Tips for using a git pre-commit hook

Here’s a few tips for using a Git pre-commit hook.

Keep your hook script in source control

Commit your hook script (say pre-commit.sh) at the root of your project and include the installation instructions in your README/documentation to encourage all developers use it.

Installation is nothing more than:

ln -s ../../pre-commit.sh .git/hooks/pre-commit

Then everyone benefits from running the same set of tests before committing and updates are picked up automatically.

Stash unstaged changes before running tests

Ensure that unstaged code isn’t tested within your pre-commit script. This is missed by many sample pre-commit scripts but is easily achieved with git stash:

# pre-commit.sh
STASH_NAME="pre-commit-$(date +%s)"
git stash save --quiet --keep-index --include-untracked $STASH_NAME

# Test prospective commit
...

STASHES=$(git stash list)
if [[ $STASHES == *"$STASH_NAME" ]]; then
  git stash pop --quiet
fi

Run your test suite before each commit

Obviously.

It’s best to have a script (say run_tests.sh) that encapsulates the standard arguments to your test runner so your pre-commit script doesn’t fall out of date. Something like:

# pre-commit.sh
git stash -q --keep-index
./run_tests.sh
RESULT=$?
git stash pop -q
[ $RESULT -ne 0 ] && exit 1
exit 0

where a sample run_tests.sh implementation for a Django project may look like:

# run_tests.sh
./manage.py test --settings=settings_test -v 2

Skip the pre-commit hook sometimes

Be aware of the --no-verify option to git commit. This bypasses the pre-commit hook when committing, which is useful if you have just manually run your test suite and don’t need to see it run again when committing.

I use git aliases to make this easy:

# ~/.bash_aliases
alias gc='git commit'
alias gcv='git commit --no-verify'

Search yo.. code for debugging code

At some point, someone will try and commit a file containing

import pdb; pdb.set_trace()

or some other debugging code. This can be easily avoided using the pre-commit.sh file to grep the staged codebase and abort the commit if forbidden strings are found.

Here’s an example that looks for console.log:

FILES_PATTERN='\.(js|coffee)(\..+)?$'
FORBIDDEN='console.log'
git diff --cached --name-only | \
    grep -E $FILES_PATTERN | \
    GREP_COLOR='4;5;37;41' xargs grep --color --with-filename -n $FORBIDDEN && echo 'COMMIT REJECTED Found "$FORBIDDEN" references. Please remove them before committing' && exit 1

It’s straightforward to extend this code block to search for other terms.