pre-commit

At Yelp we rely heavily on pre-commit hooks to find and fix common issues before changes are submitted for code review. We run our hooks before every commit to automatically point out issues like missing semicolons, whitespace problems, and testing statements in code. Automatically fixing these issues before posting code reviews allows our code reviewer to pay attention to architecture of a change and not worry about trivial errors.

As we created more libraries and projects we recognized that sharing our pre commit hooks across projects is painful. We copied and pasted bash scripts from project to project. We also had to manually change the hooks to work for different project structures.

We believe that you should always use the best industry standard linters. Some of the best linters are written in languages that you do not use in your project or have installed on your machine. For example scss-lint is a linter for SCSS written in ruby. If you're writing a project in node you should be able to use scss-lint as a pre-commit hook without adding a Gemfile to your project or understanding how to get scss-lint installed.

We built pre-commit to solve our hook issues. pre-commit is a multi-language package manager for pre-commit hooks. You specify a list of hooks you want and pre-commit manages the installation and execution of any hook written in any language before every commit. pre-commit is specifically designed to not require root access; if one of your developers doesn't have node installed but modifies a javascript file, pre-commit automatically handles downloading and building node to run jshint without root.

Installation

Before you can run hooks, you need to have the pre-commit package manager installed.

Using pip:

pip install pre-commit

Non Administrative Installation:

curl http://pre-commit.github.io/local-install.py | python

System Level Install:

sudo curl https://bootstrap.pypa.io/get-pip.py | python - pre-commit

In a Python Project, add the following to your requirements.txt (or requirements-dev.txt):

pre-commit

Adding pre-commit Plugins To Your Project

Once you have pre-commit installed, adding pre-commit plugins to your project is done with the .pre-commit-config.yaml configuration file.

Add a file called .pre-commit-config.yaml to the root of your project. The pre-commit config file describes:

repo, sha - where to get plugins (git repos).
id - What plugins from the repo you want to use.
language_version - (optional) Override the default language version for the hook. See Advanced Features: "Overriding Language Version"
files - (optional) Override the default pattern for files to run on.
exclude - (optional) File exclude pattern.
args - (optional) additional parameters to pass to the hook.

For example:

-   repo: git://github.com/pre-commit/pre-commit-hooks
    sha: 82344a4055f4e103afdc31e98a46de679fe55385
    hooks:
    -   id: trailing-whitespace

This configuration says to download the pre-commit-hooks project and run it's trailing-whitespace hook.

Usage

run pre-commit install to install pre-commit into your git hooks. pre-commit will now run on every commit. Everytime you clone a project using pre-commit running install should always be the first thing you do.

If you want to manually run all pre-commit hooks on a repository, run pre-commit run --all-files. To run individual hooks use pre-commit run <hook_id>.

The first time pre-commit runs on a file it will automatically download, install, and run the hook. Note that running a hook for the first time may be slow. For example: If the machine does not have node installed, pre-commit will download and build a copy of node.

Creating New Hooks

pre-commit currently supports hooks written in JavaScript (node), Python, Ruby and system installed scripts. As long as your git repo is an installable package (gem, npm, pypi, etc) or exposes a executable, it can be used with pre-commit. Each git repo can support as many languages/hooks as you want.

An executable must satisfy the following things:

Returncode of hook must be different between success / failures (Usually 0 for success, nonzero for failure)
It must take filenames

A git repo containing pre-commit plugins must contain a hooks.yaml file that tells pre-commit:

id - The id of the hook - used in pre-commit-config.yaml
name - The name of the hook - shown during hook execution
entry - The entry point - The executable to run
files - The pattern of files to run on.
language - The language of the hook - tells pre-commit how to install the hook
description - (optional) The description of the hook
language_version - (optional) See advanced features "Overriding Language Version"
expected_return_value - (optional) Defaults to 0

For example:

-   id: trailing-whitespace
    name: Trim Trailing Whitespace
    description: This hook trims trailing whitespace.
    entry: trailing-whitespace-fixer
    language: python
    files: \.(js|rb|md|py|sh|txt|yaml|yml)$

Popular Hooks

JSHint:

-   repo: git://github.com/pre-commit/mirrors-jshint
    sha: 8e7fa9caad6f7b2aae8d2c7b64f457611416192b
    hooks:
    -   id: jshint

SCSS-Lint:

-   repo: git://github.com/pre-commit/mirrors-scss-lint
    sha: d7266131da322d6d76a18d6a3659f21025d9ea11
    hooks:
    -   id: scss-lint

Ruby-Lint:

-   repo: git://github.com/pre-commit/mirrors-ruby-lint
    sha: f4b537e0bf868fc6baefcb61288a12b35aac2157
    hooks:
    -   id: ruby-lint

Whitespace Fixers:

-   repo: git://github.com/pre-commit/pre-commit-hooks
    sha: a751eb58f91d8fa70e8b87c9c95777c5a743a932
    hooks:
    -   id: trailing-whitespace
    -   id: end-of-file-fixer

flake8:

-   repo: git://github.com/pre-commit/pre-commit-hooks
    sha: a751eb58f91d8fa70e8b87c9c95777c5a743a932
    hooks:
    -   id: flake8

pyflakes:

-   repo: git://github.com/pre-commit/pre-commit-hooks
    sha: a751eb58f91d8fa70e8b87c9c95777c5a743a932
    hooks:
    -   id: pyflakes

Advanced features

Temporarily Disabling Hooks

Not all hooks are perfect so sometimes you may need to skip execution of one or more hooks. pre-commit solves this by querying a SKIP environment variable. The SKIP environment variable is a comma separated list of hook ids. This allows you to skip a single hook instead of --no-verifying the entire commit

$ SKIP=flake8 git commit -m "foo"

pre-commit During Commits

Running hooks on unstaged changes can lead to both false-positives and false-negatives during committing. pre-commit only runs on the staged contents of files by temporarily saving the contents of your files at commit time and stashing the unstaged changes while running hooks.

pre-commit During Merges

The biggest gripe we've had in the past with pre-commit hooks was during merge conflict resolution. When working on very large projects a merge often results in hundreds of committed files. I shouldn't need to run hooks on all of these files that I didn't even touch! This often led to running commit with --no-verify and allowed introduction of real bugs that hooks could have caught through merge-conflict resolution. pre-commit solves this by only running hooks on files that conflict or were manually edited during conflict resolution.

Passing Arguments to Hooks

Sometimes hooks require arguments to run correctly. You can pass static arguments by specifying the args property in your .pre-commit-config.yaml as follows:

-   repo: git://github.com/pre-commit/pre-commit-hooks
    sha: a751eb58f91d8fa70e8b87c9c95777c5a743a932
    hooks:
    -   id: flake8
        args: [--max-line-length=131]

This will pass --max-line-length=131 to flake8

Overriding Language Version

Sometimes you only want to run the hooks on a specific version of the language. For each language, they default to using the system installed language (So for example if I'm running python2.6 and a hook specifies python, pre-commit will run the hook using python2.6). Sometimes you don't want the default system installed version so you can override this on a per-hook basis by setting the language_version.

-   repo: git://github.com/pre-commit/mirrors-scss-lint
    sha: d7266131da322d6d76a18d6a3659f21025d9ea11
    hooks:
    -   id: scss-lint
        language_version: 1.9.3-p484

This tells pre-commit to use 1.9.3-p484 to run the scss-lint hook.

Valid values for specific languages are listed below:

python: Whatever system installed python interpreters you have. The value of this argument is passed as the -p to virtualenv
node: See https://github.com/ekalinin/nodeenv#advanced
ruby: See https://github.com/sstephenson/ruby-build/tree/master/share/ruby-build

Contributing

We're looking to grow the project and get more contributors especially to support more languages/versions. We'd also like to get the hooks.yaml files added to popular linters without maintaining forks / mirrors.

Feel free to submit Bug Reports, Pull Requests, and Feature Requests.

When submitting a pull request, please enable travis-ci for your fork.

Contributors

Anthony Sottile
Ken Struys