commit-msg git hook to add Sign-off-by trailer to every commit message automatically

commit-msg

#!/usr/bin/env bash

# Usage:
# 1. Download this file and save it as .git/hooks/commit-msg under current repo.
# 2. Run `chmod +x .git/hooks/commit-msg`

COMMIT_MSG_FILE=$1

# If the commit message is fixup! or squash! or Merge, exit without changing it.
grep -qE '^(fixup!|squash!|Merge)' "$COMMIT_MSG_FILE" && exit 0

# Add a Signed-off-by line if one does not already exist.
SIGN_OFF_BY_TRAILER=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
grep -qs "^$SIGN_OFF_BY_TRAILER" "$COMMIT_MSG_FILE" ||
    git interpret-trailers --in-place --trailer "$SIGN_OFF_BY_TRAILER" "$COMMIT_MSG_FILE"

# Detect and abort duplicate Signed-off-by lines.
test "" = "$(grep '^Signed-off-by: ' "$COMMIT_MSG_FILE" |
	 sort | uniq -c | sed -e '/^[ 	]*1[ 	]/d')" || {
	echo >&2 Duplicate Signed-off-by lines.
	exit 1
}