Commit Msg

The commit-msg hook is a client-side hook that runs after you've committed changes to your repository. It's typically used to validate or modify the commit message before it's recorded in the Git history.

The mistake it catches

git commit without -m opens an editor, and the easiest thing to type into an empty editor is the wrong thing. Every repository that enforces commit message rules does it with this hook.

mkdir -p .git/hooks
cat > .git/hooks/commit-msg <<'EOF'
#!/bin/sh
pattern='^(feat|fix|docs|chore|refactor|test): .{5,}'
if ! head -1 "$1" | grep -Eq "$pattern"; then
  echo "Commit message must start with feat:, fix:, docs:, chore:, refactor: or test:"
  echo "and be at least 5 characters long."
  exit 1
fi
EOF
chmod +x .git/hooks/commit-msg

The first line of the message is what gets checked — a commit subject is the part that ends up in git log --oneline and in release notes, so that is the part worth constraining.

Other things it does

Cap the line length, which keeps git log readable:

if [ "$(head -1 "$1" | wc -c)" -gt 72 ]; then
  echo "Keep the first line under 72 characters."
  exit 1
fi

Reject merge commits on a branch that should stay linear:

if head -1 "$1" | grep -qE '^Merge (branch|remote)'; then
  echo "Merge commits are not allowed on this branch."
  exit 1
fi

A non-zero exit aborts the commit, leaving your changes staged and nothing written.

Where hooks live

.git/hooks is per-clone and is not committed, so everyone on a team sets up their own. To share the rules, keep the script in the repository — under scripts/hooks/ or .githooks/ — and add a line to the contributing guide telling people to run an install script that points core.hooksPath at it:

git config core.hooksPath .githooks

That version travels with the project. See Pre-commit Hooks for the hook that runs before the commit rather than after.

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