Could you just exit when there are no code changes?

git diff-tree --no-commit-id --name-only -r HEAD..origin/master | grep -vqE "^Documentation/"
if [ $? != 0 ] ; then
  echo No changes to source detected.
  exit 0
fi

I was thinking of adding another check for doc changes later, similar to

DOC_CHANGES=$(git diff-tree --no-commit-id --name-only -r HEAD..origin/master | grep -E "^Documentation/" | wc -l)

and then have another if clause that handles documentation changes.

Fair enough.

HEAD..origin/master might be Semaphore specific and developers might name their git-remotes differently. Do you think we should check $SEMAPHORE=true and $CI=true? See Semaphore environment variables

If you decide to do the rm -rf only in the $CI case, you can add a similar if for the other rm -rf line 75.

We clone to ${BUILD_DIR} regardless, so we have to remove it regardless. The rm from L32 is because we assume that semaphore didn't cleanup after itself. Basically, we should just check if it's there at all and remove it before cloning it.

this check seems redundant?

How about catching changes in {ROADMAP,README,CONTRIBUTING,CHANGELOG}.md? These currently will land in SRC_CHANGES, right?

Good idea!

Semaphore complains about the ):

grep: Unmatched ) or \)