Hooks

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Overview

Questions

• How do I automate checks on my commit?
• How do I check changes before a commit?

Objectives

• Learn about using hooks to improve quality of commits

Git hooks are scripts that get run when a specific event occurs in git. The scripts can be written in any language and do anything you like, so any executauble script can be a hook.

Git hooks can trigger events on the server size or locally, and commonly used hooks include: - pre-commit: Executed before the git commit command and is usually used to check the changes with linters or tests. - prepare-commit-msg: Executed before after the commit message is - commit-msg: Validates the commit message, can be used to check whether the commit message adheres to project policies - post-commit: Runs after the commit, often used for notifications or logging. - pre-receive: Server-side hook, which runs before a push is accepted, commonly used to enfore project policies

Examples of local events that can trigger hooks include commit (pre- or post-commit hooks), checkout or rebase. Pre-commit hooks are perhaps the most common and useful ones: they trigger actions before the code is committed and if the hook script fails, then the command is aborted. This can be very powerful - you can automatically run linters, before the code is even committed.

List of pre-written pre-commit hooks: https://github.com/pre-commit/pre-commit-hooks

The executable files are stored in the .git/hooks/ directory in your project directory. A pre-commit hooks will be an executable file in this directory stored with the magic name pre-commit. Check the directory, there are already several examples. Let’s create a new one

BASH

touch .git/hooks/pre-commit
nano .git/hooks/pre-commit

And add the following text to it:

#!/usr/bin/env bash

set -eo pipefail
flake8 hello.py
echo "flake8 passed!"

Now let’s make hello.py:

BASH

touch hello.py
nano hello.py

And add some text to it:

PYTHON

print('Hello world!'')

The typo is on purpose. Add and commit it to the repository.

GitHub actions are the equivalent of server-side hooks on GitHub.

There are lots of things that can be done with GitHub actions: https://docs.github.com/en/actions

Here is an example of a simple cron job: https://github.com/mpi-astronomy/XarXiv

GitFlow 1

Materials: https://verdantfox.com/blog/how-to-use-git-pre-commit-hooks-the-hard-way-and-the-easy-way

Key Points

• Git provides a list of different hooks for you to run tasks at specific times in the commit
• Use the pre-commit hook to check for change conformity before changes are committed

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