César Ferradas

How to lint code using git hooks

21 February 2021

A fast and effective way to avoid unlinted code from sneaking into version control.

The problem

I’m sure every developer can relate with this:

  1. Write a bunch of code, potentially quite quickly and including find-and-replace operations.

  2. Commit this code to a feature branch

  3. Push to remote

  4. Linting fails in the remote CI system

What happened? Well, if you were coding fast, you might have missed linting warnings provided by your IDE. Or, if you found and replaced a lot of code, your linting might have not even run in your IDE.

Then you committed to your branch. Now you have a commit with unclean (if not buggy) code. Ideally all commits that get to your main branch should be deployable, and this one might not be.

The solution

How do you avoid this? Enter git pre-commit hooks.

You can tell git to run certain scripts at various stages of your git flow. The “pre-commit” hook as its name implies runs just before you commit files to the repo.

You can use this feature to run linting before committing any files. Let’s use Python and flake8 for this example.

First, create a .git/hooks/pre-commit file and make it executable:

touch .git/hooks/pre-commit
chmod +x .git/hooks/pre-commit

Then add this to the file:


set -u

py_files=$(git diff --cached --name-only | grep ".py" | xargs)

if [ -z "$py_files" ]; then
    echo no Python files, skipping linting
    echo linting $py_files
    flake8 $py_files

This script is slightly smarter than just running flake8 on the entire repo. That can be slow and annoying to wait for every time you commit if you’re committing often (which you probably should).

Instead, the script narrows down the files that we need to lint as much as possible. It first only looks at the files that have been changed and will be committed (using git diff --cached --name-only). Then it excludes any non-Python files (using grep .py).

The result is you only run linting on what’s exactly necessary to prevent dirty commits.

Hope this little script helps you. If it did or you have any feedback, let me know.