How to Use Git Pre-commit Hook: Simple Guide

A pre-commit hook is a script that Git runs automatically before a commit is saved. To use it, create an executable script named pre-commit inside the .git/hooks/ folder of your repository. This script can check code style, run tests, or block commits if conditions fail.

Syntax

The pre-commit hook is a script placed in the .git/hooks/pre-commit file. It must be executable and can be written in any scripting language like Bash or Python.

When you run git commit, Git executes this script before completing the commit. If the script exits with a non-zero status, the commit is stopped.

• .git/hooks/pre-commit: The hook script file
• Executable permission: The script must be executable (chmod +x)
• Exit status: 0 means success, any other value blocks the commit

sh

#!/bin/sh
# This is the pre-commit hook script

# Commands to run before commit

# Exit 0 to allow commit
exit 0

Example

This example shows a simple pre-commit hook that checks if any Python files have syntax errors before allowing the commit.

sh

#!/bin/sh

# Check Python syntax for all staged .py files
files=$(git diff --cached --name-only --diff-filter=ACM | grep '\.py$')

if [ -z "$files" ]; then
  exit 0
fi

for file in $files; do
  python3 -m py_compile "$file"
  if [ $? -ne 0 ]; then
    echo "Syntax error in $file. Commit aborted."
    exit 1
  fi
 done

exit 0

Output

Syntax error in script.py. Commit aborted.

Common Pitfalls

• Not making the pre-commit script executable will prevent it from running.
• Using commands that require unstaged changes can cause false negatives; always check staged files.
• Forgetting to exit with 0 on success or non-zero on failure will cause unexpected commit behavior.
• Long-running scripts can slow down commits and frustrate users.

sh

# Wrong: script not executable
# File created but no chmod +x

# Right: make script executable
chmod +x .git/hooks/pre-commit

Quick Reference

Step	Description
Create script	Add your script to .git/hooks/pre-commit
Make executable	Run chmod +x .git/hooks/pre-commit
Test script	Run git commit to trigger the hook
Exit codes	Use 0 to allow commit, non-zero to block
Check staged files	Use git diff --cached to inspect files

Key Takeaways

Place your pre-commit script in .git/hooks/pre-commit and make it executable.

Use exit code 0 to allow commits and non-zero to block them.

Check only staged files to avoid false errors.

Keep pre-commit scripts fast to avoid slowing down commits.

Test your hook by running git commit and observing behavior.