Git Hooks — Automate Quality Control

Module 05 45 min

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Section Objectives

• Understand what Git hooks are and how they work
• Configure pre-commit to validate code quality
• Configure commit-msg to enforce commit message format
• Share hooks with the team via Husky or pre-commit

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What are Git Hooks?

Git hooks are scripts that run automatically at specific points in the Git workflow:

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Hook Types

Hook	When	Common Use
pre-commit	Before commit	Linting, formatting, fast tests
commit-msg	After entering message	Validate message format
post-commit	After commit	Notifications, logging
pre-push	Before push	Full test suite
post-merge	After merge	Install dependencies
post-checkout	After switch	Install dependencies
pre-rebase	Before rebase	Safety checks

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Manual Hook Configuration

Hooks are stored in .git/hooks/:

# View available hook examples
ls .git/hooks/
# pre-commit.sample
# commit-msg.sample
# pre-push.sample
# ...

pre-commit Hook

Create the file .git/hooks/pre-commit with this content:

#!/bin/bash

echo "Running pre-commit checks..."

# 1. Check for Python syntax errors
if command -v python3 &>/dev/null; then
    python3 -m py_compile *.py 2>/dev/null
    if [ $? -ne 0 ]; then
        echo "Python syntax error detected!"
        exit 1
    fi
fi

# 2. Check for debug artifacts
if git diff --cached | grep -E "console\.log|debugger|pdb\.set_trace|TODO REMOVE"; then
    echo "Debug artifacts found in staged files!"
    echo "Remove console.log, debugger, pdb.set_trace before committing."
    exit 1
fi

# 3. Check for .env files accidentally staged
if git diff --cached --name-only | grep -E "\.env$|\.env\..+$|credentials"; then
    echo "Sensitive file detected in staging area!"
    echo "Remove .env or credential files before committing."
    exit 1
fi

echo "All pre-commit checks passed!"
exit 0

chmod +x .git/hooks/pre-commit

commit-msg Hook

Enforces Conventional Commits format. Create .git/hooks/commit-msg:

#!/bin/bash

COMMIT_MSG_FILE=$1
COMMIT_MSG=$(cat "$COMMIT_MSG_FILE")

# Conventional Commits pattern
PATTERN="^(feat|fix|docs|style|refactor|test|chore|perf|ci|revert|build)(\(.+\))?: .{1,100}"

if ! echo "$COMMIT_MSG" | grep -qE "$PATTERN"; then
    echo "Invalid commit message format!"
    echo ""
    echo "Expected format: type(scope): description"
    echo "Types: feat, fix, docs, style, refactor, test, chore, perf, ci, revert, build"
    echo ""
    echo "Examples:"
    echo "  feat(auth): add JWT authentication"
    echo "  fix(api): handle 404 for missing users"
    echo "  docs: update README"
    echo ""
    echo "Your message: '$COMMIT_MSG'"
    exit 1
fi

echo "Commit message format OK"
exit 0

chmod +x .git/hooks/commit-msg

pre-push Hook

Create .git/hooks/pre-push:

#!/bin/bash

echo "Running test suite before push..."

if command -v python3 &>/dev/null && [ -f "test_app.py" ]; then
    python3 test_app.py
    if [ $? -ne 0 ]; then
        echo "Tests failed! Push aborted."
        exit 1
    fi
fi

echo "All tests passed! Pushing..."
exit 0

chmod +x .git/hooks/pre-push

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Sharing Hooks with the Team

Hooks in .git/hooks/ are not committed (.git/ is not tracked). To share hooks:

Option 1: Python pre-commit Tool

pip install pre-commit

Create .pre-commit-config.yaml:

repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.5.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-json
      - id: check-merge-conflict
      - id: detect-private-key
      - id: no-commit-to-branch
        args: ['--branch', 'main']

  - repo: https://github.com/psf/black
    rev: 24.1.0
    hooks:
      - id: black

  - repo: https://github.com/pycqa/flake8
    rev: 7.0.0
    hooks:
      - id: flake8
        args: ['--max-line-length=100']

  - repo: https://github.com/compilerla/conventional-pre-commit
    rev: v3.0.0
    hooks:
      - id: conventional-pre-commit
        stages: [commit-msg]

pre-commit install
pre-commit install --hook-type commit-msg
pre-commit run --all-files

Option 2: Husky (JavaScript/Node.js projects)

npm install --save-dev husky
npx husky init

echo "npm run lint" > .husky/pre-commit
echo "npm test" > .husky/pre-push

Create .husky/commit-msg:

npx --no -- commitlint --edit $1

npm install --save-dev @commitlint/cli @commitlint/config-conventional
echo "export default { extends: ['@commitlint/config-conventional'] };" > commitlint.config.js

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Bypassing Hooks (for Emergencies)

# Skip the pre-commit hook (use with caution!)
git commit --no-verify -m "emergency: hotfix prod outage"

# Skip the pre-push hook
git push --no-verify

Use with caution

--no-verify bypasses all quality checks. Only use it for genuine production emergencies, not as a shortcut to avoid fixing issues. Document the reason in your commit message.

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Summary

Hook	Common Use
pre-commit	Linting, formatting, detect secrets
commit-msg	Validate Conventional Commits
pre-push	Run full test suite
post-merge	Run npm install after merge

Tool	Ecosystem	Description
pre-commit	Universal	YAML config, Python-based
husky	Node.js	Hooks via npm scripts
lefthook	Universal	Fast, Go-based alternative

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Next Steps

• Hands-on Lab TP6 — Hooks, tags, cherry-pick in practice