Getting to know pre-commit

Getting your whole team to use the same set of client-side hook scripts when working with Git is not a simple task. That’s why I was really grateful when I stumbled upon the pre-commit project by yelp. This project offers some tools for managing your pre-commit hooks and facilitates sharing them between multiple projects and developers.

Setup

The core idea is that you have all your hooks configured within the project’s root directory in a file called .pre-commit-config.yaml and then simply execute pre-commit install once to have pre-commit inject itself into .git/hooks/pre-commit. From now on, whenever you are about to make a commit, pre-commit will launch and execute your hook scripts if appropriate.

The scripts themselves are hosted in some other repository that you specify in that config file. To illustrate this, here a short example:

-   repo: ssh://git@our-repo-server.com/precommit-hooks.git
    sha: 1bf3eaef56059e168aac55393a7494cac727ebcd
    hooks:
        - id: flow-branches
        - id: grunt-test

This is the configuration I’m currently using for one of our work projects (as an experiment so far). It specifies that it should execute the hookscripts flow-branches and grunt-test from our precommit-hooks repository. The scripts themselves are versioned with the commit ID, so updates to the scripts repository don’t break anything.

Other team members then just have to install pre-commit and execute its install sub-command and have all the configured hook scripts enabled.

$ pip install pre-commit
$ cd /path/to/project
$ pre-commit install

This repository referred to in the configuration file has to contain a hooks.yaml file where all the exposed hook scripts are documented. The file in this example would look something like this:

-   id: flow-branches
    name: Flow Branches
    language: python
    entry: flow-branches
    files: .*
-   id: grunt-test
    name: Grunt Test
    language: python
    entry: grunt-test
    files: .*

The id signals what the script should be called in your project’s .pre-commit-config.yaml while the entry property points to the executable script itself.

Multi-language support

As this example indicates, you can write your hook scripts in various languages. For Python, for instance, this will create a virtualenv and install all the dependencies specified in the setup.py of our hooks repository for you. This is for me the single best feature of this project. You don’t have to tell your team-mates what other stuff they have to install before they can use your new fancy hook or explain virtualenvs to them if they are not Python coders but just tell them once to install pre-commit and run pre-commit install within the repository. All the rest is done by the tool itself.

If you want to write your scripts using Node.JS you can do so as well and it will also handle all the dependencies mentioned in a package.json file for you.

Only run when necessary

The example above also include a field files which is also worth mentioning. With this you can specify that a hook script should only be executed if the commit-to-be-made contains files with a specific name pattern. This way for instance a hook for running jshint on JavaScript files would not be executed if the commit affect any JavaScript files.

You can override this pattern within the .pre-commit-config.yaml if you want to.

The downsides

Sadly, at this point pre-commit has two major downsides which mean that I’m still looking for alternative implementations:

  1. As the name implies it is limited to handling pre-commit hooks. But what about when you want to also check the commit message? For that you’d have to install a script into the commit-msg hook.

  2. Internally, pre-commit using some shell utilities like xargs which means it doesn’t support Windows. There is ticket for that but that’s about it.

I’ve already looked at overcommit by Causes but this doesn’t seem to handle the sharing of scripts between projects that well and also is limited to operate on Ruby hook scripts. Especially the latter makes adapting it harder because our current language stack doesn’t include Ruby (except for Compass/SASS). From what I’ve seen it also doesn’t do anything about managing dependencies of hook scripts.

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