Today I learned that Git’s pre-commit hook is executed/triggered once per commit, not once per file included in the commit.

If you want to restrict its behavior to certain files, you need to implement that logic inside your pre-commit script (for example, by checking git diff --cached --name-only to see which files are staged).

✼ Example 1: Strip YAML front matter from README.md

#!/bin/sh

if git diff --cached --name-only | grep -q '^README.md$'; then
    if head -n1 README.md | grep -q '^---$'; then
        sed '1,/^---$/d' README.md > README.md.tmp && mv README.md.tmp README.md
        git add README.md
    fi
fi

✼ Example 2: Run a Python script to update README.md

#!/bin/sh

if git diff --cached --name-only | grep -q '^README.md$'; then
    python3 .scripts/update_readme.py
    git add README.md
fi

By adding file-based checks, you can avoid unnecessary work and speed up commit performance, especially in large projects.