provide an example for `commit-msg` hook

Hello and thank you for creating and maintaining this project.

I’ve started using it very recently and I am having issues with setting up a hook that would allow me and my colleagues to verify Git commit messages (with regex).

Reading the documentation at pre-commit.com didn’t really help; I was able to install the hook and it’s contents is below:

#!/usr/bin/env python3.7
# File generated by pre-commit: https://pre-commit.com
# ID: 138fd403232d2ddd5efb44317e38bf03
import os
import sys

# we try our best, but the shebang of this script is difficult to determine:
# - macos doesn't ship with python3
# - windows executables are almost always `python.exe`
# therefore we continue to support python2 for this small script
if sys.version_info < (3, 3):
    from distutils.spawn import find_executable as which
else:
    from shutil import which

# work around https://github.com/Homebrew/homebrew-core/issues/30445
os.environ.pop('__PYVENV_LAUNCHER__', None)

# start templated
INSTALL_PYTHON = '/Users/ivica/PATH_TO/venv/bin/python3.7'
ARGS = ['hook-impl', '--config=.pre-commit-config.yaml', '--hook-type=commit-msg']
# end templated
ARGS.extend(('--hook-dir', os.path.realpath(os.path.dirname(__file__))))
ARGS.append('--')
ARGS.extend(sys.argv[1:])

DNE = '`pre-commit` not found.  Did you forget to activate your virtualenv?'
if os.access(INSTALL_PYTHON, os.X_OK):
    CMD = [INSTALL_PYTHON, '-mpre_commit']
elif which('pre-commit'):
    CMD = ['pre-commit']
else:
    raise SystemExit(DNE)

CMD.extend(ARGS)
if sys.platform == 'win32':  # https://bugs.python.org/issue19124
    import subprocess

    if sys.version_info < (3, 7):  # https://bugs.python.org/issue25942
        raise SystemExit(subprocess.Popen(CMD).wait())
    else:
        raise SystemExit(subprocess.call(CMD))
else:
    os.execvp(CMD[0], CMD)

What i’m missing from the docs is how to use this hook. As I said before, I’d like to verify that commit messages are in a correct format with some regex rules.

There is an existing hook for black configured like so

repos:
  - repo: https://github.com/psf/black
    rev: 20.8b1
    hooks:
      - id: black
        language_version: python3

and it is working well.

I am probably missing or misunderstanding something - i’m just not sure what.

[MarcoGorelli]

Good to see --negate come in handy 😄

          - id: validate-commit-msg
            name: validate-commit-msg
            language: pygrep
            entry: ^(CDD|CDTS)-[0-9]+\s-\s
            stages: [commit-msg]
            args: [--negate]

[asottile]

cool, you can do it even easier with pygrep – then you don’t need a script at all!