Its behavior is also described in the text following this list. void (*close_func)(struct awk_input *iobuf); This function pointer should point to a function that does the “teardown.” It should release any resources allocated by XXX_take_control_of(). It may also close the file. If it does so, it should set the fd field to INVALID_HANDLE. If fd is still not INVALID_HANDLE after the call to this function, gawk calls the regular close() system call. Having a “teardown” function is optional. If your input parser does not need it, do not set this field. Then, gawk calls the regular close() system call on the file de‐ scriptor, so it should be valid. The XXX_get_record() function does the work of creating input records. The param‐ eters are as follows: char **out This is a pointer to a char * variable that is set to point to the record. gawk makes its own copy of the data, so the extension must manage this storage. struct awk_input *iobuf This is the awk_input_buf_t for the file. The fields should be used for reading data (fd) and for managing private state (opaque), if any. int *errcode If an error occurs, *errcode should be set to an appropriate code from . char **rt_start size_t *rt_len If the concept of a “record terminator” makes sense, then *rt_start should be set to point to the data to be used for RT, and *rt_len should be set to the length of the data. Otherwise, *rt_len should be set to zero. gawk makes its own copy of this data, so the extension must manage this storage. 408 | Chapter 16: Writing Extensions for gawk The return value is the length of the buffer pointed to by *out, or EOF if end-of-file was reached or an error occurred. It is guaranteed that errcode is a valid pointer, so there is no need to test for a NULL value. gawk sets *errcode to zero, so there is no need to set it unless an error occurs. If an error does occur, the function should return EOF and set *errcode to a value greater than zero. In that case, if *errcode does not equal zero, gawk automatically updates the ERRNO variable based on the value of *errcode. (In general, setting ‘*errcode = errno’ should do the right thing.) As an alternative to supplying a function that returns an input record, you may instead supply a function that simply reads bytes, and let gawk parse the data into records. If you do so, the data should be returned in the multibyte encoding of the current locale. Such a function should follow the same behavior as the read() system call, and you fill in the read_func pointer with its address in the awk_input_buf_t structure. By default, gawk sets the read_func pointer to point to the read() system call. So your extension need not set this field explicitly. You must choose one method or the other: either a function that returns a record, or one that returns raw data. In particular, if you supply a function to get a record, gawk will call it, and will never call the raw read function. gawk ships with a sample extension that reads directories, returning records for each entry in a directory (see “Reading Directories” on page 448). You may wish to use that code as a guide for writing your own input parser. When writing an input parser, you should think about (and document) how it is ex‐ pected to interact with awk code. You may want it to always be called, and to take effect as appropriate (as the readdir extension does). Or you may want it to take effect based upon the value of an awk variable, as the XML extension from the gawkextlib project does (see “The gawkextlib Project” on page 452). In the latter case, code in a BEGINFILE section can look at FILENAME and ERRNO to decide whether or not to activate an input parser (see “The BEGINFILE and ENDFILE Special Patterns” on page 147). You register your input parser with the following function: void register_input_parser(awk_input_parser_t *input_parser); Register the input parser pointed to by input_parser with gawk. API Description | 409 Customized output wrappers An output wrapper is the mirror image of an input parser. It allows an extension to take over the output to a file opened with the ‘>’ or ‘>>’ I/O redirection operators (see “Redirecting Output of print and printf” on page 100). The output wrapper is very similar to the input parser structure: typedef struct awk_output_wrapper { const char *name; /* name of the wrapper */ awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf); awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf); awk_const struct awk_output_wrapper *awk_const next; /* for gawk */ } awk_output_wrapper_t; The members are as follows: const char *name; This is the name of the output wrapper. awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf); This points to a function that examines the information in the awk_output_buf_t structure pointed to by outbuf. It should return true if the output wrapper wants to take over the file, and false otherwise. It should not change any state (variable values, etc.) within gawk. awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf); The function pointed to by this field is called when gawk decides to let the output wrapper take control of the file. It should fill in appropriate members of the awk_output_buf_t structure, as described next, and return true if successful, false otherwise. awk_const struct output_wrapper *awk_const next; This is for use by gawk; therefore it is marked awk_const so that the extension cannot modify it. The awk_output_buf_t structure looks like this: typedef struct awk_output_buf { const char *name; /* name of output file */ const char *mode; /* mode argument to fopen */ FILE *fp; /* stdio file pointer */ awk_bool_t redirected; /* true if a wrapper is active */ void *opaque; /* for use by output wrapper */ size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count, FILE *fp, void *opaque); int (*gawk_fflush)(FILE *fp, void *opaque); int (*gawk_ferror)(FILE *fp, void *opaque); int (*gawk_fclose)(FILE *fp, void *opaque); } awk_output_buf_t; 410 | Chapter 16: Writing Extensions for gawk Here too, your extension will define XXX_can_take_file() and XXX_take_con trol_of() functions that examine and update data members in the awk_output_buf_t. The data members are as follows: const char *name; The name of the output file. const char *mode; The mode string (as would be used in the second argument to fopen()) with which the file was opened. FILE *fp; The FILE pointer from . gawk opens the file before attempting to find an output wrapper. awk_bool_t redirected; This field must be set to true by the XXX_take_control_of() function. void *opaque; This pointer is opaque to gawk. The extension should use it to store a pointer to any private data associated with the file. size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count, FILE *fp, void *opaque); int (*gawk_fflush)(FILE *fp, void *opaque); int (*gawk_ferror)(FILE *fp, void *opaque); int (*gawk_fclose)(FILE *fp, void *opaque); These pointers should be set to point to functions that perform the equivalent functions as the functions do, if appropriate. gawk uses these function pointers for all output. gawk initializes the pointers to point to internal “pass- through” functions that just call the regular functions, so an extension only needs to redefine those functions that are appropriate for what it does. The XXX_can_take_file() function should make a decision based upon the name and mode fields, and any additional state (such as awk variable values) that is appropriate. When gawk calls XXX_take_control_of(), that function should fill in the other fields as appropriate, except for fp, which it should just use normally. You register your output wrapper with the following function: void register_output_wrapper(awk_output_wrapper_t *output_wrapper); Register the output wrapper pointed to by output_wrapper with gawk. API Description | 411 Customized two-way processors A two-way processor combines an input parser and an output wrapper for two-way I/O with the ‘|&’ operator (see “Redirecting Output of print and printf” on page 100). It makes identical usage of the awk_input_parser_t and awk_output_buf_t structures as described earlier. A two-way processor is represented by the following structure: typedef struct awk_two_way_processor { const char *name; /* name of the two-way processor */ awk_bool_t (*can_take_two_way)(const char *name); awk_bool_t (*take_control_of)(const char *name, awk_input_buf_t *inbuf, awk_output_buf_t *outbuf); awk_const struct awk_two_way_processor *awk_const next; /* for gawk */ } awk_two_way_processor_t; The fields are as follows: const char *name; The name of the two-way processor. awk_bool_t (*can_take_two_way)(const char *name); The function pointed to by this field should return true if it wants to take over two-way I/O for this filename. It should not change any state (variable values, etc.) within gawk. awk_bool_t (*take_control_of)(const char *name, awk_input_buf_t *inbuf, awk_output_buf_t *outbuf); The function pointed to by this field should fill in the awk_input_buf_t and awk_outut_buf_t structures pointed to by inbuf and outbuf, respectively. These structures were described earlier. awk_const struct two_way_processor *awk_const next; This is for use by gawk; therefore it is marked awk_const so that the extension cannot modify it. As with the input parser and output processor, you provide “yes I can take this” and “take over for this” functions, XXX_can_take_two_way() and XXX_take_control_of(). You register your two-way processor with the following function: void register_two_way_processor(awk_two_way_processor_t * two_way_processor); Register the two-way processor pointed to by two_way_processor with gawk. 412 | Chapter 16: Writing Extensions for gawk 3. Because the API uses only ISO C 90 features, it cannot make use of the ISO C 99 variadic macro feature to hide that parameter. More’s the pity. Printing Messages You can print different kinds of warning messages from your extension, as described here. Note that for these functions, you must pass in the extension ID received from gawk when the extension was loaded:3 void fatal(awk_ext_id_t id, const char *format, ...); Print a message and then cause gawk to exit immediately. void warning(awk_ext_id_t id, const char *format, ...); Print a warning message. void lintwarn(awk_ext_id_t id, const char *format, ...); Print a “lint warning.” Normally this is the same as printing a warning message, but if gawk was invoked with ‘--lint=fatal’, then lint warnings become fatal error messages. All of these functions are otherwise like the C printf() family of functions, where the format parameter is a string with literal characters and formatting codes intermixed. Updating ERRNO The following functions allow you to update the ERRNO variable: void update_ERRNO_int(int errno_val); Set ERRNO to the string equivalent of the error code in errno_val. The value should be one of the defined error codes in , and gawk turns it into a (possibly translated) string using the C strerror() function. void update_ERRNO_string(const char *string); Set ERRNO directly to the string value of ERRNO. gawk makes a copy of the value of string. void unset_ERRNO(void); Unset ERRNO. Requesting Values All of the functions that return values from gawk work in the same way. You pass in an awk_valtype_t value to indicate what kind of value you expect. If the actual value matches what you requested, the function returns true and fills in the awk_value_t result. Otherwise, the function returns false, and the val_type member indicates the API Description | 413 type of the actual value. You may then print an error message or reissue the request for the actual value type, as appropriate. This behavior is summarized in Table 16-1. Table 16-1. API value types returned Type of Actual Value String Number Array Undefined String String String False False Number Number if can be converted, else false Number False False Type Array False False Array False Requested Scalar Scalar Scalar False False Undefined String Number Array Undefined Value cookie False False False False Accessing and Updating Parameters Two functions give you access to the arguments (parameters) passed to your extension function. They are: awk_bool_t get_argument(size_t count, awk_valtype_t wanted, awk_value_t *result); Fill in the awk_value_t structure pointed to by result with the countth argument. Return true if the actual type matches wanted, and false otherwise. In the latter case, result->val_type indicates the actual type (see Table 16-1). Counts are zero-based —the first argument is numbered zero, the second one, and so on. wanted indicates the type of value expected. awk_bool_t set_argument(size_t count, awk_array_t array); Convert a parameter that was undefined into an array; this provides call by reference for arrays. Return false if count is too big, or if the argument’s type is not undefined. See “Array Manipulation” on page 419 for more information on creating arrays. Symbol Table Access Two sets of routines provide access to global variables, and one set allows you to create and release cached values. Variable access and update by name The following routines provide the ability to access and update global awk-level variables by name. In compiler terminology, identifiers of different kinds are termed symbols, thus the “sym” in the routines’ names. The data structure that stores information about symbols is termed a symbol table. The functions are as follows: 414 | Chapter 16: Writing Extensions for gawk awk_bool_t sym_lookup(const char *name, awk_valtype_t wanted, awk_value_t *result); Fill in the awk_value_t structure pointed to by result with the value of the variable named by the string name, which is a regular C string. wanted indicates the type of value expected. Return true if the actual type matches wanted, and false otherwise. In the latter case, result->val_type indicates the actual type (see Table 16-1). awk_bool_t sym_update(const char *name, awk_value_t *value); Update the variable named by the string name, which is a regular C string. The variable is added to gawk’s symbol table if it is not there. Return true if everything worked, and false otherwise. Changing types (scalar to array or vice versa) of an existing variable is not allowed, nor may this routine be used to update an array. This routine cannot be used to update any of the predefined variables (such as ARGC or NF). An extension can look up the value of gawk’s special variables. However, with the ex‐ ception of the PROCINFO array, an extension cannot change any of those variables. It is possible for the lookup of PROCINFO to fail. This happens if the awk program being run does not reference PROCINFO; in this case, gawk doesn’t bother to create the array and populate it. Variable access and update by cookie A scalar cookie is an opaque handle that provides access to a global variable or array. It is an optimization that avoids looking up variables in gawk’s symbol table every time access is needed. This was discussed earlier, in “General-Purpose Data Types” on page 399. The following functions let you work with scalar cookies: awk_bool_t sym_lookup_scalar(awk_scalar_t cookie, awk_valtype_t wanted, awk_value_t *result); Retrieve the current value of a scalar cookie. Once you have obtained a scalar cookie using sym_lookup(), you can use this function to get its value more efficiently. Return false if the value cannot be retrieved. awk_bool_t sym_update_scalar(awk_scalar_t cookie, awk_value_t *value); Update the value associated with a scalar cookie. Return false if the new value is not of type AWK_STRING or AWK_NUMBER. Here too, the predefined variables may not be updated. API Description | 415 4. The difference is measurable and quite real. Trust us. It is not obvious at first glance how to work with scalar cookies or what their raison d’être really is. In theory, the sym_lookup() and sym_update() routines are all you really need to work with variables. For example, you might have code that looks up the value of a variable, evaluates a condition, and then possibly changes the value of the variable based on the result of that evaluation, like so: /* do_magic --- do something really great */ static awk_value_t * do_magic(int nargs, awk_value_t *result) { awk_value_t value; if ( sym_lookup("MAGIC_VAR", AWK_NUMBER, & value) && some_condition(value.num_value)) { value.num_value += 42; sym_update("MAGIC_VAR", & value); } return make_number(0.0, result); } This code looks (and is) simple and straightforward. So what’s the problem? Well, consider what happens if awk-level code associated with your extension calls the magic() function (implemented in C by do_magic()), once per record, while processing hundreds of thousands or millions of records. The MAGIC_VAR variable is looked up in the symbol table once or twice per function call! The symbol table lookup is really pure overhead; it is considerably more efficient to get a cookie that represents the variable, and use that to get the variable’s value and update it as needed.4 Thus, the way to use cookies is as follows. First, install your extension’s variable in gawk’s symbol table using sym_update(), as usual. Then get a scalar cookie for the variable using sym_lookup(): static awk_scalar_t magic_var_cookie; /* cookie for MAGIC_VAR */ static void my_extension_init() { awk_value_t value; /* install initial value */ sym_update("MAGIC_VAR", make_number(42.0, & value)); 416 | Chapter 16: Writing Extensions for gawk 5. Numeric values are clearly less problematic, requiring only a C double to store. /* get the cookie */ sym_lookup("MAGIC_VAR", AWK_SCALAR, & value); /* save the cookie */ magic_var_cookie = value.scalar_cookie; … } Next, use the routines in this section for retrieving and updating the value through the cookie. Thus, do_magic() now becomes something like this: /* do_magic --- do something really great */ static awk_value_t * do_magic(int nargs, awk_value_t *result) { awk_value_t value; if ( sym_lookup_scalar(magic_var_cookie, AWK_NUMBER, & value) && some_condition(value.num_value)) { value.num_value += 42; sym_update_scalar(magic_var_cookie, & value); } … return make_number(0.0, result); } The previous code omitted error checking for presentation purpos‐ es. Your extension code should be more robust and carefully check the return values from the API functions. Creating and using cached values The routines in this section allow you to create and release cached values. Like scalar cookies, in theory, cached values are not necessary. You can create numbers and strings using the functions in “Constructor Functions” on page 403. You can then assign those values to variables using sym_update() or sym_update_scalar(), as you like. However, you can understand the point of cached values if you remember that every string value’s storage must come from gawk_malloc(), gawk_calloc(), or gawk_realloc(). If you have 20 variables, all of which have the same string value, you must create 20 identical copies of the string.5 API Description | 417 It is clearly more efficient, if possible, to create a value once and then tell gawk to reuse the value for multiple variables. That is what the routines in this section let you do. The functions are as follows: awk_bool_t create_value(awk_value_t *value, awk_value_cookie_t *result); Create a cached string or numeric value from value for efficient later assignment. Only values of type AWK_NUMBER and AWK_STRING are allowed. Any other type is rejected. AWK_UNDEFINED could be allowed, but doing so would result in inferior performance. awk_bool_t release_value(awk_value_cookie_t vc); Release the memory associated with a value cookie obtained from create_value(). You use value cookies in a fashion similar to the way you use scalar cookies. In the extension initialization routine, you create the value cookie: static awk_value_cookie_t answer_cookie; /* static value cookie */ static void my_extension_init() { awk_value_t value; char *long_string; size_t long_string_len; /* code from earlier */ … /* … fill in long_string and long_string_len … */ make_malloced_string(long_string, long_string_len, & value); create_value(& value, & answer_cookie); /* create cookie */ … } Once the value is created, you can use it as the value of any number of variables: static awk_value_t * do_magic(int nargs, awk_value_t *result) { awk_value_t new_value; … /* as earlier */ value.val_type = AWK_VALUE_COOKIE; value.value_cookie = answer_cookie; sym_update("VAR1", & value); sym_update("VAR2", & value); … sym_update("VAR100", & value); … } 418 | Chapter 16: Writing Extensions for gawk 6. OK, the only data structure. 7. It is also a “cookie,” but the gawk developers did not wish to overuse this term. Using value cookies in this way saves considerable storage, as all of VAR1 through VAR100 share the same value. You might be wondering, “Is this sharing problematic? What happens if awk code assigns a new value to VAR1; are all the others changed too?” That’s a great question. The answer is that no, it’s not a problem. Internally, gawk uses reference-counted strings. This means that many variables can share the same string value, and gawk keeps track of the usage. When a variable’s value changes, gawk simply decrements the reference count on the old value and updates the variable to use the new value. Finally, as part of your cleanup action (see “Registering an exit callback function” on page 405) you should release any cached values that you created, using release_value(). Array Manipulation The primary data structure6 in awk is the associative array (see Chapter 8). Extensions need to be able to manipulate awk arrays. The API provides a number of data structures for working with arrays, functions for working with individual elements, and functions for working with arrays as a whole. This includes the ability to “flatten” an array so that it is easy for C code to traverse every element in an array. The array data structures integrate nicely with the data structures for values to make it easy to both work with and create true arrays of arrays (see “General-Purpose Data Types” on page 399). Array data types The data types associated with arrays are as follows: typedef void *awk_array_t; If you request the value of an array variable, you get back an awk_array_t value. This value is opaque7 to the extension; it uniquely identifies the array but can only be used by passing it into API functions or receiving it from API functions. This is very similar to way ‘FILE *’ values are used with the library routines. typedef struct awk_element { /* convenience linked list pointer, not used by gawk */ struct awk_element *next; enum { AWK_ELEMENT_DEFAULT = 0, /* set by gawk */ API Description | 419 AWK_ELEMENT_DELETE = 1 /* set by extension */ } flags; awk_value_t index; awk_value_t value; } awk_element_t; The awk_element_t is a “flattened” array element. awk produces an array of these inside the awk_flat_array_t (see the next item). Individual elements may be marked for deletion. New elements must be added individually, one at a time, using the separate API for that purpose. The fields are as follows: struct awk_element *next; This pointer is for the convenience of extension writers. It allows an extension to create a linked list of new elements that can then be added to an array in a loop that traverses the list. enum { … } flags; A set of flag values that convey information between the extension and gawk. Currently there is only one: AWK_ELEMENT_DELETE. Setting it causes gawk to delete the element from the original array upon release of the flattened array. index value The index and value of the element, respectively. All memory pointed to by index and value belongs to gawk. typedef struct awk_flat_array { awk_const void *awk_const opaque1; /* for use by gawk */ awk_const void *awk_const opaque2; /* for use by gawk */ awk_const size_t count; /* how many elements */ awk_element_t elements[1]; /* will be extended */ } awk_flat_array_t; This is a flattened array. When an extension gets one of these from gawk, the elements array is of actual size count. The opaque1 and opaque2 pointers are for use by gawk; therefore they are marked awk_const so that the extension cannot modify them. Array functions The following functions relate to individual array elements: awk_bool_t get_element_count(awk_array_t a_cookie, size_t *count); For the array represented by a_cookie, place in *count the number of elements it contains. A subarray counts as a single element. Return false if there is an error. 420 | Chapter 16: Writing Extensions for gawk awk_bool_t get_array_element(awk_array_t a_cookie, const awk_value_t *const index, awk_valtype_t wanted, awk_value_t *result); For the array represented by a_cookie, return in *result the value of the element whose index is index. wanted specifies the type of value you wish to retrieve. Return false if wanted does not match the actual type or if index is not in the array (see Table 16-1). The value for index can be numeric, in which case gawk converts it to a string. Using nonintegral values is possible, but requires that you understand how such values are converted to strings (see “Conversion of Strings and Numbers” on page 117); thus, using integral values is safest. As with all strings passed into gawk from an extension, the string value of index must come from gawk_malloc(), gawk_calloc(), or gawk_realloc(), and gawk releases the storage. awk_bool_t set_array_element(awk_array_t a_cookie, const awk_value_t *const index, const awk_value_t *const value); In the array represented by a_cookie, create or modify the element whose index is given by index. The ARGV and ENVIRON arrays may not be changed, although the PROCINFO array can be. awk_bool_t set_array_element_by_elem(awk_array_t a_cookie, awk_element_t element); Like set_array_element(), but take the index and value from element. This is a convenience macro. awk_bool_t del_array_element(awk_array_t a_cookie, const awk_value_t* const index); Remove the element with the given index from the array represented by a_cookie. Return true if the element was removed, or false if the element did not exist in the array. The following functions relate to arrays as a whole: awk_array_t create_array(void); Create a new array to which elements may be added. See “How to create and pop‐ ulate arrays” on page 425 for a discussion of how to create a new array and add elements to it. API Description | 421 awk_bool_t clear_array(awk_array_t a_cookie); Clear the array represented by a_cookie. Return false if there was some kind of problem, true otherwise. The array remains an array, but after calling this function, it has no elements. This is equivalent to using the delete statement (see “The delete Statement” on page 184). awk_bool_t flatten_array(awk_array_t a_cookie, awk_flat_array_t **data); For the array represented by a_cookie, create an awk_flat_array_t structure and fill it in. Set the pointer whose address is passed as data to point to this structure. Return true upon success, or false otherwise. See the next section for a discussion of how to flatten an array and work with it. awk_bool_t release_flattened_array(awk_array_t a_cookie, awk_flat_array_t *data); When done with a flattened array, release the storage using this function. You must pass in both the original array cookie and the address of the created awk_flat_array_t structure. The function returns true upon success, false other‐ wise. Working with all the elements of an array To flatten an array is to create a structure that represents the full array in a fashion that makes it easy for C code to traverse the entire array. Some of the code in extension/ testext.c does this, and also serves as a nice example showing how to use the APIs. We walk through that part of the code one step at a time. First, the gawk script that drives the test extension: @load "testext" BEGIN { n = split("blacky rusty sophie raincloud lucky", pets) printf("pets has %d elements\n", length(pets)) ret = dump_array_and_delete("pets", "3") printf("dump_array_and_delete(pets) returned %d\n", ret) if ("3" in pets) printf("dump_array_and_delete() did NOT remove index \"3\"!\n") else printf("dump_array_and_delete() did remove index \"3\"!\n") print "" } This code creates an array with split() (see “String-Manipulation Functions” on page 194) and then calls dump_array_and_delete(). That function looks up the array whose name is passed as the first argument, and deletes the element at the index passed in the second argument. The awk code then prints the return value and checks if the element was indeed deleted. Here is the C code that implements dump_array_and_delete(). It has been edited slightly for presentation. 422 | Chapter 16: Writing Extensions for gawk The first part declares variables, sets up the default return value in result, and checks that the function was called with the correct number of arguments: static awk_value_t * dump_array_and_delete(int nargs, awk_value_t *result) { awk_value_t value, value2, value3; awk_flat_array_t *flat_array; size_t count; char *name; int i; assert(result != NULL); make_number(0.0, result); if (nargs != 2) { printf("dump_array_and_delete: nargs not right " "(%d should be 2)\n", nargs); goto out; } The function then proceeds in steps, as follows. First, retrieve the name of the array, passed as the first argument, followed by the array itself. If either operation fails, print an error message and return: /* get argument named array as flat array and print it */ if (get_argument(0, AWK_STRING, & value)) { name = value.str_value.str; if (sym_lookup(name, AWK_ARRAY, & value2)) printf("dump_array_and_delete: sym_lookup of %s passed\n", name); else { printf("dump_array_and_delete: sym_lookup of %s failed\n", name); goto out; } } else { printf("dump_array_and_delete: get_argument(0) failed\n"); goto out; } For testing purposes and to make sure that the C code sees the same number of elements as the awk code, the second step is to get the count of elements in the array and print it: if (! get_element_count(value2.array_cookie, & count)) { printf("dump_array_and_delete: get_element_count failed\n"); goto out; } printf("dump_array_and_delete: incoming size is %lu\n", (unsigned long) count); API Description | 423 The third step is to actually flatten the array, and then to double-check that the count in the awk_flat_array_t is the same as the count just retrieved: if (! flatten_array(value2.array_cookie, & flat_array)) { printf("dump_array_and_delete: could not flatten array\n"); goto out; } if (flat_array->count != count) { printf("dump_array_and_delete: flat_array->count (%lu)" " != count (%lu)\n", (unsigned long) flat_array->count, (unsigned long) count); goto out; } The fourth step is to retrieve the index of the element to be deleted, which was passed as the second argument. Remember that argument counts passed to get_argument() are zero-based, and thus the second argument is numbered one: if (! get_argument(1, AWK_STRING, & value3)) { printf("dump_array_and_delete: get_argument(1) failed\n"); goto out; } The fifth step is where the “real work” is done. The function loops over every element in the array, printing the index and element values. In addition, upon finding the element with the index that is supposed to be deleted, the function sets the AWK_ELEMENT_DELETE bit in the flags field of the element. When the array is released, gawk traverses the flattened array, and deletes any elements that have this flag bit set: for (i = 0; i < flat_array->count; i++) { printf("\t%s[\"%.*s\"] = %s\n", name, (int) flat_array->elements[i].index.str_value.len, flat_array->elements[i].index.str_value.str, valrep2str(& flat_array->elements[i].value)); if (strcmp(value3.str_value.str, flat_array->elements[i].index.str_value.str) == 0) { flat_array->elements[i].flags |= AWK_ELEMENT_DELETE; printf("dump_array_and_delete: marking element \"%s\" " "for deletion\n", flat_array->elements[i].index.str_value.str); } } The sixth step is to release the flattened array. This tells gawk that the extension is no longer using the array, and that it should delete any elements marked for deletion. gawk also frees any storage that was allocated, so you should not use the pointer (flat_array in this code) once you have called release_flattened_array(): 424 | Chapter 16: Writing Extensions for gawk if (! release_flattened_array(value2.array_cookie, flat_array)) { printf("dump_array_and_delete: could not release flattened array\n"); goto out; } Finally, because everything was successful, the function sets the return value to success, and returns: make_number(1.0, result); out: return result; } Here is the output from running this part of the test: pets has 5 elements dump_array_and_delete: sym_lookup of pets passed dump_array_and_delete: incoming size is 5 pets["1"] = "blacky" pets["2"] = "rusty" pets["3"] = "sophie" dump_array_and_delete: marking element "3" for deletion pets["4"] = "raincloud" pets["5"] = "lucky" dump_array_and_delete(pets) returned 1 dump_array_and_delete() did remove index "3"! How to create and populate arrays Besides working with arrays created by awk code, you can create arrays and populate them as you see fit, and then awk code can access them and manipulate them. There are two important points about creating arrays from extension code: • You must install a new array into gawk’s symbol table immediately upon creating it. Once you have done so, you can then populate the array. Similarly, if installing a new array as a subarray of an existing array, you must add the new array to its parent before adding any elements to it. Thus, the correct way to build an array is to work “top down.” Create the array, and immediately install it in gawk’s symbol table using sym_update(), or install it as an element in a previously existing array using set_array_element(). We show ex‐ ample code shortly. • Due to gawk internals, after using sym_update() to install an array into gawk, you have to retrieve the array cookie from the value passed in to sym_update() before doing anything else with it, like so: awk_value_t value; awk_array_t new_array; API Description | 425 new_array = create_array(); val.val_type = AWK_ARRAY; val.array_cookie = new_array; /* install array in the symbol table */ sym_update("array", & val); new_array = val.array_cookie; /* YOU MUST DO THIS */ If installing an array as a subarray, you must also retrieve the value of the array cookie after the call to set_element(). The following C code is a simple test extension to create an array with two regular elements and with a subarray. The leading #include directives and boilerplate variable declarations (see “Boilerplate Code” on page 429) are omitted for brevity. The first step is to create a new array and then install it in the symbol table: /* create_new_array --- create a named array */ static void create_new_array() { awk_array_t a_cookie; awk_array_t subarray; awk_value_t index, value; a_cookie = create_array(); value.val_type = AWK_ARRAY; value.array_cookie = a_cookie; if (! sym_update("new_array", & value)) printf("create_new_array: sym_update(\"new_array\") failed!\n"); a_cookie = value.array_cookie; Note how a_cookie is reset from the array_cookie field in the value structure. The second step is to install two regular values into new_array: (void) make_const_string("hello", 5, & index); (void) make_const_string("world", 5, & value); if (! set_array_element(a_cookie, & index, & value)) { printf("fill_in_array: set_array_element failed\n"); return; } (void) make_const_string("answer", 6, & index); (void) make_number(42.0, & value); if (! set_array_element(a_cookie, & index, & value)) { printf("fill_in_array: set_array_element failed\n"); return; } 426 | Chapter 16: Writing Extensions for gawk The third step is to create the subarray and install it: (void) make_const_string("subarray", 8, & index); subarray = create_array(); value.val_type = AWK_ARRAY; value.array_cookie = subarray; if (! set_array_element(a_cookie, & index, & value)) { printf("fill_in_array: set_array_element failed\n"); return; } subarray = value.array_cookie; The final step is to populate the subarray with its own element: (void) make_const_string("foo", 3, & index); (void) make_const_string("bar", 3, & value); if (! set_array_element(subarray, & index, & value)) { printf("fill_in_array: set_array_element failed\n"); return; } } Here is a sample script that loads the extension and then dumps the array: @load "subarray" function dumparray(name, array, i) { for (i in array) if (isarray(array[i])) dumparray(name "[\"" i "\"]", array[i]) else printf("%s[\"%s\"] = %s\n", name, i, array[i]) } BEGIN { dumparray("new_array", new_array); } Here is the result of running the script: $ AWKLIBPATH=$PWD ./gawk -f subarray.awk new_array["subarray"]["foo"] = bar new_array["hello"] = world new_array["answer"] = 42 (See “How gawk Finds Extensions” on page 431 for more information on the AWKLIB PATH environment variable.) API Description | 427 API Variables The API provides two sets of variables. The first provides information about the version of the API (both with which the extension was compiled, and with which gawk was compiled). The second provides information about how gawk was invoked. API version constants and variables The API provides both a “major” and a “minor” version number. The API versions are available at compile time as constants: GAWK_API_MAJOR_VERSION The major version of the API GAWK_API_MINOR_VERSION The minor version of the API The minor version increases when new functions are added to the API. Such new func‐ tions are always added to the end of the API struct. The major version increases (and the minor version is reset to zero) if any of the data types change size or member order, or if any of the existing functions change signature. It could happen that an extension may be compiled against one version of the API but loaded by a version of gawk using a different version. For this reason, the major and minor API versions of the running gawk are included in the API struct as read-only constant integers: api->major_version The major version of the running gawk api->minor_version The minor version of the running gawk It is up to the extension to decide if there are API incompatibilities. Typically, a check like this is enough: if (api->major_version != GAWK_API_MAJOR_VERSION || api->minor_version < GAWK_API_MINOR_VERSION) { fprintf(stderr, "foo_extension: version mismatch with gawk!\n"); fprintf(stderr, "\tmy version (%d, %d), gawk version (%d, %d)\n", GAWK_API_MAJOR_VERSION, GAWK_API_MINOR_VERSION, api->major_version, api->minor_version); exit(1); } Such code is included in the boilerplate dl_load_func() macro provided in gawkapi.h (discussed in “Boilerplate Code” on page 429). 428 | Chapter 16: Writing Extensions for gawk Informational variables The API provides access to several variables that describe whether the corresponding command-line options were enabled when gawk was invoked. The variables are: do_debug This variable is true if gawk was invoked with --debug option. do_lint This variable is true if gawk was invoked with --lint option. do_mpfr This variable is true if gawk was invoked with --bignum option. do_profile This variable is true if gawk was invoked with --profile option. do_sandbox This variable is true if gawk was invoked with --sandbox option. do_traditional This variable is true if gawk was invoked with --traditional option. The value of do_lint can change if awk code modifies the LINT predefined variable (see “Predefined Variables” on page 160). The others should not change during execution. Boilerplate Code As mentioned earlier (see “How It Works at a High Level” on page 394), the function definitions as presented are really macros. To use these macros, your extension must provide a small amount of boilerplate code (variables and functions) toward the top of your source file, using predefined names as described here. The boilerplate needed is also provided in comments in the gawkapi.h header file: /* Boilerplate code: */ int plugin_is_GPL_compatible; static gawk_api_t *const api; static awk_ext_id_t ext_id; static const char *ext_version = NULL; /* or … = "some string" */ static awk_ext_func_t func_table[] = { { "name", do_name, 1 }, /* … */ }; /* EITHER: */ static awk_bool_t (*init_func)(void) = NULL; API Description | 429 /* OR: */ static awk_bool_t init_my_extension(void) { … } static awk_bool_t (*init_func)(void) = init_my_extension; dl_load_func(func_table, some_name, "name_space_in_quotes") These variables and functions are as follows: int plugin_is_GPL_compatible; This asserts that the extension is compatible with the GNU GPL. If your extension does not have this, gawk will not load it (see “Extension Licensing” on page 394). static gawk_api_t *const api; This global static variable should be set to point to the gawk_api_t pointer that gawk passes to your dl_load() function. This variable is used by all of the macros. static awk_ext_id_t ext_id; This global static variable should be set to the awk_ext_id_t value that gawk passes to your dl_load() function. This variable is used by all of the macros. static const char *ext_version = NULL; /* or … = "some string" */ This global static variable should be set either to NULL, or to point to a string giving the name and version of your extension. static awk_ext_func_t func_table[] = { … }; This is an array of one or more awk_ext_func_t structures, as described earlier (see “Registering an extension function” on page 404). It can then be looped over for multiple calls to add_ext_func(). static awk_bool_t (*init_func)(void) = NULL; OR static awk_bool_t init_my_extension(void) { … } static awk_bool_t (*init_func)(void) = init_my_extension; If you need to do some initialization work, you should define a function that does it (creates variables, opens files, etc.) and then define the init_func pointer to point to your function. The function should return awk_false upon failure, or awk_true if everything goes well. If you don’t need to do any initialization, define the pointer and initialize it to NULL. 430 | Chapter 16: Writing Extensions for gawk dl_load_func(func_table, some_name, "name_space_in_quotes") This macro expands to a dl_load() function that performs all the necessary initializations. The point of all the variables and arrays is to let the dl_load() function (from the dl_load_func() macro) do all the standard work. It does the following: 1. Check the API versions. If the extension major version does not match gawk’s, or if the extension minor version is greater than gawk’s, it prints a fatal error message and exits. 2. Load the functions defined in func_table. If any of them fails to load, it prints a warning message but continues on. 3. If the init_func pointer is not NULL, call the function it points to. If it returns awk_false, print a warning message. 4. If ext_version is not NULL, register the version string with gawk. How gawk Finds Extensions Compiled extensions have to be installed in a directory where gawk can find them. If gawk is configured and built in the default fashion, the directory in which to find ex‐ tensions is /usr/local/lib/gawk. You can also specify a search path with a list of di‐ rectories to search for compiled extensions. See “The AWKLIBPATH Environment Variable” on page 32 for more information. Example: Some File Functions No matter where you go, there you are. —Buckaroo Banzai Two useful functions that are not in awk are chdir() (so that an awk program can change its directory) and stat() (so that an awk program can gather information about a file). In order to illustrate the API in action, this section implements these functions for gawk in an extension. Using chdir() and stat() This section shows how to use the new functions at the awk level once they’ve been integrated into the running gawk interpreter. Using chdir() is very straightforward. It takes one argument, the new directory to change to: @load "filefuncs" … How gawk Finds Extensions | 431 newdir = "/home/arnold/funstuff" ret = chdir(newdir) if (ret < 0) { printf("could not change to %s: %s\n", newdir, ERRNO) > "/dev/stderr" exit 1 } … The return value is negative if the chdir() failed, and ERRNO (see “Predefined Vari‐ ables” on page 160) is set to a string indicating the error. Using stat() is a bit more complicated. The C stat() function fills in a structure that has a fair amount of information. The right way to model this in awk is to fill in an associative array with the appropriate information: file = "/home/arnold/.profile" ret = stat(file, fdata) if (ret < 0) { printf("could not stat %s: %s\n", file, ERRNO) > "/dev/stderr" exit 1 } printf("size of %s is %d bytes\n", file, fdata["size"]) The stat() function always clears the data array, even if the stat() fails. It fills in the following elements: "name" The name of the file that was stat()ed. "dev" "ino" The file’s device and inode numbers, respectively. "mode" The file’s mode, as a numeric value. This includes both the file’s type and its permissions. "nlink" The number of hard links (directory entries) the file has. "uid" "gid" The numeric user and group ID numbers of the file’s owner. "size" The size in bytes of the file. 432 | Chapter 16: Writing Extensions for gawk "blocks" The number of disk blocks the file actually occupies. This may not be a function of the file’s size if the file has holes. "atime" "mtime" "ctime" The file’s last access, modification, and inode update times, respectively. These are numeric timestamps, suitable for formatting with strftime() (see “Time Func‐ tions” on page 211). "pmode" The file’s “printable mode.” This is a string representation of the file’s type and permissions, such as is produced by ‘ls -l’—for example, "drwxr-xr-x". "type" A printable string representation of the file’s type. The value is one of the following: "blockdev" "chardev" The file is a block or character device (“special file”). "directory" The file is a directory. "fifo" The file is a named pipe (also known as a FIFO). "file" The file is just a regular file. "socket" The file is an AF_UNIX (“Unix domain”) socket in the filesystem. "symlink" The file is a symbolic link. "devbsize" The size of a block for the element indexed by "blocks". This information is derived from either the DEV_BSIZE constant defined in on most systems, or the S_BLKSIZE constant in on BSD systems. For some other systems, a priori knowledge is used to provide a value. Where no value can be determined, it defaults to 512. Several additional elements may be present, depending upon the operating system and the type of the file. You can test for them in your awk program by using the in operator (see “Referring to an Array Element” on page 175): Example: Some File Functions | 433 8. This version is edited slightly for presentation. See extension/filefuncs.c in the gawk distribution for the complete version. "blksize" The preferred block size for I/O to the file. This field is not present on all POSIX- like systems in the C stat structure. "linkval" If the file is a symbolic link, this element is the name of the file the link points to (i.e., the value of the link). "rdev" "major" "minor" If the file is a block or character device file, then these values represent the numeric device number and the major and minor components of that number, respectively. C Code for chdir() and stat() Here is the C code for these extensions.8 The file includes a number of standard header files, and then includes the gawkapi.h header file, which provides the API definitions. Those are followed by the necessary variable declarations to make use of the API macros and boilerplate code (see “Boiler‐ plate Code” on page 429): #ifdef HAVE_CONFIG_H #include #endif #include #include #include #include #include #include #include #include #include "gawkapi.h" #include "gettext.h" #define _(msgid) gettext(msgid) #define N_(msgid) msgid #include "gawkfts.h" #include "stack.h" 434 | Chapter 16: Writing Extensions for gawk static const gawk_api_t *api; /* for convenience macros to work */ static awk_ext_id_t *ext_id; static awk_bool_t init_filefuncs(void); static awk_bool_t (*init_func)(void) = init_filefuncs; static const char *ext_version = "filefuncs extension: version 1.0"; int plugin_is_GPL_compatible; By convention, for an awk function foo(), the C function that implements it is called do_foo(). The function should have two arguments. The first is an int, usually called nargs, that represents the number of actual arguments for the function. The second is a pointer to an awk_value_t structure, usually named result: /* do_chdir --- provide dynamically loaded chdir() function for gawk */ static awk_value_t * do_chdir(int nargs, awk_value_t *result) { awk_value_t newdir; int ret = -1; assert(result != NULL); if (do_lint && nargs != 1) lintwarn(ext_id, _("chdir: called with incorrect number of arguments, " "expecting 1")); The newdir variable represents the new directory to change to, which is retrieved with get_argument(). Note that the first argument is numbered zero. If the argument is retrieved successfully, the function calls the chdir() system call. If the chdir() fails, ERRNO is updated: if (get_argument(0, AWK_STRING, & newdir)) { ret = chdir(newdir.str_value.str); if (ret < 0) update_ERRNO_int(errno); } Finally, the function returns the return value to the awk level: return make_number(ret, result); } The stat() extension is more involved. First comes a function that turns a numeric mode into a printable representation (e.g., octal 0644 becomes ‘-rw-r--r--’). This is omitted here for brevity: /* format_mode --- turn a stat mode field into something readable */ Example: Some File Functions | 435 static char * format_mode(unsigned long fmode) { … } Next comes a function for reading symbolic links, which is also omitted here for brevity: /* read_symlink --- read a symbolic link into an allocated buffer. … */ static char * read_symlink(const char *fname, size_t bufsize, ssize_t *linksize) { … } Two helper functions simplify entering values in the array that will contain the result of the stat(): /* array_set --- set an array element */ static void array_set(awk_array_t array, const char *sub, awk_value_t *value) { awk_value_t index; set_array_element(array, make_const_string(sub, strlen(sub), & index), value); } /* array_set_numeric --- set an array element with a number */ static void array_set_numeric(awk_array_t array, const char *sub, double num) { awk_value_t tmp; array_set(array, sub, make_number(num, & tmp)); } The following function does most of the work to fill in the awk_array_t result array with values obtained from a valid struct stat. This work is done in a separate function to support the stat() function for gawk and also to support the fts() extension, which is included in the same file but whose code is not shown here (see “File-Related Func‐ tions” on page 442). The first part of the function is variable declarations, including a table to map file types to strings: 436 | Chapter 16: Writing Extensions for gawk /* fill_stat_array --- do the work to fill an array with stat info */ static int fill_stat_array(const char *name, awk_array_t array, struct stat *sbuf) { char *pmode; /* printable mode */ const char *type = "unknown"; awk_value_t tmp; static struct ftype_map { unsigned int mask; const char *type; } ftype_map[] = { { S_IFREG, "file" }, { S_IFBLK, "blockdev" }, { S_IFCHR, "chardev" }, { S_IFDIR, "directory" }, #ifdef S_IFSOCK { S_IFSOCK, "socket" }, #endif #ifdef S_IFIFO { S_IFIFO, "fifo" }, #endif #ifdef S_IFLNK { S_IFLNK, "symlink" }, #endif #ifdef S_IFDOOR /* Solaris weirdness */ { S_IFDOOR, "door" }, #endif /* S_IFDOOR */ }; int j, k; The destination array is cleared, and then code fills in various elements based on values in the struct stat: /* empty out the array */ clear_array(array); /* fill in the array */ array_set(array, "name", make_const_string(name, strlen(name), & tmp)); array_set_numeric(array, "dev", sbuf->st_dev); array_set_numeric(array, "ino", sbuf->st_ino); array_set_numeric(array, "mode", sbuf->st_mode); array_set_numeric(array, "nlink", sbuf->st_nlink); array_set_numeric(array, "uid", sbuf->st_uid); array_set_numeric(array, "gid", sbuf->st_gid); array_set_numeric(array, "size", sbuf->st_size); array_set_numeric(array, "blocks", sbuf->st_blocks); array_set_numeric(array, "atime", sbuf->st_atime); array_set_numeric(array, "mtime", sbuf->st_mtime); array_set_numeric(array, "ctime", sbuf->st_ctime); /* for block and character devices, add rdev, Example: Some File Functions | 437 major and minor numbers */ if (S_ISBLK(sbuf->st_mode) || S_ISCHR(sbuf->st_mode)) { array_set_numeric(array, "rdev", sbuf->st_rdev); array_set_numeric(array, "major", major(sbuf->st_rdev)); array_set_numeric(array, "minor", minor(sbuf->st_rdev)); } The latter part of the function makes selective additions to the destination array, de‐ pending upon the availability of certain members and/or the type of the file. It then returns zero, for success: #ifdef HAVE_STRUCT_STAT_ST_BLKSIZE array_set_numeric(array, "blksize", sbuf->st_blksize); #endif /* HAVE_STRUCT_STAT_ST_BLKSIZE */ pmode = format_mode(sbuf->st_mode); array_set(array, "pmode", make_const_string(pmode, strlen(pmode), & tmp)); /* for symbolic links, add a linkval field */ if (S_ISLNK(sbuf->st_mode)) { char *buf; ssize_t linksize; if ((buf = read_symlink(name, sbuf->st_size, & linksize)) != NULL) array_set(array, "linkval", make_malloced_string(buf, linksize, & tmp)); else warning(ext_id, _("stat: unable to read symbolic link `%s'"), name); } /* add a type field */ type = "unknown"; /* shouldn't happen */ for (j = 0, k = sizeof(ftype_map)/sizeof(ftype_map[0]); j < k; j++) { if ((sbuf->st_mode & S_IFMT) == ftype_map[j].mask) { type = ftype_map[j].type; break; } } array_set(array, "type", make_const_string(type, strlen(type), & tmp)); return 0; } The third argument to stat() was not discussed previously. This argument is optional. If present, it causes do_stat() to use the stat() system call instead of the lstat() system call. This is done by using a function pointer: statfunc. statfunc is initialized to point to lstat() (instead of stat()) to get the file information, in case the file is a 438 | Chapter 16: Writing Extensions for gawk symbolic link. However, if the third argument is included, statfunc is set to point to stat(), instead. Here is the do_stat() function, which starts with variable declarations and argument checking: /* do_stat --- provide a stat() function for gawk */ static awk_value_t * do_stat(int nargs, awk_value_t *result) { awk_value_t file_param, array_param; char *name; awk_array_t array; int ret; struct stat sbuf; /* default is lstat() */ int (*statfunc)(const char *path, struct stat *sbuf) = lstat; assert(result != NULL); if (nargs != 2 && nargs != 3) { if (do_lint) lintwarn(ext_id, _("stat: called with wrong number of arguments")); return make_number(-1, result); } Then comes the actual work. First, the function gets the arguments. Next, it gets the information for the file. If the called function (lstat() or stat()) returns an error, the code sets ERRNO and returns: /* file is first arg, array to hold results is second */ if ( ! get_argument(0, AWK_STRING, & file_param) || ! get_argument(1, AWK_ARRAY, & array_param)) { warning(ext_id, _("stat: bad parameters")); return make_number(-1, result); } if (nargs == 3) { statfunc = stat; } name = file_param.str_value.str; array = array_param.array_cookie; /* always empty out the array */ clear_array(array); /* stat the file; if error, set ERRNO and return */ ret = statfunc(name, & sbuf); if (ret < 0) { Example: Some File Functions | 439 update_ERRNO_int(errno); return make_number(ret, result); } The tedious work is done by fill_stat_array(), shown earlier. When done, the func‐ tion returns the result from fill_stat_array(): ret = fill_stat_array(name, array, & sbuf); return make_number(ret, result); } Finally, it’s necessary to provide the “glue” that loads the new function(s) into gawk. The filefuncs extension also provides an fts() function, which we omit here (see “File-Related Functions” on page 442). For its sake, there is an initialization function: /* init_filefuncs --- initialization routine */ static awk_bool_t init_filefuncs(void) { … } We are almost done. We need an array of awk_ext_func_t structures for loading each function into gawk: static awk_ext_func_t func_table[] = { { "chdir", do_chdir, 1 }, { "stat", do_stat, 2 }, #ifndef __MINGW32__ { "fts", do_fts, 3 }, #endif }; Each extension must have a routine named dl_load() to load everything that needs to be loaded. It is simplest to use the dl_load_func() macro in gawkapi.h: /* define the dl_load() function using the boilerplate macro */ dl_load_func(func_table, filefuncs, "") And that’s it! Integrating the Extensions Now that the code is written, it must be possible to add it at runtime to the running gawk interpreter. First, the code must be compiled. Assuming that the functions are in 440 | Chapter 16: Writing Extensions for gawk 9. In practice, you would probably want to use the GNU Autotools (Automake, Autoconf, Libtool, and get- text) to configure and build your libraries. Instructions for doing so are beyond the scope of this book. See “The gawkextlib Project” on page 452 for Internet links to the tools. a file named filefuncs.c, and idir is the location of the gawkapi.h header file, the following steps9 create a GNU/Linux shared library: $ gcc -fPIC -shared -DHAVE_CONFIG_H -c -O -g -Iidir filefuncs.c $ gcc -o filefuncs.so -shared filefuncs.o Once the library exists, it is loaded by using the @load keyword: # file testff.awk @load "filefuncs" BEGIN { "pwd" | getline curdir # save current directory close("pwd") chdir("/tmp") system("pwd") # test it chdir(curdir) # go back print "Info for testff.awk" ret = stat("testff.awk", data) print "ret =", ret for (i in data) printf "data[\"%s\"] = %s\n", i, data[i] print "testff.awk modified:", strftime("%m %d %Y %H:%M:%S", data["mtime"]) print "\nInfo for JUNK" ret = stat("JUNK", data) print "ret =", ret for (i in data) printf "data[\"%s\"] = %s\n", i, data[i] print "JUNK modified:", strftime("%m %d %Y %H:%M:%S", data["mtime"]) } The AWKLIBPATH environment variable tells gawk where to find extensions (see “How gawk Finds Extensions” on page 431). We set it to the current directory and run the program: $ AWKLIBPATH=$PWD gawk -f testff.awk /tmp Info for testff.awk ret = 0 data["blksize"] = 4096 data["devbsize"] = 512 data["mtime"] = 1412004710 data["mode"] = 33204 Example: Some File Functions | 441 data["type"] = file data["dev"] = 2053 data["gid"] = 1000 data["ino"] = 10358899 data["ctime"] = 1412004710 data["blocks"] = 8 data["nlink"] = 1 data["name"] = testff.awk data["atime"] = 1412004716 data["pmode"] = -rw-rw-r-- data["size"] = 666 data["uid"] = 1000 testff.awk modified: 09 29 2014 18:31:50 Info for JUNK ret = -1 JUNK modified: 01 01 1970 02:00:00 The Sample Extensions in the gawk Distribution This section provides a brief overview of the sample extensions that come in the gawk distribution. Some of them are intended for production use (e.g., the filefuncs, read dir, and inplace extensions). Others mainly provide example code that shows how to use the extension API. File-Related Functions The filefuncs extension provides three different functions, as follows. The usage is: @load "filefuncs" This is how you load the extension. result = chdir("/some/directory") The chdir() function is a direct hook to the chdir() system call to change the current directory. It returns zero upon success or a value less than zero upon error. In the latter case, it updates ERRNO. result = stat("/some/path", statdata [, follow]) The stat() function provides a hook into the stat() system call. It returns zero upon success or a value less than zero upon error. In the latter case, it updates ERRNO. By default, it uses the lstat() system call. However, if passed a third argument, it uses stat() instead. In all cases, it clears the statdata array. When the call is successful, stat() fills the statdata array with information retrieved from the filesystem, as follows: 442 | Chapter 16: Writing Extensions for gawk Subscript Field in struct stat File type "name" The filename All "dev" st_dev All "ino" st_ino All "mode" st_mode All "nlink" st_nlink All "uid" st_uid All "gid" st_gid All "size" st_size All "atime" st_atime All "mtime" st_mtime All "ctime" st_ctime All "rdev" st_rdev Device files "major" st_major Device files "minor" st_minor Device files "blksize" st_blksize All "pmode" A human-readable version of the mode value, like that printed by ls (for example, "-rwxr-xr-x") All "linkval" The value of the symbolic link Symbolic links "type" The type of the file as a string—one of "file", "blockdev", "chardev", "directory", "socket", "fifo", "symlink", "door", or "unknown" (not all systems support all file types) All flags = or(FTS_PHYSICAL, ...) result = fts(pathlist, flags, filedata) Walk the file trees provided in pathlist and fill in the filedata array, as described next. flags is the bitwise OR of several predefined values, also described in a mo‐ ment. Return zero if there were no errors, otherwise return −1. The fts() function provides a hook to the C library fts() routines for traversing file hierarchies. Instead of returning data about one file at a time in a stream, it fills in a multidimensional array with data about each file and directory encountered in the re‐ quested hierarchies. The arguments are as follows: pathlist An array of filenames. The element values are used; the index values are ignored. The Sample Extensions in the gawk Distribution | 443 flags This should be the bitwise OR of one or more of the following predefined constant flag values. At least one of FTS_LOGICAL or FTS_PHYSICAL must be provided; other‐ wise fts() returns an error value and sets ERRNO. The flags are: FTS_LOGICAL Do a “logical” file traversal, where the information returned for a symbolic link refers to the linked-to file, and not to the symbolic link itself. This flag is mu‐ tually exclusive with FTS_PHYSICAL. FTS_PHYSICAL Do a “physical” file traversal, where the information returned for a symbolic link refers to the symbolic link itself. This flag is mutually exclusive with FTS_LOGICAL. FTS_NOCHDIR As a performance optimization, the C library fts() routines change directory as they traverse a file hierarchy. This flag disables that optimization. FTS_COMFOLLOW Immediately follow a symbolic link named in pathlist, whether or not FTS_LOGICAL is set. FTS_SEEDOT By default, the C library fts() routines do not return entries for . (dot) and .. (dot-dot). This option causes entries for dot-dot to also be included. (The extension always includes an entry for dot; more on this in a moment.) FTS_XDEV During a traversal, do not cross onto a different mounted filesystem. filedata The filedata array holds the results. fts() first clears it. Then it creates an element in filedata for every element in pathlist. The index is the name of the directory or file given in pathlist. The element for this index is itself an array. There are two cases: The path is a file In this case, the array contains two or three elements: "path" The full path to this file, starting from the “root” that was given in the pathlist array. 444 | Chapter 16: Writing Extensions for gawk "stat" This element is itself an array, containing the same information as provided by the stat() function described earlier for its statdata argument. The element may not be present if the stat() system call for the file failed. "error" If some kind of error was encountered, the array will also contain an ele‐ ment named "error", which is a string describing the error. The path is a directory In this case, the array contains one element for each entry in the directory. If an entry is a file, that element is the same as for files, just described. If the entry is a directory, that element is (recursively) an array describing the subdirectory. If FTS_SEEDOT was provided in the flags, then there will also be an element named "..". This element will be an array containing the data as provided by stat(). In addition, there will be an element whose index is ".". This element is an array containing the same two or three elements as for a file: "path", "stat", and "error". The fts() function returns zero if there were no errors. Otherwise, it returns −1. The fts() extension does not exactly mimic the interface of the C library fts() routines, choosing instead to provide an interface that is based on associative arrays, which is more comfortable to use from an awk program. This includes the lack of a comparison function, because gawk already provides powerful array sorting facilities. Al‐ though an fts_read()-like interface could have been provided, this felt less natural than simply creating a multidimensional array to represent the file hierarchy and its information. See test/fts.awk in the gawk distribution for an example use of the fts() extension function. Interface to fnmatch() This extension provides an interface to the C library fnmatch() function. The usage is: @load "fnmatch" This is how you load the extension. result = fnmatch(pattern, string, flags) The return value is zero on success, FNM_NOMATCH if the string did not match the pattern, or a different nonzero value if an error occurred. The Sample Extensions in the gawk Distribution | 445 In addition to the fnmatch() function, the fnmatch extension adds one constant (FNM_NOMATCH), and an array of flag values named FNM. The arguments to fnmatch() are: pattern The filename wildcard to match string The filename string flag Either zero, or the bitwise OR of one or more of the flags in the FNM array The flags are as follows: Array element Corresponding flag defined by fnmatch() FNM["CASEFOLD"] FNM_CASEFOLD FNM["FILE_NAME"] FNM_FILE_NAME FNM["LEADING_DIR"] FNM_LEADING_DIR FNM["NOESCAPE"] FNM_NOESCAPE FNM["PATHNAME"] FNM_PATHNAME FNM["PERIOD"] FNM_PERIOD Here is an example: @load "fnmatch" … flags = or(FNM["PERIOD"], FNM["NOESCAPE"]) if (fnmatch("*.a", "foo.c", flags) == FNM_NOMATCH) print "no match" Interface to fork(), wait(), and waitpid() The fork extension adds three functions, as follows: @load "fork" This is how you load the extension. pid = fork() This function creates a new process. The return value is zero in the child and the process ID number of the child in the parent, or −1 upon error. In the latter case, ERRNO indicates the problem. In the child, PROCINFO["pid"] and PROCINFO["ppid"] are updated to reflect the correct values. 446 | Chapter 16: Writing Extensions for gawk ret = waitpid(pid) This function takes a numeric argument, which is the process ID to wait for. The return value is that of the waitpid() system call. ret = wait() This function waits for the first child to die. The return value is that of the wait() system call. There is no corresponding exec() function. Here is an example: @load "fork" … if ((pid = fork()) == 0) print "hello from the child" else print "hello from the parent" Enabling In-Place File Editing The inplace extension emulates GNU sed’s -i option, which performs “in-place” ed‐ iting of each input file. It uses the bundled inplace.awk include file to invoke the ex‐ tension properly: # inplace --- load and invoke the inplace extension. @load "inplace" # Please set INPLACE_SUFFIX to make a backup copy. For example, you may # want to set INPLACE_SUFFIX to .bak on the command line or in a BEGIN rule. BEGINFILE { inplace_begin(FILENAME, INPLACE_SUFFIX) } ENDFILE { inplace_end(FILENAME, INPLACE_SUFFIX) } For each regular file that is processed, the extension redirects standard output to a temporary file configured to have the same owner and permissions as the original. After the file has been processed, the extension restores standard output to its original des‐ tination. If INPLACE_SUFFIX is not an empty string, the original file is linked to a backup filename created by appending that suffix. Finally, the temporary file is renamed to the original filename. If any error occurs, the extension issues a fatal error to terminate processing immediately without damaging the original file. The Sample Extensions in the gawk Distribution | 447 Here are some simple examples: $ gawk -i inplace '{ gsub(/foo/, "bar") }; { print }' file1 file2 file3 To keep a backup copy of the original files, try this: $ gawk -i inplace -v INPLACE_SUFFIX=.bak '{ gsub(/foo/, "bar") } > { print }' file1 file2 file3 Character and Numeric values: ord() and chr() The ordchr extension adds two functions, named ord() and chr(), as follows: @load "ordchr" This is how you load the extension. number = ord(string) Return the numeric value of the first character in string. char = chr(number) Return a string whose first character is that represented by number. These functions are inspired by the Pascal language functions of the same name. Here is an example: @load "ordchr" … printf("The numeric value of 'A' is %d\n", ord("A")) printf("The string value of 65 is %s\n", chr(65)) Reading Directories The readdir extension adds an input parser for directories. The usage is as follows: @load "readdir" When this extension is in use, instead of skipping directories named on the command line (or with getline), they are read, with each entry returned as a record. The record consists of three fields. The first two are the inode number and the filename, separated by a forward slash character. On systems where the directory entry contains the file type, the record has a third field (also separated by a slash), which is a single letter indicating the type of the file. The letters and their corresponding file types are shown in Table 16-2. Table 16-2. File types returned by the readdir extension Letter File type b Block device c Character device d Directory 448 | Chapter 16: Writing Extensions for gawk Letter File type f Regular file l Symbolic link p Named pipe (FIFO) s Socket u Anything else (unknown) On systems without the file type information, the third field is always ‘u’. On GNU/Linux systems, there are filesystems that don’t support the d_type entry (see the readdir(3) manual page), and so the file type is always ‘u’. You can use the filefuncs extension to call stat() in order to get correct type information. Here is an example: @load "readdir" ... BEGIN { FS = "/" } { print "filename is", $2 } Reversing Output The revoutput extension adds a simple output wrapper that reverses the characters in each output line. Its main purpose is to show how to write an output wrapper, although it may be mildly amusing for the unwary. Here is an example: @load "revoutput" BEGIN { REVOUT = 1 print "don't panic" > "/dev/stdout" } The output from this program is ‘cinap t'nod’. Two-Way I/O Example The revtwoway extension adds a simple two-way processor that reverses the characters in each line sent to it for reading back by the awk program. Its main purpose is to show how to write a two-way processor, although it may also be mildly amusing. The following example shows how to use it: @load "revtwoway" BEGIN { The Sample Extensions in the gawk Distribution | 449 cmd = "/magic/mirror" print "don't panic" |& cmd cmd |& getline result print result close(cmd) } The output from this program also is: ‘cinap t'nod’. Dumping and Restoring an Array The rwarray extension adds two functions, named writea() and reada(), as follows: @load "rwarray" This is how you load the extension. ret = writea(file, array) This function takes a string argument, which is the name of the file to which to dump the array, and the array itself as the second argument. writea() understands arrays of arrays. It returns one on success, or zero upon failure. ret = reada(file, array) reada() is the inverse of writea(); it reads the file named as its first argument, filling in the array named as the second argument. It clears the array first. Here too, the return value is one on success and zero upon failure. The array created by reada() is identical to that written by writea() in the sense that the contents are the same. However, due to implementation issues, the array traversal order of the re-created array is likely to be different from that of the original array. As array traversal order in awk is by default undefined, this is (technically) not a problem. If you need to guarantee a particular traversal order, use the array sorting features in gawk to do so (see “Controlling Array Traversal and Array Sorting” on page 328). The file contains binary data. All integral values are written in network byte order. However, double-precision floating-point values are written as native binary data. Thus, arrays containing only string data can theoretically be dumped on systems with one byte order and restored on systems with a different one, but this has not been tried. Here is an example: @load "rwarray" … ret = writea("arraydump.bin", array) … ret = reada("arraydump.bin", array) Reading an Entire File The readfile extension adds a single function named readfile(), and an input parser: 450 | Chapter 16: Writing Extensions for gawk @load "readfile" This is how you load the extension. result = readfile("/some/path") The argument is the name of the file to read. The return value is a string containing the entire contents of the requested file. Upon error, the function returns the empty string and sets ERRNO. BEGIN { PROCINFO["readfile"] = 1 } In addition, the extension adds an input parser that is activated if PROCINFO["read file"] exists. When activated, each input file is returned in its entirety as $0. RT is set to the null string. Here is an example: @load "readfile" … contents = readfile("/path/to/file"); if (contents == "" && ERRNO != "") { print("problem reading file", ERRNO) > "/dev/stderr" ... } Extension Time Functions The time extension adds two functions, named gettimeofday() and sleep(), as follows: @load "time" This is how you load the extension. the_time = gettimeofday() Return the time in seconds that has elapsed since 1970-01-01 UTC as a floating- point value. If the time is unavailable on this platform, return −1 and set ERRNO. The returned time should have sub-second precision, but the actual precision may vary based on the platform. If the standard C gettimeofday() system call is available on this platform, then it simply returns the value. Otherwise, if on MS-Windows, it tries to use GetSystemTimeAsFileTime(). result = sleep(seconds) Attempt to sleep for seconds seconds. If seconds is negative, or the attempt to sleep fails, return −1 and set ERRNO. Otherwise, return zero after sleeping for the indicated amount of time. Note that seconds may be a floating-point (nonintegral) value. Implementation details: depending on platform availability, this function tries to use nanosleep() or select() to implement the delay. The Sample Extensions in the gawk Distribution | 451 API Tests The testext extension exercises parts of the extension API that are not tested by the other samples. The extension/testext.c file contains both the C code for the exten‐ sion and awk test code inside C comments that run the tests. The testing framework extracts the awk code and runs the tests. See the source file for more information. The gawkextlib Project The gawkextlib project provides a number of gawk extensions, including one for pro‐ cessing XML files. This is the evolution of the original xgawk (XML gawk) project. As of this writing, there are seven extensions: • errno extension • GD graphics library extension • PDF extension • PostgreSQL extension • MPFR library extension (this provides access to a number of MPFR functions that gawk’s native MPFR support does not) • Redis extension • XML parser extension, using the Expat XML parsing library You can check out the code for the gawkextlib project using the Git distributed source code control system. The command is as follows: git clone git://git.code.sf.net/p/gawkextlib/code gawkextlib-code You will need to have the Expat XML parser library installed in order to build and use the XML extension. In addition, you must have the GNU Autotools installed (Autoconf, Automake, Lib‐ tool, and GNU gettext). The simple recipe for building and testing gawkextlib is as follows. First, build and install gawk: cd .../path/to/gawk/code ./configure --prefix=/tmp/newgawk Install in /tmp/newgawk for now make && make check Build and check that all is OK make install Install gawk Next, build gawkextlib and test it: cd .../path/to/gawkextlib-code ./update-autotools Generate configure, etc. 452 | Chapter 16: Writing Extensions for gawk You may have to run this command twice ./configure --with-gawk=/tmp/newgawk Configure, point at ``installed'' gawk make && make check Build and check that all is OK make install Install the extensions If you have installed gawk in the standard way, then you will likely not need the --with- gawk option when configuring gawkextlib. You may need to use the sudo utility to install both gawk and gawkextlib, depending upon how your system works. If you write an extension that you wish to share with other gawk users, consider doing so through the gawkextlib project. See the project’s website for more information. Summary • You can write extensions (sometimes called plug-ins) for gawk in C or C++ using the application programming interface (API) defined by the gawk developers. • Extensions must have a license compatible with the GNU General Public License (GPL), and they must assert that fact by declaring a variable named plugin_is_GPL_compatible. • Communication between gawk and an extension is two-way. gawk passes a struct to the extension that contains various data fields and function pointers. The ex‐ tension can then call into gawk via the supplied function pointers to accomplish certain tasks. • One of these tasks is to “register” the name and implementation of new awk-level functions with gawk. The implementation takes the form of a C function pointer with a defined signature. By convention, implementation functions are named do_XXXX() for some awk-level function XXXX(). • The API is defined in a header file named gawkapi.h. You must include a number of standard header files before including it in your source file. • API function pointers are provided for the following kinds of operations: — Allocating, reallocating, and releasing memory — Registration functions (you may register extension functions, exit callbacks, a version string, input parsers, output wrappers, and two-way processors) — Printing fatal, warning, and “lint” warning messages — Updating ERRNO, or unsetting it — Accessing parameters, including converting an undefined parameter into an array — Symbol table access (retrieving a global variable, creating one, or changing one) Summary | 453 — Creating and releasing cached values; this provides an efficient way to use values for multiple variables and can be a big performance win — Manipulating arrays (retrieving, adding, deleting, and modifying elements; get‐ ting the count of elements in an array; creating a new array; clearing an array; and flattening an array for easy C-style looping over all its indices and elements) • The API defines a number of standard data types for representing awk values, array elements, and arrays. • The API provides convenience functions for constructing values. It also provides memory management functions to ensure compatibility between memory allocated by gawk and memory allocated by an extension. • All memory passed from gawk to an extension must be treated as read-only by the extension. • All memory passed from an extension to gawk must come from the API’s memory allocation functions. gawk takes responsibility for the memory and releases it when appropriate. • The API provides information about the running version of gawk so that an ex‐ tension can make sure it is compatible with the gawk that loaded it. • It is easiest to start a new extension by copying the boilerplate code described in this chapter. Macros in the gawkapi.h header file make this easier to do. • The gawk distribution includes a number of small but useful sample extensions. The gawkextlib project includes several more (larger) extensions. If you wish to write an extension and contribute it to the community of gawk users, the gawkextlib project is the place to do so. 454 | Chapter 16: Writing Extensions for gawk PART IV Appendices Part IV contains three appendices, the last of which is the license that covers the gawk source code: • Appendix A, The Evolution of the awk Language • Appendix B, Installing gawk • Appendix C, GNU General Public License APPENDIX A The Evolution of the awk Language This book describes the GNU implementation of awk, which follows the POSIX speci‐ fication. Many longtime awk users learned awk programming with the original awk implementation in Version 7 Unix. (This implementation was the basis for awk in Berkeley Unix, through 4.3-Reno. Subsequent versions of Berkeley Unix, and, for a while, some systems derived from 4.4BSD-Lite, used various versions of gawk for their awk.) This chapter briefly describes the evolution of the awk language, with cross- references to other parts of the book where you can find more information. To save space, we have omitted information on the history of features in gawk from this edition. You can find it in the online documentation. Major Changes Between V7 and SVR3.1 The awk language evolved considerably between the release of Version 7 Unix (1978) and the new version that was first made generally available in System V Release 3.1 (1987). This section summarizes the changes, with cross-references to further details: • The requirement for ‘;’ to separate rules on a line (see “awk Statements Versus Lines” on page 16) • User-defined functions and the return statement (see “User-Defined Functions” on page 220) • The delete statement (see “The delete Statement” on page 184) • The do-while statement (see “The do-while Statement” on page 151) • The built-in functions atan2(), cos(), sin(), rand(), and srand() (see “Numeric Functions” on page 192) 457 • The built-in functions gsub(), sub(), and match() (see “String-Manipulation Functions” on page 194) • The built-in functions close() and system() (see “Input/Output Functions” on page 208) • The ARGC, ARGV, FNR, RLENGTH, RSTART, and SUBSEP predefined variables (see “Pre‐ defined Variables” on page 160) • Assignable $0 (see “Changing the Contents of a Field” on page 62) • The conditional expression using the ternary operator ‘?:’ (see “Conditional Ex‐ pressions” on page 134) • The expression ‘indx in array’ outside of for statements (see “Referring to an Array Element” on page 175) • The exponentiation operator ‘^’ (see “Arithmetic Operators” on page 119) and its assignment operator form ‘^=’ (see “Assignment Expressions” on page 122) • C-compatible operator precedence, which breaks some old awk programs (see “Operator Precedence (How Operators Nest)” on page 136) • Regexps as the value of FS (see “Specifying How Fields Are Separated” on page 65) and as the third argument to the split() function (see “String-Manipulation Functions” on page 194), rather than using only the first character of FS • Dynamic regexps as operands of the ‘~’ and ‘!~’ operators (see “Using Dynamic Regexps” on page 49) • The escape sequences ‘\b’, ‘\f’, and ‘\r’ (see “Escape Sequences” on page 40) • Redirection of input for the getline function (see “Explicit Input with getline” on page 77) • Multiple BEGIN and END rules (see “The BEGIN and END Special Patterns” on page 145) • Multidimensional arrays (see “Multidimensional Arrays” on page 185) Changes Between SVR3.1 and SVR4 The System V Release 4 (1989) version of Unix awk added these features (some of which originated in gawk): • The ENVIRON array (see “Predefined Variables” on page 160) • Multiple -f options on the command line (see “Command-Line Options” on page 22) 458 | Appendix A: The Evolution of the awk Language • The -v option for assigning variables before program execution begins (see “Command-Line Options” on page 22) • The -- signal for terminating command-line options • The ‘\a’, ‘\v’, and ‘\x’ escape sequences (see “Escape Sequences” on page 40) • A defined return value for the srand() built-in function (see “Numeric Func‐ tions” on page 192) • The toupper() and tolower() built-in string functions for case translation (see “String-Manipulation Functions” on page 194) • A cleaner specification for the ‘%c’ format-control letter in the printf function (see “Format-Control Letters” on page 94) • The ability to dynamically pass the field width and precision ("%*.*d") in the ar‐ gument list of printf and sprintf() (see “Format-Control Letters” on page 94) • The use of regexp constants, such as /foo/, as expressions, where they are equivalent to using the matching operator, as in ‘$0 ~ /foo/’ (see “Using Regular Expression Constants” on page 114) • Processing of escape sequences inside command-line variable assignments (see “Assigning variables on the command line” on page 116) Changes Between SVR4 and POSIX awk The POSIX Command Language and Utilities standard for awk (1992) introduced the following changes into the language: • The use of -W for implementation-specific options (see “Command-Line Op‐ tions” on page 22) • The use of CONVFMT for controlling the conversion of numbers to strings (see “Con‐ version of Strings and Numbers” on page 117) • The concept of a numeric string and tighter comparison rules to go with it (see “Variable Typing and Comparison Expressions” on page 128) • The use of predefined variables as function parameter names is forbidden (see “Function Definition Syntax” on page 221) • More complete documentation of many of the previously undocumented features of the language In 2012, a number of extensions that had been commonly available for many years were finally added to POSIX. They are: Changes Between SVR4 and POSIX awk | 459 • The fflush() built-in function for flushing buffered output (see “Input/Output Functions” on page 208) • The nextfile statement (see “The nextfile Statement” on page 158) • The ability to delete all of an array at once with ‘delete array’ (see “The delete Statement” on page 184) See “Common Extensions Summary” on page 463 for a list of common extensions not permitted by the POSIX standard. The 2008 POSIX standard can be found online at http://www.opengroup.org/online pubs/9699919799/. Extensions in Brian Kernighan’s awk Brian Kernighan has made his version available via his home page (see “Other Freely Available awk Implementations” on page 485). This section describes common extensions that originally appeared in his version of awk: • The ‘**’ and ‘**=’ operators (see “Arithmetic Operators” on page 119 and “Assign‐ ment Expressions” on page 122) • The use of func as an abbreviation for function (see “Function Definition Syn‐ tax” on page 221) • The fflush() built-in function for flushing buffered output (see “Input/Output Functions” on page 208) See “Common Extensions Summary” on page 463 for a full list of the extensions available in his awk. Extensions in gawk Not in POSIX awk The GNU implementation, gawk, adds a large number of features. They can all be dis‐ abled with either the --traditional or --posix options (see “Command-Line Op‐ tions” on page 22). A number of features have come and gone over the years. This section summarizes the additional features over POSIX awk that are in the current version of gawk. • Additional predefined variables: — The ARGIND, BINMODE, ERRNO, FIELDWIDTHS, FPAT, IGNORECASE, LINT, PROCINFO, RT, and TEXTDOMAIN variables (see “Predefined Variables” on page 160) • Special files in I/O redirections: 460 | Appendix A: The Evolution of the awk Language — The /dev/stdin, /dev/stdout, /dev/stderr, and /dev/fd/N special filenames (see “Special Filenames in gawk” on page 104) — The /inet, /inet4, and ‘/inet6’ special files for TCP/IP networking using ‘|&’ to specify which version of the IP protocol to use (see “Using gawk for Network Programming” on page 337) • Changes and/or additions to the language: — The ‘\x’ escape sequence (see “Escape Sequences” on page 40) — Full support for both POSIX and GNU regexps (see Chapter 3) — The ability for FS and for the third argument to split() to be null strings (see “Making Each Character a Separate Field” on page 67) — The ability for RS to be a regexp (see “How Input Is Split into Records” on page 55) — The ability to use octal and hexadecimal constants in awk program source code (see “Octal and hexadecimal numbers” on page 112) — The ‘|&’ operator for two-way I/O to a coprocess (see “Two-Way Communica‐ tions with Another Process” on page 334) — Indirect function calls (see “Indirect Function Calls” on page 231) — Directories on the command line produce a warning and are skipped (see “Di‐ rectories on the Command Line” on page 87) • New keywords: — The BEGINFILE and ENDFILE special patterns (see “The BEGINFILE and END‐ FILE Special Patterns” on page 147) — The switch statement (see “The switch Statement” on page 154) • Changes to standard awk functions: — The optional second argument to close() that allows closing one end of a two- way pipe to a coprocess (see “Two-Way Communications with Another Pro‐ cess” on page 334) — POSIX compliance for gsub() and sub() with --posix — The length() function accepts an array argument and returns the number of elements in the array (see “String-Manipulation Functions” on page 194) — The optional third argument to the match() function for capturing text- matching subexpressions within a regexp (see “String-Manipulation Func‐ tions” on page 194) — Positional specifiers in printf formats for making translations easier (see “Re‐ arranging printf Arguments” on page 351) Extensions in gawk Not in POSIX awk | 461 — The split() function’s additional optional fourth argument, which is an array to hold the text of the field separators (see “String-Manipulation Functions” on page 194) • Additional functions only in gawk: — The gensub(), patsplit(), and strtonum() functions for more powerful text manipulation (see “String-Manipulation Functions” on page 194) — The asort() and asorti() functions for sorting arrays (see “Controlling Array Traversal and Array Sorting” on page 328) — The mktime(), systime(), and strftime() functions for working with time‐ stamps (see “Time Functions” on page 211) — The and(), compl(), lshift(), or(), rshift(), and xor() functions for bit manipulation (see “Bit-Manipulation Functions” on page 217) — The isarray() function to check if a variable is an array or not (see “Getting Type Information” on page 219) — The bindtextdomain(), dcgettext(), and dcngettext() functions for inter‐ nationalization (see “Internationalizing awk Programs” on page 348) • Changes and/or additions in the command-line options: — The AWKPATH environment variable for specifying a path search for the -f command-line option (see “Command-Line Options” on page 22) — The AWKLIBPATH environment variable for specifying a path search for the -l command-line option (see “Command-Line Options” on page 22) — The -b, -c, -C, -d, -D, -e, -E, -g, -h, -i, -l, -L, -M, -n, -N, -o, -O, -p, -P, -r, -S, -t, and -V short options. Also, the ability to use GNU-style long-named options that start with --; and the --assign, --bignum, --characters-as-bytes, --copyright, --debug, --dump-variables, --exec, --field-separator, --file, --gen-pot, --help, --include, --lint, --lint-old, --load, --non-decimal-data, --optimize, --posix, --pretty-print, --profile, --re-interval, --sandbox, --source, --traditional, --use-lc-numeric, and --version long options (see “Command-Line Options” on page 22) • Support for the following obsolete systems was removed from the code and the documentation for gawk version 4.0: — Amiga — Atari — BeOS — Cray — MIPS RiscOS 462 | Appendix A: The Evolution of the awk Language — MS-DOS with the Microsoft Compiler — MS-Windows with the Microsoft Compiler — NeXT — SunOS 3.x, Sun 386 (Road Runner) — Tandem (non-POSIX) — Prestandard VAX C compiler for VAX/VMS — GCC for VAX and Alpha has not been tested for a while. • Support for the following obsolete system was removed from the code for gawk version 4.1: — Ultrix Common Extensions Summary The following table summarizes the common extensions supported by gawk, Brian Kernighan’s awk, and mawk, the three most widely used freely available versions of awk (see “Other Freely Available awk Implementations” on page 485). Feature BWK awk mawk gawk Now standard ‘\x’ escape sequence ✓ ✓ ✓ FS as null string ✓ ✓ ✓ /dev/stdin special file ✓ ✓ ✓ /dev/stdout special file ✓ ✓ ✓ /dev/stderr special file ✓ ✓ ✓ delete without subscript ✓ ✓ ✓ ✓ fflush() function ✓ ✓ ✓ ✓ length() of an array ✓ ✓ ✓ nextfile statement ✓ ✓ ✓ ✓ ** and **= operators ✓ ✓ func keyword ✓ ✓ BINMODE variable ✓ ✓ RS as regexp ✓ ✓ Time-related functions ✓ ✓ Common Extensions Summary | 463 1. And Life was good. Regexp Ranges and Locales: A Long Sad Story This section describes the confusing history of ranges within regular expressions and their interactions with locales, and how this affected different versions of gawk. The original Unix tools that worked with regular expressions defined character ranges (such as ‘[a-z]’) to match any character between the first character in the range and the last character in the range, inclusive. Ordering was based on the numeric value of each character in the machine’s native character set. Thus, on ASCII-based systems, ‘[a-z]’ matched all the lowercase letters, and only the lowercase letters, as the numeric values for the letters from ‘a’ through ‘z’ were contiguous. (On an EBCDIC system, the range ‘[a-z]’ includes additional nonalphabetic characters as well.) Almost all introductory Unix literature explained range expressions as working in this fashion, and in particular, would teach that the “correct” way to match lowercase letters was with ‘[a-z]’, and that ‘[A-Z]’ was the “correct” way to match uppercase letters. And indeed, this was true.1 The 1992 POSIX standard introduced the idea of locales (see “Where You Are Makes a Difference” on page 138). Because many locales include other letters besides the plain 26 letters of the English alphabet, the POSIX standard added character classes (see “Using Bracket Expressions” on page 46) as a way to match different kinds of characters besides the traditional ones in the ASCII character set. However, the standard changed the interpretation of range expressions. In the "C" and "POSIX" locales, a range expression like ‘[a-dx-z]’ is still equivalent to ‘[abcdxyz]’, as in ASCII. But outside those locales, the ordering was defined to be based on collation order. What does that mean? In many locales, ‘A’ and ‘a’ are both less than ‘B’. In other words, these locales sort characters in dictionary order, and ‘[a-dx-z]’ is typically not equiv‐ alent to ‘[abcdxyz]’; instead, it might be equivalent to ‘[ABCXYabcdxyz]’, for example. This point needs to be emphasized: much literature teaches that you should use ‘[a- z]’ to match a lowercase character. But on systems with non-ASCII locales, this also matches all of the uppercase characters except ‘A’ or ‘Z’! This was a continuous cause of confusion, even well into the twenty-first century. To demonstrate these issues, the following example uses the sub() function, which does text replacement (see “String-Manipulation Functions” on page 194). Here, the intent is to remove trailing uppercase characters: $ echo something1234abc | gawk-3.1.8 '{ sub("[A-Z]*$", ""); print }' something1234a 464 | Appendix A: The Evolution of the awk Language 2. And thus was born the Campaign for Rational Range Interpretation (or RRI). A number of GNU tools have already implemented this change, or will soon. Thanks to Karl Berry for coining the phrase “Rational Range Interpretation.” 3. See the standard and its rationale. This output is unexpected, as the ‘bc’ at the end of ‘something1234abc’ should not normally match ‘[A-Z]*’. This result is due to the locale setting (and thus you may not see it on your system). Similar considerations apply to other ranges. For example, ‘["-/]’ is perfectly valid in ASCII, but is not valid in many Unicode locales, such as en_US.UTF-8. Early versions of gawk used regexp matching code that was not locale-aware, so ranges had their traditional interpretation. When gawk switched to using locale-aware regexp matchers, the problems began; es‐ pecially as both GNU/Linux and commercial Unix vendors started implementing non- ASCII locales, and making them the default. Perhaps the most frequently asked question became something like, “Why does ‘[A-Z]’ match lowercase letters?!?” This situation existed for close to 10 years, if not more, and the gawk maintainer grew weary of trying to explain that gawk was being nicely standards-compliant, and that the issue was in the user’s locale. During the development of version 4.0, he modified gawk to always treat ranges in the original, pre-POSIX fashion, unless --posix was used (see “Command-Line Options” on page 22).2 Fortunately, shortly before the final release of gawk 4.0, the maintainer learned that the 2008 standard had changed the definition of ranges, such that outside the "C" and "POSIX" locales, the meaning of range expressions was undefined.3 By using this lovely technical term, the standard gives license to implementors to im‐ plement ranges in whatever way they choose. The gawk maintainer chose to apply the pre-POSIX meaning both with the default regexp matching and when --traditional or --posix are used. In all cases gawk remains POSIX-compliant. Major Contributors to gawk Always give credit where credit is due. —Anonymous This section names the major contributors to gawk and/or this book, in approximate chronological order: Major Contributors to gawk | 465 • Dr. Alfred V. Aho, Dr. Peter J. Weinberger, and Dr. Brian W. Kernighan, all of Bell Laboratories, designed and implemented Unix awk, from which gawk gets the ma‐ jority of its feature set. • Paul Rubin did the initial design and implementation in 1986, and wrote the first draft (around 40 pages) of this book. • Jay Fenlason finished the initial implementation. • Diane Close revised the first draft of this book, bringing it to around 90 pages. • Richard Stallman helped finish the implementation and the initial draft of this book. He is also the founder of the FSF and the GNU Project. • John Woods contributed parts of the code (mostly fixes) in the initial version of gawk. • In 1988, David Trueman took over primary maintenance of gawk, making it com‐ patible with “new” awk, and greatly improving its performance. • Conrad Kwok, Scott Garfinkle, and Kent Williams did the initial ports to MS-DOS with various versions of MSC. • Pat Rankin provided the VMS port and its documentation. • Hal Peterson provided help in porting gawk to Cray systems. (This is no longer supported.) • Kai Uwe Rommel provided the initial port to OS/2 and its documentation. • Michal Jaegermann provided the port to Atari systems and its documentation. (This port is no longer supported.) He continues to provide portability checking, and has done a lot of work to make sure gawk works on non-32-bit systems. • Fred Fish provided the port to Amiga systems and its documentation. (With Fred’s sad passing, this is no longer supported.) • Scott Deifik currently maintains the MS-DOS port using DJGPP. • Eli Zaretskii currently maintains the MS-Windows port using MinGW. • Juan Grigera provided a port to Windows32 systems. (This is no longer supported.) • For many years, Dr. Darrel Hankerson acted as coordinator for the various ports to different PC platforms and created binary distributions for various PC operating systems. He was also instrumental in keeping the documentation up to date for the various PC platforms. • Christos Zoulas provided the extension() built-in function for dynamically adding new functions. (This was obsoleted at gawk 4.1.) • Jürgen Kahrs contributed the initial version of the TCP/IP networking code and documentation, and motivated the inclusion of the ‘|&’ operator. 466 | Appendix A: The Evolution of the awk Language • Stephen Davies provided the initial port to Tandem systems and its documentation. (However, this is no longer supported.) He was also instrumental in the initial work to integrate the byte-code internals into the gawk code base. • Matthew Woehlke provided improvements for Tandem’s POSIX-compliant systems. • Martin Brown provided the port to BeOS and its documentation. (This is no longer supported.) • Arno Peters did the initial work to convert gawk to use GNU Automake and GNU gettext. • Alan J. Broder provided the initial version of the asort() function as well as the code for the optional third argument to the match() function. • Andreas Buening updated the gawk port for OS/2. • Isamu Hasegawa, of IBM in Japan, contributed support for multibyte characters. • Michael Benzinger contributed the initial code for switch statements. • Patrick T.J. McPhee contributed the code for dynamic loading in Windows32 en‐ vironments. (This is no longer supported.) • Anders Wallin helped keep the VMS port going for several years. • Assaf Gordon contributed the code to implement the --sandbox option. • John Haque made the following contributions: — The modifications to convert gawk into a byte-code interpreter, including the debugger — The addition of true arrays of arrays — The additional modifications for support of arbitrary-precision arithmetic — The initial text of Chapter 15 — The work to merge the three versions of gawk into one, for the 4.1 release — Improved array internals for arrays indexed by integers — The improved array sorting features were also driven by John, together with Pat Rankin • Panos Papadopoulos contributed the original text for “Including Other Files into Your Program” on page 35. • Efraim Yawitz contributed the original text for Chapter 14. • The development of the extension API first released with gawk 4.1 was driven pri‐ marily by Arnold Robbins and Andrew Schorr, with notable contributions from the rest of the development team. Major Contributors to gawk | 467 • John Malmberg contributed significant improvements to the OpenVMS port and the related documentation. • Antonio Giovanni Colombo rewrote a number of examples in the early chapters that were severely dated, for which I am incredibly grateful. • Arnold Robbins has been working on gawk since 1988, at first helping David True‐ man, and as the primary maintainer since around 1994. Summary • The awk language has evolved over time. The first release was with V7 Unix, circa 1978. In 1987, for System V Release 3.1, major additions, including user-defined functions, were made to the language. Additional changes were made for System V Release 4, in 1989. Since then, further minor changes have happened under the auspices of the POSIX standard. • Brian Kernighan’s awk provides a small number of extensions that are implemented in common with other versions of awk. • gawk provides a large number of extensions over POSIX awk. They can be disabled with either the --traditional or --posix options. • The interaction of POSIX locales and regexp matching in gawk has been confusing over the years. Today, gawk implements Rational Range Interpretation, where rang‐ es of the form ‘[a-z]’ match only the characters numerically between ‘a’ through ‘z’ in the machine’s native character set. Usually this is ASCII, but it can be EBCDIC on IBM S/390 systems. • Many people have contributed to gawk development over the years. We hope that the list provided in this chapter is complete and gives the appropriate credit where credit is due. 468 | Appendix A: The Evolution of the awk Language APPENDIX B Installing gawk This appendix provides instructions for installing gawk on the various platforms that are supported by the developers. The primary developer supports GNU/Linux (and Unix), whereas the other ports are contributed. See “Reporting Problems and Bugs” on page 484 for the email addresses of the people who maintain the respective ports. The gawk Distribution This section describes how to get the gawk distribution, how to extract it, and then what is in the various files and subdirectories. Getting the gawk Distribution There are two ways to get GNU software: • Copy it from someone else who already has it. • Retrieve gawk from the Internet host ftp.gnu.org, in the directory /gnu/gawk. Both anonymous ftp and http access are supported. If you have the wget program, you can use a command like the following: wget http://ftp.gnu.org/gnu/gawk/gawk-4.1.2.tar.gz The GNU software archive is mirrored around the world. The up-to-date list of mirror sites is available from the main FSF website. Try to use one of the mirrors; they will be less busy, and you can usually find one closer to your site. 469 Extracting the Distribution gawk is distributed as several tar files compressed with different compression programs: gzip, bzip2, and xz. For simplicity, the rest of these instructions assume you are using the one compressed with the GNU Gzip program (gzip). Once you have the distribution (e.g., gawk-4.1.2.tar.gz), use gzip to expand the file and then use tar to extract it. You can use the following pipeline to produce the gawk distribution: gzip -d -c gawk-4.1.2.tar.gz | tar -xvpf - On a system with GNU tar, you can let tar do the decompression for you: tar -xvpzf gawk-4.1.2.tar.gz Extracting the archive creates a directory named gawk-4.1.2 in the current directory. The distribution filename is of the form gawk-V.R.P.tar.gz. The V represents the major version of gawk, the R represents the current release of version V, and the P represents a patch level, meaning that minor bugs have been fixed in the release. The current patch level is 2, but when retrieving distributions, you should get the version with the highest version, release, and patch level. (Note, however, that patch levels greater than or equal to 70 denote “beta” or nonproduction software; you might not want to retrieve such a version unless you don’t mind experimenting.) If you are not on a Unix or GNU/Linux system, you need to make other arrangements for getting and extracting the gawk distribution. You should consult a local expert. Contents of the gawk Distribution The gawk distribution has a number of C source files, documentation files, subdirec‐ tories, and files related to the configuration process (see “Compiling and Installing gawk on Unix-Like Systems” on page 474), as well as several subdirectories related to different non-Unix operating systems: Various ‘.c’, ‘.y’, and ‘.h’ files These files contain the actual gawk source code. ABOUT-NLS A file containing information about GNU gettext and translations. AUTHORS A file with some information about the authorship of gawk. It exists only to satisfy the pedants at the Free Software Foundation. 470 | Appendix B: Installing gawk README README_d/README.* Descriptive files: README for gawk under Unix and the rest for the various hardware and software combinations. INSTALL A file providing an overview of the configuration and installation process. ChangeLog A detailed list of source code changes as bugs are fixed or improvements made. ChangeLog.0 An older list of source code changes. NEWS A list of changes to gawk since the last release or patch. NEWS.0 An older list of changes to gawk. COPYING The GNU General Public License. POSIX.STD A description of behaviors in the POSIX standard for awk that are left undefined, or where gawk may not comply fully, as well as a list of things that the POSIX standard should describe but does not. doc/awkforai.txt Pointers to the original draft of a short article describing why gawk is a good lan‐ guage for artificial intelligence (AI) programming. doc/bc_notes A brief description of gawk’s “byte code” internals. doc/README.card doc/ad.block doc/awkcard.in doc/cardfonts doc/colors doc/macros doc/no.colors doc/setter.outline The troff source for a five-color awk reference card. A modern version of troff such as GNU troff (groff) is needed to produce the color version. See the file README.card for instructions if you have an older troff. The gawk Distribution | 471 doc/gawk.1 The troff source for a manual page describing gawk. This is distributed for the convenience of Unix users. doc/gawktexi.in doc/sidebar.awk The Texinfo source file for this book. It should be processed by doc/sidebar.awk before processing with texi2dvi or texi2pdf to produce a printed document, and with makeinfo to produce an Info or HTML file. The Makefile takes care of this processing and produces printable output via texi2dvi or texi2pdf. doc/gawk.texi The file produced after processing gawktexi.in with sidebar.awk. doc/gawk.info The generated Info file for this book. doc/gawkinet.texi The Texinfo source file for TCP/IP Internetworking with gawk. It should be pro‐ cessed with TeX (via texi2dvi or texi2pdf) to produce a printed document and with makeinfo to produce an Info or HTML file. doc/gawkinet.info The generated Info file for TCP/IP Internetworking with gawk. doc/igawk.1 The troff source for a manual page describing the igawk program presented in “An Easy Way to Use Library Functions” on page 314. doc/Makefile.in The input file used during the configuration process to generate the actual Make file for creating the documentation. Makefile.am */Makefile.am Files used by the GNU Automake software for generating the Makefile.in files used by Autoconf and configure. Makefile.in aclocal.m4 bisonfix.awk config.guess configh.in configure.ac configure 472 | Appendix B: Installing gawk custom.h depcomp install-sh missing_d/* mkinstalldirs m4/* These files and subdirectories are used when configuring and compiling gawk for various Unix systems. Most of them are explained in “Compiling and Installing gawk on Unix-Like Systems” on page 474. The rest are there to support the main infrastructure. po/* The po library contains message translations. awklib/extract.awk awklib/Makefile.am awklib/Makefile.in awklib/eg/* The awklib directory contains a copy of extract.awk (see “Extracting Programs from Texinfo Source Files” on page 309), which can be used to extract the sample programs from the Texinfo source file for this book. It also contains a Make file.in file, which configure uses to generate a Makefile. Makefile.am is used by GNU Automake to create Makefile.in. The library functions from Chap‐ ter 10 and the igawk program from “An Easy Way to Use Library Functions” on page 314 are included as ready-to-use files in the gawk distribution. They are in‐ stalled as part of the installation process. The rest of the programs in this book are available in appropriate subdirectories of awklib/eg. extension/* The source code, manual pages, and infrastructure files for the sample extensions included with gawk. See Chapter 16 for more information. posix/* Files needed for building gawk on POSIX-compliant systems. pc/* Files needed for building gawk under MS-Windows (see “Installation on PC Op‐ erating Systems” on page 477 for details). vms/* Files needed for building gawk under Vax/VMS and OpenVMS (see “Compiling and Installing gawk on Vax/VMS and OpenVMS” on page 479 for details). The gawk Distribution | 473 test/* A test suite for gawk. You can use ‘make check’ from the top-level gawk directory to run your version of gawk against the test suite. If gawk successfully passes ‘make check’, then you can be confident of a successful port. Compiling and Installing gawk on Unix-Like Systems Usually, you can compile and install gawk by typing only two commands. However, if you use an unusual system, you may need to configure gawk for your system yourself. Compiling gawk for Unix-Like Systems The normal installation steps should work on all modern commercial Unix-derived systems, GNU/Linux, BSD-based systems, and the Cygwin environment for MS-Windows. After you have extracted the gawk distribution, cd to gawk-4.1.2. As with most GNU software, you configure gawk for your system by running the configure program. This program is a Bourne shell script that is generated automatically using GNU Autoconf. (The Autoconf software is described fully in Autoconf—Generating Automatic Config‐ uration Scripts, which can be found online at the Free Software Foundation’s website.) To configure gawk, simply run configure: sh ./configure This produces a Makefile and config.h tailored to your system. The config.h file describes various facts about your system. You might want to edit the Makefile to change the CFLAGS variable, which controls the command-line options that are passed to the C compiler (such as optimization levels or compiling for debugging). Alternatively, you can add your own values for most make variables on the command line, such as CC and CFLAGS, when running configure: CC=cc CFLAGS=-g sh ./configure See the file INSTALL in the gawk distribution for all the details. After you have run configure and possibly edited the Makefile, type: make Shortly thereafter, you should have an executable version of gawk. That’s all there is to it! To verify that gawk is working properly, run ‘make check’. All of the tests should succeed. If these steps do not work, or if any of the tests fail, check the files in the README_d directory to see if you’ve found a known problem. If the failure is not described there, send in a bug report (see “Reporting Problems and Bugs” on page 484). 474 | Appendix B: Installing gawk Of course, once you’ve built gawk, it is likely that you will wish to install it. To do so, you need to run the command ‘make install’, as a user with the appropriate permis‐ sions. How to do this varies by system, but on many systems you can use the sudo command to do so. The command then becomes ‘sudo make install’. It is likely that you will be asked for your password, and you will have to have been set up previously as a user who is allowed to run the sudo command. Additional Configuration Options There are several additional options you may use on the configure command line when compiling gawk from scratch, including: --disable-extensions Disable configuring and building the sample extensions in the extension directory. This is useful for cross-compiling. The default action is to dynamically check if the extensions can be configured and compiled. --disable-lint Disable all lint checking within gawk. The --lint and --lint-old options (see “Command-Line Options” on page 22) are accepted, but silently do nothing. Sim‐ ilarly, setting the LINT variable (see “Built-in Variables That Control awk” on page 160) has no effect on the running awk program. When used with the GNU Compiler Collection’s (GCC’s) automatic dead code elimination, this option cuts almost 23K bytes off the size of the gawk executable on GNU/Linux x86_64 systems. Results on other systems and with other compilers are likely to vary. Using this option may bring you some slight performance im‐ provement. Using this option will cause some of the tests in the test suite to fail. This option may be removed at a later date. --disable-nls Disable all message-translation facilities. This is usually not desirable, but it may bring you some slight performance improvement. --with-whiny-user-strftime Force use of the included version of the C strftime() function for deficient systems. Use the command ‘./configure --help’ to see the full list of options supplied by configure. Compiling and Installing gawk on Unix-Like Systems | 475 The Configuration Process This section is of interest only if you know something about using the C language and Unix-like operating systems. The source code for gawk generally attempts to adhere to formal standards wherever possible. This means that gawk uses library routines that are specified by the ISO C standard and by the POSIX operating system interface standard. The gawk source code requires using an ISO C compiler (the 1990 standard). Many Unix systems do not support all of either the ISO or the POSIX standards. The missing_d subdirectory in the gawk distribution contains replacement versions of those functions that are most likely to be missing. The config.h file that configure creates contains definitions that describe features of the particular operating system where you are attempting to compile gawk. The three things described by this file are: what header files are available, so that they can be correctly included, what (supposedly) standard functions are actually available in your C libraries, and various miscellaneous facts about your operating system. For example, there may not be an st_blksize element in the stat structure. In this case, ‘HAVE_STRUCT_STAT_ST_BLKSIZE’ is undefined. It is possible for your C compiler to lie to configure. It may do so by not exiting with an error when a library function is not available. To get around this, edit the cus tom.h file. Use an ‘#ifdef’ that is appropriate for your system, and either #define any constants that configure should have defined but didn’t, or #undef any constants that configure defined and should not have. The custom.h file is automatically included by the config.h file. It is also possible that the configure program generated by Autoconf will not work on your system in some other fashion. If you do have a problem, the configure.ac file is the input for Autoconf. You may be able to change this file and generate a new version of configure that works on your system (see “Reporting Problems and Bugs” on page 484 for information on how to report problems in configuring gawk). The same mecha‐ nism may be used to send in updates to configure.ac and/or custom.h. Installation on Other Operating Systems This section describes how to install gawk on various non-Unix systems. 476 | Appendix B: Installing gawk Installation on PC Operating Systems This section covers installation and usage of gawk on Intel architecture machines run‐ ning MS-DOS and any version of MS-Windows. In this section, the term “Windows32” refers to any of Microsoft Windows 95/98/ME/NT/2000/XP/Vista/7/8. The limitations of MS-DOS (and MS-DOS shells under the other operating systems) have meant that various “DOS extenders” are often used with programs such as gawk. The varying capabilities of Microsoft Windows 3.1 and Windows32 can add to the confusion. For an overview of the considerations, refer to README_d/README.pc in the distribution. Compiling gawk for PC operating systems gawk can be compiled for MS-DOS and Windows32 using the GNU development tools from DJ Delorie (DJGPP: MS-DOS only) or MinGW (Windows32). The file README_d/ README.pc in the gawk distribution contains additional notes, and pc/Makefile con‐ tains important information on compilation options. To build gawk for MS-DOS and Windows32, copy the files in the pc directory (except for ChangeLog) to the directory with the rest of the gawk sources, then invoke make with the appropriate target name as an argument to build gawk. The Makefile copied from the pc directory contains a configuration section with comments and may need to be edited in order to work with your make utility. The Makefile supports a number of targets for building various MS-DOS and Win‐ dows32 versions. A list of targets is printed if the make command is given without a target. As an example, to build gawk using the DJGPP tools, enter ‘make djgpp’. (The DJGPP tools needed for the build may be found at ftp://ftp.delorie.com/pub/djgpp/ current/v2gnu/.) To build a native MS-Windows binary of gawk using the MinGW tools, type ‘make mingw32’. Testing gawk on PC operating systems Using make to run the standard tests and to install gawk requires additional Unix-like tools, including sh, sed, and cp. In order to run the tests, the test/*.ok files may need to be converted so that they have the usual MS-DOS-style end-of-line markers. Alter‐ natively, run make check CMP="diff -a" to use GNU diff in text mode instead of cmp to compare the resulting files. Using gawk on PC operating systems Under MS-DOS and MS-Windows, the Cygwin and MinGW environments support both the ‘|&’ operator and TCP/IP networking (see “Using gawk for Network Program‐ ming” on page 337). Installation on Other Operating Systems | 477 The MS-DOS and MS-Windows versions of gawk search for program files as described in “The AWKPATH Environment Variable” on page 31. However, semicolons (rather than colons) separate elements in the AWKPATH variable. If AWKPATH is not set or is empty, then the default search path is ‘.;c:/lib/awk;c:/gnu/lib/awk’. An sh-like shell (as opposed to command.com under MS-DOS or cmd.exe under MS- Windows) may be useful for awk programming. The DJGPP collection of tools includes an MS-DOS port of Bash. Under MS-Windows and MS-DOS, gawk (and many other text programs) silently translates end-of-line ‘\r\n’ to ‘\n’ on input and ‘\n’ to ‘\r\n’ on output. A special BINMODE variable (c.e.) allows control over these translations and is interpreted as follows: • If BINMODE is "r" or one, then binary mode is set on read (i.e., no translations on reads). • If BINMODE is "w" or two, then binary mode is set on write (i.e., no translations on writes). • If BINMODE is "rw" or "wr" or three, binary mode is set for both read and write. • BINMODE=non-null-string is the same as ‘BINMODE=3’ (i.e., no translations on reads or writes). However, gawk issues a warning message if the string is not one of "rw" or "wr". The modes for standard input and standard output are set one time only (after the command line is read, but before processing any of the awk program). Setting BIN MODE for standard input or standard output is accomplished by using an appropriate ‘-v BINMODE=N ’ option on the command line. BINMODE is set at the time a file or pipe is opened and cannot be changed midstream. The name BINMODE was chosen to match mawk (see “Other Freely Available awk Imple‐ mentations” on page 485). mawk and gawk handle BINMODE similarly; however, mawk adds a ‘-W BINMODE=N ’ option and an environment variable that can set BINMODE, RS, and ORS. The files binmode[1-3].awk (under gnu/lib/awk in some of the prepared binary distributions) have been chosen to match mawk’s ‘-W BINMODE=N ’ option. These can be changed or discarded; in particular, the setting of RS giving the fewest “surprises” is open to debate. mawk uses ‘RS = "\r\n"’ if binary mode is set on read, which is appropriate for files with the MS-DOS-style end-of-line. To illustrate, the following examples set binary mode on writes for standard output and other files, and set ORS as the “usual” MS-DOS-style end-of-line: gawk -v BINMODE=2 -v ORS="\r\n" … 478 | Appendix B: Installing gawk or: gawk -v BINMODE=w -f binmode2.awk … These give the same result as the ‘-W BINMODE=2’ option in mawk. The following changes the record separator to "\r\n" and sets binary mode on reads, but does not affect the mode on standard input: gawk -v RS="\r\n" -e "BEGIN { BINMODE = 1 }" … or: gawk -f binmode1.awk … With proper quoting, in the first example the setting of RS can be moved into the BEGIN rule. Using gawk in the Cygwin environment gawk can be built and used “out of the box” under MS-Windows if you are using the Cygwin environment. This environment provides an excellent simulation of GNU/ Linux, using Bash, GCC, GNU Make, and other GNU programs. Compilation and in‐ stallation for Cygwin is the same as for a Unix system: tar -xvpzf gawk-4.1.2.tar.gz cd gawk-4.1.2 ./configure make && make check When compared to GNU/Linux on the same system, the ‘configure’ step on Cygwin takes considerably longer. However, it does finish, and then the ‘make’ proceeds as usual. Using gawk in the MSYS environment In the MSYS environment under MS-Windows, gawk automatically uses binary mode for reading and writing files. Thus, there is no need to use the BINMODE variable. This can cause problems with other Unix-like components that have been ported to MS- Windows that expect gawk to do automatic translation of "\r\n", because it won’t. Compiling and Installing gawk on Vax/VMS and OpenVMS This subsection describes how to compile and install gawk under VMS. The older des‐ ignation “VMS” is used throughout to refer to OpenVMS. Compiling gawk on VMS To compile gawk under VMS, there is a DCL command procedure that issues all the necessary CC and LINK commands. There is also a Makefile for use with the MMS and MMK utilities. From the source directory, use either: Installation on Other Operating Systems | 479 1. The IA64 architecture is also known as “Itanium.” $ @[.vms]vmsbuild.com or: $ MMS/DESCRIPTION=[.vms]descrip.mms gawk or: $ MMK/DESCRIPTION=[.vms]descrip.mms gawk MMK is an open source, free, near-clone of MMS and can better handle ODS-5 volumes with upper- and lowercase filenames. MMK is available from https://github.com/endless software/mmk. With ODS-5 volumes and extended parsing enabled, the case of the target parameter may need to be exact. gawk has been tested under VAX/VMS 7.3 and Alpha/VMS 7.3-1 using Compaq C V6.4, and under Alpha/VMS 7.3, Alpha/VMS 7.3-2, and IA64/VMS 8.3. The most recent builds used HP C V7.3 on Alpha VMS 8.3 and both Alpha and IA64 VMS 8.4 used HP C 7.3.1 See “The VMS GNV project” on page 483 for information on building gawk as a PCSI kit that is compatible with the GNV product. Compiling gawk dynamic extensions on VMS The extensions that have been ported to VMS can be built using one of the following commands: $ MMS/DESCRIPTION=[.vms]descrip.mms extensions or: $ MMK/DESCRIPTION=[.vms]descrip.mms extensions gawk uses AWKLIBPATH as either an environment variable or a logical name to find the dynamic extensions. Dynamic extensions need to be compiled with the same compiler options for floating- point, pointer size, and symbol name handling as were used to compile gawk itself. Alpha and Itanium should use IEEE floating point. The pointer size is 32 bits, and the symbol name handling should be exact case with CRC shortening for symbols longer than 32 bits. For Alpha and Itanium: /name=(as_is,short) /float=ieee/ieee_mode=denorm_results 480 | Appendix B: Installing gawk For VAX: /name=(as_is,short) Compile-time macros need to be defined before the first VMS-supplied header file is included, as follows: #if (__CRTL_VER >= 70200000) && !defined (__VAX) #define _LARGEFILE 1 #endif #ifndef __VAX #ifdef __CRTL_VER #if __CRTL_VER >= 80200000 #define _USE_STD_STAT 1 #endif #endif #endif If you are writing your own extensions to run on VMS, you must supply these definitions yourself. The config.h file created when building gawk on VMS does this for you; if instead you use that file or a similar one, then you must remember to include it before any VMS-supplied header files. Installing gawk on VMS To use gawk, all you need is a “foreign” command, which is a DCL symbol whose value begins with a dollar sign. For example: $ GAWK :== $disk1:[gnubin]gawk Substitute the actual location of gawk.exe for ‘$disk1:[gnubin]’. The symbol should be placed in the login.com of any user who wants to run gawk, so that it is defined every time the user logs on. Alternatively, the symbol may be placed in the system-wide sylogin.com procedure, which allows all users to run gawk. If your gawk was installed by a PCSI kit into the GNV$GNU: directory tree, the program will be known as GNV$GNU:[bin]gnv$gawk.exe and the help file will be GNV$GNU: [vms_help]gawk.hlp. The PCSI kit also installs a GNV$GNU:[vms_bin]gawk_verb.cld file that can be used to add gawk and awk as DCL commands. For just the current process you can use: $ set command gnv$gnu:[vms_bin]gawk_verb.cld Or the system manager can use GNV$GNU:[vms_bin]gawk_verb.cld to add the gawk and awk to the system-wide ‘DCLTABLES’. The DCL syntax is documented in the gawk.hlp file. Installation on Other Operating Systems | 481 Optionally, the gawk.hlp entry can be loaded into a VMS help library: $ LIBRARY/HELP sys$help:helplib [.vms]gawk.hlp (You may want to substitute a site-specific help library rather than the standard VMS library ‘HELPLIB’.) After loading the help text, the command: $ HELP GAWK provides information about both the gawk implementation and the awk programming language. The logical name ‘AWK_LIBRARY’ can designate a default location for awk program files. For the -f option, if the specified filename has no device or directory path information in it, gawk looks in the current directory first, then in the directory specified by the translation of ‘AWK_LIBRARY’ if the file is not found. If, after searching in both directories, the file still is not found, gawk appends the suffix ‘.awk’ to the filename and retries the file search. If ‘AWK_LIBRARY’ has no definition, a default value of ‘SYS$LIBRARY:’ is used for it. Running gawk on VMS Command-line parsing and quoting conventions are significantly different on VMS, so examples in this book or from other sources often need minor changes. They are minor though, and all awk programs should run correctly. Here are a couple of trivial tests: $ gawk -- "BEGIN {print ""Hello, World!""}" $ gawk -"W" version ! could also be -"W version" or "-W version" Note that uppercase and mixed-case text must be quoted. The VMS port of gawk includes a DCL-style interface in addition to the original shell- style interface (see the help entry for details). One side effect of dual command-line parsing is that if there is only a single parameter (as in the quoted string program), the command becomes ambiguous. To work around this, the normally optional -- flag is required to force Unix-style parsing rather than DCL parsing. If any other dash-type options (or multiple parameters such as datafiles to process) are present, there is no ambiguity and -- can be omitted. The exit value is a Unix-style value and is encoded into a VMS exit status value when the program exits. The VMS severity bits will be set based on the exit value. A failure is indicated by 1, and VMS sets the ERROR status. A fatal error is indicated by 2, and VMS sets the FATAL status. All other values will have the SUCCESS status. The exit value is encoded to comply with VMS coding standards and will have the C_FACILITY_NO of 0x350000 with 482 | Appendix B: Installing gawk the constant 0xA000 added to the number shifted over by 3 bits to make room for the severity codes. To extract the actual gawk exit code from the VMS status, use: unix_status = (vms_status .and. &x7f8) / 8 A C program that uses exec() to call gawk will get the original Unix-style exit value. Older versions of gawk for VMS treated a Unix exit code 0 as 1, a failure as 2, a fatal error as 4, and passed all the other numbers through. This violated the VMS exit status coding requirements. VAX/VMS floating point uses unbiased rounding. See “Rounding Numbers” on page 247. VMS reports time values in GMT unless one of the SYS$TIMEZONE_RULE or TZ logical names is set. Older versions of VMS, such as VAX/VMS 7.3, do not set these logical names. The default search path, when looking for awk program files specified by the -f option, is "SYS$DISK:[],AWK_LIBRARY:". The logical name AWKPATH can be used to override this default. The format of AWKPATH is a comma-separated list of directory specifications. When defining it, the value should be quoted so that it retains a single translation and not a multitranslation RMS searchlist. The VMS GNV project The VMS GNV package provides a build environment similar to POSIX with ports of a collection of open source tools. The gawk found in the GNV base kit is an older port. Currently, the GNV project is being reorganized to supply individual PCSI packages for each component. See https://sourceforge.net/p/gnv/wiki/InstallingGNVPackages/. The normal build procedure for gawk produces a program that is suitable for use with GNV. The file vms/gawk_build_steps.txt in the distribution documents the procedure for building a VMS PCSI kit that is compatible with GNV. Some VMS systems have an old version of gawk Some versions of VMS have an old version of gawk. To access it, define a symbol, as follows: $ gawk :== $sys$common:[syshlp.examples.tcpip.snmp]gawk.exe This is apparently version 2.15.6, which is extremely old. We recommend compiling and using the current version. Installation on Other Operating Systems | 483 Reporting Problems and Bugs There is nothing more dangerous than a bored archaeologist. —Douglas Adams, The Hitchhiker’s Guide to the Galaxy If you have problems with gawk or think that you have found a bug, report it to the developers; we cannot promise to do anything, but we might well want to fix it. Before reporting a bug, make sure you have really found a genuine bug. Carefully reread the documentation and see if it says you can do what you’re trying to do. If it’s not clear whether you should be able to do something or not, report that too; it’s a bug in the documentation! Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible awk program and input datafile that reproduce the problem. Then send us the program and datafile, some idea of what kind of Unix system you’re using, the compiler you used to compile gawk, and the exact results gawk gave you. Also say what you expected to occur; this helps us decide whether the problem is really in the documentation. Make sure to include the version number of gawk you are using. You can get this in‐ formation with the command ‘gawk --version’. Once you have a precise problem description, send email to bug-gawk@gnu.org. The gawk maintainers subscribe to this address, and thus they will receive your bug report. Although you can send mail to the maintainers directly, the bug reporting ad‐ dress is preferred because the email list is archived at the GNU Project. All email must be in English. This is the only language understood in common by all the maintainers. Do not try to report bugs in gawk by posting to the Usenet/Inter‐ net newsgroup comp.lang.awk. The gawk developers do occasion‐ ally read this newsgroup, but there is no guarantee that we will see your posting. The steps described here are the only officially recog‐ nized way for reporting bugs. Really. 484 | Appendix B: Installing gawk Many distributions of GNU/Linux and the various BSD-based oper‐ ating systems have their own bug reporting systems. If you report a bug using your distribution’s bug reporting system, you should also send a copy to bug-gawk@gnu.org. This is for two reasons. First, although some distributions forward bug reports “upstream” to the GNU mailing list, many don’t, so there is a good chance that the gawk maintainers won’t even see the bug report! Second, mail to the GNU list is archived, and having every‐ thing at the GNU Project keeps things self-contained and not depen‐ dent on other organizations. Non-bug suggestions are always welcome as well. If you have questions about things that are unclear in the documentation or are just obscure features, ask on the bug list; we will try to help you out if we can. If you find bugs in one of the non-Unix ports of gawk, send an email to the bug list, with a copy to the person who maintains that port. The maintainers are named in the fol‐ lowing list, as well as in the README file in the gawk distribution. Information in the README file should be considered authoritative if it conflicts with this book. The people maintaining the various gawk ports are: Unix and POSIX systems Arnold Robbins, arnold@skeeve.com MS-DOS with DJGPP Scott Deifik, scottd.mail@sbcglobal.net MS-Windows with MinGW Eli Zaretskii, eliz@gnu.org OS/2 Andreas Buening, andreas.buening@nexgo.de VMS John Malmberg, wb8tyw@qsl.net z/OS (OS/390) Dave Pitts, dpitts@cozx.com If your bug is also reproducible under Unix, send a copy of your report to the bug- gawk@gnu.org email list as well. Other Freely Available awk Implementations It’s kind of fun to put comments like this in your awk code: // Do C++ comments work? answer: yes! of course —Michael Brennan There are a number of other freely available awk implementations. This section briefly describes where to get them: Other Freely Available awk Implementations | 485 Unix awk Brian Kernighan, one of the original designers of Unix awk, has made his imple‐ mentation of awk freely available. You can retrieve this version via his home page. It is available in several archive formats: Shell archive http://www.cs.princeton.edu/~bwk/btl.mirror/awk.shar Compressed tar file http://www.cs.princeton.edu/~bwk/btl.mirror/awk.tar.gz Zip file http://www.cs.princeton.edu/~bwk/btl.mirror/awk.zip You can also retrieve it from GitHub: git clone git://github.com/onetrueawk/awk bwkawk This command creates a copy of the Git repository in a directory named bwkawk. If you leave that argument off the git command line, the repository copy is created in a directory named awk. This version requires an ISO C (1990 standard) compiler; the C compiler from GCC (the GNU Compiler Collection) works quite nicely. See “Common Extensions Summary” on page 463 for a list of extensions in this awk that are not in POSIX awk. As a side note, Dan Bornstein has created a Git repository tracking all the versions of BWK awk that he could find. It’s available at git://github.com/danfuzz/one-true- awk. mawk Michael Brennan wrote an independent implementation of awk, called mawk. It is available under the GPL, just as gawk is. The original distribution site for the mawk source code no longer has it. A copy is available at http://www.skeeve.com/gawk/mawk1.3.3.tar.gz. In 2009, Thomas Dickey took on mawk maintenance. Basic information is available on the project’s web page. The download URL is http://invisible-island.net/datafiles/ release/mawk.tar.gz. Once you have it, gunzip may be used to decompress this file. Installation is similar to gawk’s (see “Compiling and Installing gawk on Unix-Like Systems” on page 474). See “Common Extensions Summary” on page 463 for a list of extensions in mawk that are not in POSIX awk. 486 | Appendix B: Installing gawk awka Written by Andrew Sumner, awka translates awk programs into C, compiles them, and links them with a library of functions that provide the core awk functionality. It also has a number of extensions. The awk translator is released under the GPL, and the library is under the LGPL. To get awka, go to http://sourceforge.net/projects/awka. The project seems to be frozen; no new code changes have been made since ap‐ proximately 2001. pawk Nelson H.F. Beebe at the University of Utah has modified BWK awk to provide timing and profiling information. It is different from gawk with the --profile option (see “Profiling Your awk Programs” on page 338) in that it uses CPU-based profiling, not line-count profiling. You may find it at either ftp://ftp.math.utah.edu/ pub/pawk/pawk-20030606.tar.gz or http://www.math.utah.edu/pub/pawk/ pawk-20030606.tar.gz. BusyBox awk BusyBox is a GPL-licensed program providing small versions of many applications within a single executable. It is aimed at embedded systems. It includes a full im‐ plementation of POSIX awk. When building it, be careful not to do ‘make install’ as it will overwrite copies of other applications in your /usr/local/bin. For more information, see the project’s home page. The OpenSolaris POSIX awk The versions of awk in /usr/xpg4/bin and /usr/xpg6/bin on Solaris are more or less POSIX-compliant. They are based on the awk from Mortice Kern Systems for PCs. We were able to make this code compile and work under GNU/Linux with 1– 2 hours of work. Making it more generally portable (using GNU Autoconf and/or Automake) would take more work, and this has not been done, at least to our knowledge. The source code used to be available from the OpenSolaris website. However, that project was ended and the website shut down. Fortunately, the Illumos project makes this implementation available. You can view the files one at a time from https://github.com/joyent/illumos-joyent/blob/master/usr/src/cmd/awk_xpg4. jawk This is an interpreter for awk written in Java. It claims to be a full interpreter, al‐ though because it uses Java facilities for I/O and for regexp matching, the language it supports is different from POSIX awk. More information is available on the project’s home page. Other Freely Available awk Implementations | 487 Libmawk This is an embeddable awk interpreter derived from mawk. For more information, see http://repo.hu/projects/libmawk/. pawk This is a Python module that claims to bring awk-like features to Python. See https:// github.com/alecthomas/pawk for more information. (This is not related to Nelson Beebe’s modified version of BWK awk, described earlier.) QSE awk This is an embeddable awk interpreter. For more information, see http:// code.google.com/p/qse/ and http://awk.info/?tools/qse. QTawk This is an independent implementation of awk distributed under the GPL. It has a large number of extensions over standard awk and may not be 100% syntactically compatible with it. See http://www.quiktrim.org/QTawk.html for more informa‐ tion, including the manual and a download link. The project may also be frozen; no new code changes have been made since ap‐ proximately 2008. Other versions See also the “Versions and implementations” section of the Wikipedia article on awk for information on additional versions. Summary • The gawk distribution is available from the GNU Project’s main distribution site, ftp.gnu.org. The canonical build recipe is: wget http://ftp.gnu.org/gnu/gawk/gawk-4.1.2.tar.gz tar -xvpzf gawk-4.1.2.tar.gz cd gawk-4.1.2 ./configure && make && make check • gawk may be built on non-POSIX systems as well. The currently supported systems are MS-Windows using DJGPP, MSYS, MinGW, and Cygwin, and both Vax/VMS and OpenVMS. Instructions for each system are included in this appendix. 488 | Appendix B: Installing gawk • Bug reports should be sent via email to bug-gawk@gnu.org. Bug reports should be in English and should include the version of gawk, how it was compiled, and a short program and datafile that demonstrate the problem. • There are a number of other freely available awk implementations. Many are POSIX- compliant; others are less so. Summary | 489 APPENDIX C GNU General Public License Version 3, 29 June 2007 Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The GNU General Public License is a free, copyleft license for software and other kinds of works. The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program— to make sure it remains free software for all its users. We, the Free Software Foundation, use the GNU General Public License for most of our software; it applies also to any other work released this way by its authors. You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. 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A product is a consumer product 496 | Appendix C: GNU General Public License regardless of whether the product has substantial commercial, industrial or non- consumer uses, unless such uses represent the only significant mode of use of the product. “Installation Information” for a User Product means any methods, procedures, au‐ thorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made. 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Access to a network may be denied when the mod‐ ification itself materially and adversely affects the operation of the network or vi‐ olates the rules and protocols for communication across the network. Corresponding Source conveyed, and Installation Information provided, in accord with this section must be in a format that is publicly documented (and with an implementation available to the public in source code form), and must require no special password or key for unpacking, reading or copying. 7. Additional Terms. “Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are applicable to the entire Program shall be treated as though they were included in this License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those permissions, but the entire Program remains governed by this License without regard to the additional permissions. When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it. (Additional permis‐ sions may be written to require their own removal in certain cases when you modify the work.) You may place additional permissions on material, added by you to a covered work, for which you have or can give appropriate copyright permission. TERMS AND CONDITIONS | 497 Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms: a. Disclaiming warranty or limiting liability differently from the terms of sections 15 and 16 of this License; or b. Requiring preservation of specified reasonable legal notices or author attribu‐ tions in that material or in the Appropriate Legal Notices displayed by works containing it; or c. Prohibiting misrepresentation of the origin of that material, or requiring that modified versions of such material be marked in reasonable ways as different from the original version; or d. Limiting the use for publicity purposes of names of licensors or authors of the material; or e. Declining to grant rights under trademark law for use of some trade names, trademarks, or service marks; or f. Requiring indemnification of licensors and authors of that material by anyone who conveys the material (or modified versions of it) with contractual assump‐ tions of liability to the recipient, for any liability that these contractual assump‐ tions directly impose on those licensors and authors. All other non-permissive additional terms are considered “further restrictions” within the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term. If a license document contains a further restriction but permits relicensing or conveying under this License, you may add to a covered work material governed by the terms of that license document, provided that the further restriction does not survive such relicensing or conveying. If you add terms to a covered work in accord with this section, you must place, in the relevant source files, a statement of the additional terms that apply to those files, or a notice indicating where to find the applicable terms. Additional terms, permissive or non-permissive, may be stated in the form of a separately written license, or stated as exceptions; the above requirements apply either way. 8. Termination. You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will automatically terminate your rights under this License (including any patent licenses granted under the third paragraph of section 11). 498 | Appendix C: GNU General Public License However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright hold‐ er explicitly and finally terminates your license, and (b) permanently, if the copy‐ right holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, you do not qualify to receive new licenses for the same material under section 10. 9. Acceptance Not Required for Having Copies. You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a conse‐ quence of using peer-to-peer transmission to receive a copy likewise does not re‐ quire acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so. 10. Automatic Licensing of Downstream Recipients. Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License. You are not responsible for enforcing compliance by third parties with this License. An “entity transaction” is a transaction transferring control of an organization, or substantially all assets of one, or subdividing an organization, or merging organi‐ zations. If propagation of a covered work results from an entity transaction, each party to that transaction who receives a copy of the work also receives whatever licenses to the work the party’s predecessor in interest had or could give under the previous paragraph, plus a right to possession of the Corresponding Source of the work from the predecessor in interest, if the predecessor has it or can get it with reasonable efforts. You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License. For example, you may not impose a license fee, royalty, or other charge for exercise of rights granted under this License, and you may not initiate litigation (including a cross-claim or counterclaim in a lawsuit) TERMS AND CONDITIONS | 499 alleging that any patent claim is infringed by making, using, selling, offering for sale, or importing the Program or any portion of it. 11. Patents. A “contributor” is a copyright holder who authorizes use under this License of the Program or a work on which the Program is based. The work thus licensed is called the contributor’s “contributor version”. A contributor’s “essential patent claims” are all patent claims owned or controlled by the contributor, whether already acquired or hereafter acquired, that would be infringed by some manner, permitted by this License, of making, using, or selling its contributor version, but do not include claims that would be infringed only as a consequence of further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the requirements of this License. Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor’s essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version. In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission to practice a patent or covenant not to sue for patent infringement). To “grant” such a patent license to a party means to make such an agreement or com‐ mitment not to enforce a patent against the party. If you convey a covered work, knowingly relying on a patent license, and the Cor‐ responding Source of the work is not available for anyone to copy, free of charge and under the terms of this License, through a publicly available network server or other readily accessible means, then you must either (1) cause the Corresponding Source to be so available, or (2) arrange to deprive yourself of the benefit of the patent license for this particular work, or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream re‐ cipients. “Knowingly relying” means you have actual knowledge that, but for the patent license, your conveying the covered work in a country, or your recipient’s use of the covered work in a country, would infringe one or more identifiable patents in that country that you have reason to believe are valid. If, pursuant to or in connection with a single transaction or arrangement, you con‐ vey, or propagate by procuring conveyance of, a covered work, and grant a patent license to some of the parties receiving the covered work authorizing them to use, propagate, modify or convey a specific copy of the covered work, then the patent license you grant is automatically extended to all recipients of the covered work and works based on it. 500 | Appendix C: GNU General Public License A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the rights that are specifically granted under this License. You may not convey a covered work if you are a party to an arrangement with a third party that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license (a) in connection with copies of the covered work conveyed by you (or copies made from those copies), or (b) primarily for and in connection with specific products or compilations that contain the cov‐ ered work, unless you entered into that arrangement, or that patent license was granted, prior to 28 March 2007. Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law. 12. No Surrender of Others’ Freedom. If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obli‐ gations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program. 13. Use with the GNU Affero General Public License. Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such. 14. Revised Versions of this License. The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later ver‐ sion” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software TERMS AND CONDITIONS | 501 Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation. If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Program. Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version. 15. Disclaimer of Warranty. THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PER‐ MITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EX‐ PRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DE‐ FECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, RE‐ PAIR OR CORRECTION. 16. Limitation of Liability. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. 17. Interpretation of Sections 15 and 16. If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in con‐ nection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee. 502 | Appendix C: GNU General Public License END OF TERMS AND CONDITIONS How to Apply These Terms to Your New Programs If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found. one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/. Also add information on how to contact you by electronic and paper mail. If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode: program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’. This is free software, and you are welcome to redistribute it under certain conditions; type ‘show c’ for details. The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”. You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/. END OF TERMS AND CONDITIONS | 503 The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html. 504 | Appendix C: GNU General Public License We’d like to hear your suggestions for improving our indexes. Send email to index@oreilly.com. Index Symbols ! (exclamation mark) != (not equal) operator, 63, 129, 137 !~ operator, 39, 49, 113, 129, 137, 142 not operator, 132, 137, 286 " (quotation marks, double) denoting null strings, 8 enclosing string constants, 112 escaping in string constants, 40 mixing with single quotes, 10 shell rules for, 9 # (number sign) #! script mechanism, 6, 277 format specification modifiers, 96 prefacing awk comments, 7 $ (dollar sign) $0 value in BEGIN and END rules, 146 set by getline, 79 understanding, 64 end-of-string matching in regular expres‐ sions, 44, 59 field references in awk, 60, 137 % (percent sign) %= (modulus and assignment) operator, 125, 137 remainder operator, 120, 137 & (ampersand) && (Boolean and) operator, 132, 137 with sub(), gsub(), and gensub() functions, 205 ' (quotation marks, single), 5, 8 enclosing awk program on command line, 4 mixing with double quotes, 10 modifier for printf formats, 97 \' operator, 51 ( ) (parentheses) function arguments in, 135 grouping with, 45, 137 in functions, 191 using in string concatenation, 121 * (asterisk) ** (exponentiation) operator, 120, 137 **= (exponentiation and assignment) opera‐ tor, 125, 137 *= (multiplication and assignment) operator, 125, 137 multiplication operator, 61, 120, 137 regular expression metacharacter, 45 using with positional specifiers, 351 + (plus sign) ++ (increment) operator, 84, 126, 137, 151, 298 += (addition and assignment) operator, 124, 137, 298 addition operator, 120, 137 regular expression metacharacter, 45 unary plus operator, 120, 137 505 used before width modifier, 96 , (comma) as decimal point character, 118 separati