Bash Shell 速查表
这个速查表的主要主题包括了：shell介绍, navigating around the shell,公共命令, 环境变量, connectors, piping, I/O redirection, 权限, 和快捷键
Introduction to the Shell
The shell is a program, in our case, called "bash" which stands for Bourne Again Shell. How the shell works is it takes your commands and gives them to the operating system to perform. In order to interact with the shell, we use "terminal emulators" such as the gnome-terminal, eterm, nxterm, etc.
Navigating Around The Shell
On a Linux system, files are organized in a hierarchical directory structure. This means there is a starting directory called the root directory. This directory contains files and subdirectories that lead into other subdirectories.
pwd
Thepwdcommand, short for print working directory, displays your current location in the directory structure.
cd
Thecdcommand allows you to enter a new directory.
| Syntax | Explanation |
|---|---|
| cd | navigate to home directory |
| cd ~ | navigate to home directory |
| cd .. | navigate backwards to parent directory |
| cd - | navigate to previous working directory |
| cd Directory1 | navigate to directory named Directory1 |
| cd Directory1/Directory2 | navigate to directory, Directory2, through path |
mkdir
Themkdircommand makes a new directory in your current directory.
Common Commands
man
Themancommand directs you to the command manuals.
For example, the following command gives us all the information we need about the commandcat.
$ man cat
cat
Thecatcommand reads a file passed as a parameter and by default print its contents to standard output.
Passing multiply files as parameters concatenates the files and then prints to standard output.
echo
Theechocommand prints its arguments to standard output.
$ echo Hello World Hello World
If you callechowithout any parameters, the command prints a new line.
head
Theheadcommand reads the first 10 lines of any passed in text and prints its contents to standard output. You can change the default 10 lines to any number by manually passing in the desired size. For example, the following prints all 50 lines of the file.
$ head -50 test.txt
tail
Thetailcommand reads the last 10 lines of any passed in text and prints its contents to standard output. You can change the default 10 lines to any number by manually passing in the desired size. For example, the following prints all 50 lines of the file.
$ tail -50 test.txt
You can also view in real time any text appended to the file with the-fflag.
$ tail -f test.txt
less
Thelesscommand gives you a way to navigate through a passed file or block of text. Unlike themorecommand, less allows you to move backward through the file as well.
$ less test.txt $ ps aux | less
| Common less keyboard shortcuts | Description |
|---|---|
| G | Moves to end of file |
| g | Moves to beginning of file |
| :50 | Moves to the 50th line of the file |
| q | Exits less |
| /searchterm | Searches for any string matching 'searchterm' below the current line |
| / | Moves you to the next match for your previous 'searchterm' below the current line |
| ?searchterm | Searches for any string matching 'searchterm' above the current line |
| ? | Moves you to the next match for your previous 'searchterm' above the current line |
| up | Moves up a line |
| down | Moves down a line |
| pageup | Moves up a page |
| pagedown | Moves down a page |
true
Thetruecommand always returns the exit status zero to indicate success.
false
Thefalsecommand always returns the exit status non-zero to indicate failure.
$?
$?is a variable that will return the exit code of the last command you ran.
$ true $ echo $? 0 $ false $ echo $? 1
grep
Thegrepcommand is a search function.
Passing a string and a file searches the file for the given string and prints the occurrences to standard output.
$ cat users.txt user:student password:123 user:teacher password:321 $ grep 'student` file1.txt user:student password:123
grepcan take multiple files as parameters and regular expressions to specify a pattern in text.
| Common flags | Description |
|---|---|
| -i | remove case sensitivity |
| -r | search recursively through directories |
| -w | search only whole words |
| -c | prints number of times found |
| -n | prints line found on with phrase |
| -v | prints invert match |
sed
Thesedcommand is a stream editor that performs text transformations on an input.
Common use of this command is to replace expressions which takes the forms/regexp/replacement/gFor example, the following replaces all occurrences of the phrase "Hello" with "Hi".
$ cat test.txt Hello World $ sed 's/Hello/Hi/g' test.txt Hi World
history
Thehistorycommand prints out an incremented command line history.
It is common to use thegrepcommand with thehistorycommand in order to search for a particular command. For example, the following searches your history for all occurrences of the stringg++.
$ history | grep g++ 155 g++ file1.txt 159 g++ file2.txt
export
Theexportcommand sets an environment variable to be passed to child processes in the environment.
For example, the following exports the variable "name" with the value "student".
$ export name=student
ps
Thepscommand, short for process status, prints out information about the processes running.
$ ps PID TTY TIME CMD 35346 pts/2 00:00:00 bash
There are four items displayed:
- process identification number (PID)
- terminal type (TTY),
- how long process has been running (TIME)
- name of command that launched the process (CMD)
awk
Theawkcommand finds and replaces text by searching through files for lines that have a pattern.
Syntax:awk 'pattern {action}' test.txt
wget
Thewgetcommand downloads files from the web and stores it in the current working directory.
$ wget https://github.com/mikeizbicki/ucr-cs100
nc
Thenccommand, short for netcat, is a utility used to debug and investigate the network.
See nc tutorial
ping
Thepingcommand tests a network connection.
$ ping google.com PING google.com (74.125.224.34) 56(84) bytes of data. 64 bytes from lax17s01-in-f2.1e100.net (74.125.224.34): icmp_req=1 ttl=57 time=7.82 ms --- google.com ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 8ms rtt min/avg/max/mdev = 7.794/8.422/10.792/0.699 ms
The statistics at the end show an overview of how many connections went through before we called^Cand how long it took.
git
Gitis a version control system that is commonly used in the industry and in open source projects.
See git tutorial
Environment Variables
Environment variables are named variables that contain values used by one or more applications.
ThePATHvariable contains a list of directories where systems look for executable files.
TheHOMEvariable contains the path to the home directory of the current user.
ThePS1variable is the default prompt to control appearances of the command prompt.
Connectors
Connectors allow you to run multiple commands at once.
| Connector | Description |
|---|---|
| && | first command always executes and the next command will only execute if the one before it succeeds |
| || | first command always executes and the next command will only execute if the one before it fails |
| ; | first command and the following commands always execute |
$ true && echo Hello Hello $ false || echo Hello Hello $ echo Hello ; ls Hello test.txt file1.txt file2.txt
Piping
Pipes connect multiple commands together by sending the stdout of the first command to the stdin of the next command. For example, the following sends thelsoutput toheadso that only the top 10 items get printed.
$ ls -l | head
Input/Output Redirection
Output Redirection
Standard output redirection uses the symbols>and>>.
For example, the following sends the output oflsinto the file instead of printing to the screen.
ls > files.txt $ cat files.txt file1.cpp sample.txt
If the file isn't already in your working directory, the file gets created. If the file already exists, then the contents of the command overwrites what is already in the file.
To avoid overwriting a file, the>>command appends to the end of the file instead.
Input Redirection
Standard input redirection uses the symbol<.
For example, the following causessortto access its input from the file instead of the keyboard.
$ cat files.txt c b $ sort < files.txt b c
Thesortcommand prints the contents of the file and prints to the screen because we haven't redirected its output. But we can combine I/O redirection into one command line, such as:
$ sort < files.txt > files_sorted.txt
Advanced Redirection
Adding a&with the>symbol results in redirecting both standard out and standard error. For example, thetest.cppfile prints the string "stdout" withcoutand the string "stderr" withcerr.
$ g++ test.cpp $ ./a.out >& test.txt $ cat test.txt stdout stderr
The>symbol alone only redirects standard output.
If you only want to redirect a specific file descriptor you can attach the file descriptor number to>.
| Name | File Descriptor | Description |
|---|---|---|
| stdin | 0 | standard input stream |
| stdout | 1 | standard output stream |
| stderr | 2 | standard error output stream |
For example, if I only wanted to redirect "stderr" to the filetest.txtfrom the above example, I would do the following:
$ g++ test.cpp $ ./a.out 2> test.txt stdout $ cat test.txt stderr
Permissions
The commandls -lprints out a lot of information about each file that is informative about the permissions.
$ ls -l test.txt -rw-rw-r-- 1 user group 1097374 January 26 2:48 test.txt
| Output from example above | Description/Possible Outputs |
|---|---|
| - | File type: -= regular file d= directory |
| rw- | Permissions for owner of file |
| rw- | Permissions for members of the group owning the file |
| r-- | Permissions for all other users |
| user | name of user owning the file |
| group | name of group owning the file |
chmod
Thechmodcommand, short for change mode, changes the permissions of a file.
There is a combination of letters that need to be known in order to change specific users' permission.
| Letter | User |
|---|---|
| u | User who owns it |
| g | Users in the group |
| a | All users |
You callchmodby describing which actions you want to perform and to which file. The-symbol represents taking away permissions while the+symbol represents adding permissions. The following example makes the file readable and writable to the user who owns it and the group.
$ chmod ug+rw test.txt $ ls -l test.txt -rw-rw---- 1 user group 1097374 January 26 2:48 test.txt
Alternatively, we can usechmodwith hex numbers. You can think of each permission setting as a bit where it is a1if there is permission for the file and0otherwise.
rwx = 111 = 7 rw- = 110 = 6 r-x = 101 = 5 r-- = 100 = 4
Each set of permissions represents a single digit so the following commands have the same outcome as above.
$ chmod 660 test.txt
Keyboard Shortcuts
| Shortcut | Description |
|---|---|
| CTRL-A | Move cursor to beginning of line |
| CTRL-E | Move cursor to end of line |
| CTRL-R | Search bash history |
| CTRL-W | Cut the last word |
| CTRL-U | Cut everything before the cursor |
| CTRL-K | Cut everything after the cursor |
| CTRL-Y | Paste the last thing to be cut |
| CTRL-_ | Undo |
| CTRL-L | Clears terminal screen |