Blocking Bad Commits with a pre-commit Hook

A hook is just an executable file with a name Git recognizes, sitting in .git/hooks. No config file, no registration — the presence and the exit code are the entire mechanism.

1) A pre-commit hook that inspects staged content

$ cat .git/hooks/pre-commit

#!/bin/sh
if git diff --cached --name-only | xargs grep -l "console.log" 2>/dev/null; then
  echo "pre-commit: found console.log in a staged file, remove it before committing" >&2
  exit 1
fi
exit 0

git diff --cached --name-only lists staged files — exactly what’s about to be committed, not the whole working tree. The hook greps them for a forbidden string and exits 1 if it finds one.

2) A commit that trips the hook

$ printf 'export function debugAdd(a, b) {\n  console.log("adding", a, b);\n  return a + b;\n}\n' > debug.mjs
$ git add debug.mjs
$ git commit -m "Add debugAdd helper"

debug.mjs
pre-commit: found console.log in a staged file, remove it before committing

Git ran the hook before doing anything else, saw exit code 1, and aborted — no commit object was created. git log --oneline -1 still shows the previous commit; debug.mjs is still staged, waiting to be fixed.

3) Fix the file, and the same commit succeeds

$ printf 'export function debugAdd(a, b) {\n  return a + b;\n}\n' > debug.mjs
$ git add debug.mjs
$ git commit -m "Add debugAdd helper"

[main b31842c] Add debugAdd helper
 1 file changed, 3 insertions(+)
 create mode 100644 debug.mjs

Same hook, same commit message, different staged content — this time grep finds nothing, the hook exits 0, and Git proceeds normally.

Practice

1. Make the hook executable and confirm the difference: chmod -x .git/hooks/pre-commit, attempt a commit with console.log staged, and observe Git silently skips a non-executable hook instead of blocking the commit.

   Solution

   Git only runs hook files that are both present and executable. A non-executable pre-commit is treated as if it doesn’t exist — no warning, no error, the commit just goes through.

2. Add a second check to the hook (for example, blocking a staged file over some size) using the same git diff --cached pattern, and confirm both checks can independently block a commit.

3. Bypass the hook deliberately with git commit --no-verify on a commit that would normally be blocked, and confirm it succeeds — useful to know, and worth explaining to a team why it should be used sparingly.

4. Write a commit-msg hook (receives the commit message file path as $1) that rejects a commit message under 10 characters, and test it with a real short message.

5. Clone the repository into a fresh directory and confirm .git/hooks/pre-commit does not exist in the clone — hooks are local, not part of the repository’s tracked history.