How to use git pre-commit hooks, the hard way and the easy way


September 15, 2021 / 12 min read / 39639 , 24 , 2

Last updated: September 30, 2022

Tags: git, git hooks


pre-commit-with-text

If you've heard of git pre-commit hooks, but you aren't sure what they are or how to get started with them, you are in luck! In this guide, we'll talk about what git pre-commit hooks are and why you should consider using them. We'll then talk about how to write your own git pre-commit hooks, and then we'll talk about the pre-commit framework which can make setting up git pre-commit hooks easy.

What is a git pre-commit hook?

Git hooks in general, are scripts that get run when a specific event occurs in git. They can be written in any language and do anything you like. The only requirement for a git hook script is that it is an executable file.

What events in git can trigger a git hook? Git hook events can trigger on the server-side (where the code is stored remotely -- GitHub, GitLab, etc.) or locally (on your computer). Local git events that can trigger a hook script to run include:

  • pre-commit (occurs before a git commit is accepted)
  • post-commit (occurs immediately after a git commit is accepted)
  • post-checkout (occurs before a git checkout)
  • pre-rebase (occurs before a git rebase)

There are several other local git hook triggers, and also several server-side git hook triggers. But for this guide, we're specifically going to talk about git pre-commit hooks. What is special about the pre-commit hook? This hook runs when you run the command git commit. The hook runs before a commit is accepted to git, and if the hook script fails (ie if it returns a non-zero exit code) the commit is aborted. This is powerful! It means you can automatically run linters to statically analyze your code and make sure it is high quality and syntax-error-free before the code is committed. If your code doesn't meet your linters' quality standards, your commit will be rejected (for now). Your linters will tell you what, specifically, needs fixing, and in some cases, you can have your code fixed automatically, on the spot.

How to write a git pre-commit hook (the hard way)

Now that we know what a git pre-commit hook is, how can we write one? All git hooks are stored in the .git/hooks/ directory under your project root. A pre-commit hook is just an executable file stored in this folder with the magic name pre-commit.

Note that some code editors make the .git/ folder hidden by default. If you don't see it in your project root directory of your editor's file tree, try playing around with your code editor's settings to make the .git/ folder visible. For example, in VS Code, type "Files: Exclude" into the "preferences" menu to see .git/ is hidden.

Once you can access your .git/ directory, open up the .git/hooks/ directory. You will see a bunch of files named hook-type.sample. You should see one named pre-commit.sample. If you open this file, you will see an example pre-commit hook, provided by git. You can use this as an actual pre-commit hook by simply removing ".sample" from pre-commit.sample. The provided sample pre-commit hook is a bit confusing. Let's write our own, more straightforward hook.

Example pre-commit hook for a python project:

#!/usr/bin/env bash
# ^ Note the above "shebang" line. This says "This is an executable shell script"
# Name this script "pre-commit" and place it in the ".git/hooks/" directory

# If any command fails, exit immediately with that command's exit status
set -eo pipefail

# Run flake8 against all code in the `source_code` directory
flake8 source_code
echo "flake8 passed!"

# Run black against all code in the `source_code` directory
black source_code --check
echo "black passed!"

The above pre-commit hook, when placed in the .git/hooks/ directory, will run when you perform a git commit command. First, flake8 will scan your python code for its style requirements, and then black will scan your python code for its style requirements. These are two awesome python code linters I highly recommend you add to any python project. If either python linter fails, the script will immediately exit with a non-zero status code (due to set -eo pipefail). Therefore, your commit will abort with a message from the failed linter. Once you fix those errors and try to commit again, the script will succeed and exit with status code 0, and then your commit will succeed.

The above pre-commit hook script is a nice start. But note, we can also call git commands in a hook script. Therefore, we can see specifically which files are to be committed in this commit and only run our linters against those files. How might that look?

Modified pre-commit script that only runs against modified files:

#!/usr/bin/env bash

# If any command fails, exit immediately with that command's exit status
set -eo pipefail

# Find all changed files for this commit
# Compute the diff only once to save a small amount of time.
CHANGED_FILES=$(git diff --name-only --cached --diff-filter=ACMR)
# Get only changed files that match our file suffix pattern
get_pattern_files() {
    pattern=$(echo "$*" | sed "s/ /\$\\\|/g")
    echo "$CHANGED_FILES" | { grep "$pattern$" || true; }
}
# Get all changed python files
PY_FILES=$(get_pattern_files .py)

if [[ -n "$PY_FILES" ]]
then
    # Run black against changed python files for this commit
    black --check $PY_FILES
    echo "black passes all altered python sources."
    # Run flake8 against changed python files for this commit
    flake8 $PY_FILES
    echo "flake8 passed!"
fi

As I stated earlier, pre-commit hooks are very flexible. We wrote the above scripts in bash, but the scripts could have been written in python, ruby, node JS, or any other scripting language. Furthermore, a script can call other scripts in your code. This is a great way to chain together lots of hooks into one file. Simply write a base pre-commit hook script that calls all of your other pre-commit hook scripts. Those scripts could also run in any language, or even start docker containers that perform checks. Finally, these scripts can even modify (fix) your code as they run. If you do modify your code in a pre-commit script, make sure to exit with a non-zero exit status. Changes made won't be staged, and thus won't be committed (plus you might want to view those changes before re-committing).

You are now equipped to write your own custom pre-commit hooks. But there is an easier way! In the next section, we'll discuss the pre-commit framework, which can make managing pre-commit scripts much easier.

The pre-commit framework (pre-commit scripts made easy)

Pre-commit

The pre-commit framework bills itself as "A framework for managing and maintaining multi-language pre-commit hooks." Under the hood, it runs on python, but you can use the framework on any project, regardless of your project's primary language. Once installed, you're going to add a pre-commit configuration file to your project root named .pre-commit-config.yaml. In that config file, you will specify which scripts pre-commit will run when your pre-commit hook is triggered by a git commit command. Additionally, you can run the pre-commit scripts any time outside of a git commit call -- more on that later. The configuration file will look something like this (example taken from the pre-commit documentation):

Example .pre-commit-config.yaml file:

repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v2.3.0
    hooks:
    -   id: check-yaml
    -   id: end-of-file-fixer
    -   id: trailing-whitespace
-   repo: https://github.com/psf/black
    rev: 19.3b0
    hooks:
    -   id: black

The above file specifies that 4 scripts will run:

  • check-yaml (lints yaml file syntax)
  • end-of-file-fixer (makes sure files end in a newline and only one newline)
  • trailing-whitespace (trims trailing whitespace)
  • black (checks and fixes python code style)

The first 3 scripts were pulled from the repository https://github.com/pre-commit/pre-commit-hooks and the last script was pulled from the repository https://github.com/psf/black. For each repository, you must specify a rev (revision) which makes sure the script behaves according to that revision instead of always updating to the latest. Pre-commit scripts from these remote repositories can be written in any language. All of their requirements are specified in their repository. The pre-commit framework will read those requirements and build an appropriate environment to run that script. This might mean pre-commit will install a specific python version for an isolated environment to run black, or it might mean pre-commit will install a specific npm package in an isolated environment to run a node script. This is great for you, the developer since you don't have to prepare any of these special environments yourself, and it means you can use any script that is useful to you, regardless of the language it is written in.

Once you have the pre-commit framework installed and your .pre-commit-config.yaml file is ready, run pre-commit install to install/set up the hooks specified in your configuration file. At this point, pre-commit is ready to go and will automatically run when git commit is called. For most hooks, this means running specifically against files updated by your git commit. You can also run pre-commit run --all-files at any time to run your pre-commit hooks against all files in your repository. I highly suggest running this command immediately after any pre-commit install to fix all of your files according to your installed hooks.

Finding supported pre-commit hooks and rolling your own hooks

Where can you find pre-commit-framework-ready repositories/hooks to use in your .pre-commit-config.yaml file? Check out this list of supported hooks maintained by pre-commit. There are a lot of repositories with hooks compatible with the pre-commit framework. Browse the list to see if your favorite tool has a supported hook. When you click on a repository, read through its documentation on its pre-commit hook. It will likely tell you exactly how to format its section of the .pre-commit-config.yaml file.

What if you can't find a supported pre-commit hook for your lint tool-of-choice or what if you want to run a project-specific custom script during pre-commit? Fear not! pre-commit allows for this. This use case is documented in "Repository local hooks". To summarize, a local hook must define id, name, language, entry, and files/types. I found that it is simplest to define id and name as the same thing. files or types refer to either the specific files or the type of files to check for and run the script against. I also found that the easiest way to get custom hooks to work is to set the language to script with an entry pointing at a local script (./your/script/location) relative to your project root. In this case, no special environment will be created by pre-commit install, so you will need to have your local environment already appropriately set up to run the script (for instance have the correct version and packages of python, npm, etc, installed on your local system). Note that pre-commit will run your custom script against every file matched by files/types individually, so design your script accordingly.

Here is an example configuration with a few local hooks, taken directly from pre-commit's documentation on this subject:

-   repo: local
    hooks:
    -   id: pylint
        name: pylint
        entry: pylint
        language: system
        types: [python]
    -   id: check-x
        name: Check X
        entry: ./bin/check-x.sh
        language: script
        files: \.x$
    -   id: scss-lint
        name: scss-lint
        entry: scss-lint
        language: ruby
        language_version: 2.1.5
        types: [scss]
        additional_dependencies: ['scss_lint:0.52.0']

Conclusions

In this post, first, we answered the question "What is a git pre-commit hook?". We touched on what git hooks are in general and how the pre-commit hook fits into other hooks. We discussed how the pre-commit hook is triggered, what happens when it is triggered, and why pre-commit hooks are awesome. Then we discussed "how to write a git pre-commit hook (the hard way)". We talked about where pre-commit hooks live, and how to write them with examples. Next, we talked about "the pre-commit framework (pre-commit scripts made easy)". We discussed why the pre-commit framework is awesome and easy to use, and we saw an example of how to set up some hooks using the framework. Finally, we discussed "where to find supported pre-commit hooks and rolling your own hooks". We discussed how to get supported pre-commit hooks from remote repositories, and in cases where that's not possible, we discussed how to add your own scripts and custom hooks to your .pre-commit-config.yaml file.

I hope you enjoyed this introduction to pre-commit hooks, and I hope it helps you get started using pre-commit hooks in your code. If you were already familiar with pre-commit hooks but hadn't heard about the pre-commit framework, I hope you're excited to give the framework a try. Happy coding!


About the author


Theodore Williams

Hi, my name is Teddy Williams. I'm a software developer with a special love for python programming. 🐍👨‍💻 I have a wide range of programming interests including web development, hobby video game development, IoT, data science and just writing scripts to automate everyday boring tasks. I'd love it if you check out some of my other posts or take a look at my portfolio! :)

Thanks for reading this post! 💚 If you like the post, let me know by hitting the icon below, and if you have any questions or comments I'd love to hear them in the comments section. Thanks, and happy coding! 🎉

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Rob | October 19, 2022 01:22 PM

Thanks a lot, very informative and well written!

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User avatar not found verdantfox | October 19, 2022 02:40 PM

Thanks for the positive feedback, Rob. Glad it was helpful!