The Comprehensive Guide to `pre-commit`

pre-commit.md

The Comprehensive Guide to pre-commit

Table of Contents

1. Introduction: What is pre-commit?
2. Why Use pre-commit?
3. How pre-commit Works
4. Installing and Setting Up pre-commit
5. Configuring .pre-commit-config.yaml
6. Built-in and Community Hooks
7. How to Run and Use pre-commit
8. Popular Use Cases
9. Writing Custom Hooks
10. Advanced Configuration and Features
11. Integrating with CI/CD
12. Best Practices
13. Troubleshooting and Common Issues
14. Alternatives and Related Tools
15. Resources

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1. Introduction: What is pre-commit?

pre-commit is an open-source framework for managing and maintaining multi-language pre-commit hooks. In software development, Git hooks are scripts that run automatically on specific Git events (e.g., commit, push). pre-commit makes it easy to install and run hooks for code quality, security, style, and more—before code gets committed.

It is language-agnostic and supports everything from Python, JavaScript, Go, and Shell scripts, to Rust, Ruby, and more.

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2. Why Use pre-commit?

• Automate Code Quality: Catch formatting errors, linting issues, and security flaws before they reach your repo.
• Consistency Across Teams: Ensures everyone runs the same checks, reducing “works on my machine” problems.
• Developer Productivity: Automates repetitive checks, freeing up dev time.
• Multi-language Support: Works across codebases with multiple languages and ecosystems.
• Easy CI/CD Integration: Hooks can be run manually or in CI to ensure consistency.

---

3. How pre-commit Works

• You define a list of hooks (scripts) in a .pre-commit-config.yaml file.
• pre-commit installs the required environments for each hook (virtualenvs, Node, etc.).
• When you run git commit, pre-commit runs all the enabled hooks on the files you’re committing.
• If a hook fails, the commit is blocked.

---

4. Installing and Setting Up pre-commit

Installation

• Python users:

  pip install pre-commit

• Homebrew (macOS/Linux):

  brew install pre-commit

• Other: Install via conda, pipx, or download from GitHub.

Setting Up

1. Add to your project:

   pre-commit sample-config > .pre-commit-config.yaml

2. Install the Git hook scripts:

   pre-commit install

   This sets up .git/hooks/pre-commit to run pre-commit on every commit.

3. Run on all files (initial run):

   pre-commit run --all-files

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5. Configuring .pre-commit-config.yaml

This YAML file tells pre-commit which hooks to use and how to run them.

Example

repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.5.0  # Use the latest stable tag
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml

  - repo: https://github.com/psf/black
    rev: 24.3.0
    hooks:
      - id: black

Key Fields:

• repo: URL or path to the repo with hooks
• rev: Version tag or commit hash
• hooks: List of hook definitions (id, args, files, etc.)

Hook Options

• id: The hook’s unique name.
• args: Custom arguments for the hook.
• files: Regex for files to include.
• exclude: Regex for files to exclude.
• language_version: Set the language version (e.g., python3.11).

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6. Built-in and Community Hooks

There’s a huge list of pre-commit hooks:

• Official: pre-commit/pre-commit-hooks
  • E.g., trailing whitespace, debug statements, end-of-file fixes.
• Language Formatters:
  • Black (Python)
  • isort (import sorting)
  • Prettier (JS/TS/HTML/CSS)
• Linters:
  • flake8, eslint, shellcheck
• Security:
  • bandit, detect-secrets
• Others: License checks, secret detection, file size checks, etc.

---

7. How to Run and Use pre-commit

• On every commit: Automatically runs via the installed hook.

• Manually on staged files:

  pre-commit run

• On all files:

  pre-commit run --all-files

• On a specific hook:

  pre-commit run black --all-files

• Update all hooks:

  pre-commit autoupdate

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8. Popular Use Cases

• Enforce code style (formatters, linters)
• Check for secrets (AWS keys, passwords)
• Fix whitespace, newlines, EOF issues
• Enforce commit message format
• Check for large files or merge conflicts
• Security vulnerability checks

---

9. Writing Custom Hooks

You can write your own hooks in any language.

Local Hook Example (in .pre-commit-config.yaml):

repos:
  - repo: local
    hooks:
      - id: my-linter
        name: My Custom Linter
        entry: ./scripts/my_linter.sh
        language: script
        files: \.py$

Local Python Hook Example

      - id: check-todo
        name: Check for TODOs
        entry: python scripts/check_todo.py
        language: python
        files: .*

Hook script must accept a list of filenames as arguments. Return 0 for pass, non-zero for fail.

---

10. Advanced Configuration and Features

• Staged-only or all files: Use always_run, pass_filenames, and stages options.
• Multiple stages: Use stages: [commit, push, manual].
• Parallel execution: By default, hooks run in parallel.
• Failing gracefully: Hooks can be set as required or optional.
• Excluding files: Use the exclude regex for large/irrelevant files.
• Languages: Hooks support many languages (python, node, golang, docker, system, etc.).
• Hook ordering: Hooks run in the order they’re listed.

---

11. Integrating with CI/CD

• Why: Ensures code quality checks run in CI, not just on developer machines.

• How:

  • In your CI job, install pre-commit and run:

    pre-commit run --all-files --show-diff-on-failure

  • For some CIs (e.g., GitHub Actions), there are pre-built actions: pre-commit/action

• Fail the build if hooks fail.

---

12. Best Practices

• Pin hook versions: Always use specific rev values, never master/main.

• Run pre-commit run --all-files after adding new hooks.

• Add .pre-commit-config.yaml to source control.

• Document setup in CONTRIBUTING.md.

• Keep hooks fast—slow hooks slow down developer flow.

• Autoupdate hooks regularly:

  pre-commit autoupdate

• Review hook changes on update (some hooks change behavior across versions).

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13. Troubleshooting and Common Issues

• Hook not running? Did you run pre-commit install?

• Hook fails but works manually? Check environment—pre-commit uses isolated environments for each hook.

• Can’t find a hook or dependency? Sometimes you need to manually install system packages (e.g., libyaml-dev).

• Files modified by hooks? By default, if a hook modifies files (e.g., a formatter), you must git add them and recommit.

• Want to skip pre-commit just once?

  git commit --no-verify

• Speed up repeated runs: Use pre-commit run --files ... or only staged files.

• Cross-platform issues: Some hooks may have trouble on Windows—test in your team’s environments.

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14. Alternatives and Related Tools

• Husky (JavaScript-centric, popular in frontend projects)
• lefthook (Go, cross-platform, fast)
• Overcommit (Ruby)
• git hooks (native, but more manual)

pre-commit is the most widely used and language-agnostic solution.

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15. Resources

• pre-commit.com documentation
• Official hook index
• Writing custom hooks
• Awesome pre-commit
• Sample .pre-commit-config.yaml files

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TL;DR / Executive Summary

• pre-commit is a framework for managing and running Git hooks.
• It is configured via .pre-commit-config.yaml and supports many languages.
• You install it, configure hooks, and let it run checks/formatters automatically on commit.
• It boosts code quality, developer experience, and consistency.

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Have a specific use case or problem with pre-commit?

Let me know—I'm happy to go even deeper into any area, write advanced examples, or help troubleshoot!

https://git-scm.com/book/ms/v2/Customizing-Git-Git-Hooks

https://pre-commit.com/