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.

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:

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

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

pre-commit

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.

run `pre-commit install` to install pre-commit into your git hooks. pre-commit

will now run on every commit. Every time you clone a project using pre-commit

running `pre-commit 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.

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 an 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)$

- `node`

- `python`

- `ruby`

- `pcre` - "Perl Compatible Regular Expression" Specify the regex as the `entry`

- `script` - A script existing inside of a repository

- `system` - Executables available at the system level

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

By default, if you have existing hooks `pre-commit install` will install in

a migration mode which runs both your existing hooks and hooks for pre-commit.

To disable this behavior, simply pass `-f` / `--overwrite` to the `install`

command. If you decide not to use pre-commit, `pre-commit uninstall` will

restore your hooks to the state prior to installation.

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-verify`ing

the entire commit

$ SKIP=flake8 git commit -m "foo"

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.

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.

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`

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

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.

- Anthony Sottile

- Ken Struys