git commitmsg hook: Idempotently add ticket number in the footer part of a commit message

commit-msg.sh

#!/bin/bash
#
# DESCRIPTION:
#   Add the ticket identifier found in the branch name in the footer part of
#   the commit message. This action is idempotent (meaning if the ticket is already
#   in the message, it will not be added again).
#   Please note that this apply to «git commit» and its variations
#   (most notably «--amend»). Il will not apply to «reword» actions during an
#   interactive rebase, for instance.
#
# REQUIREMENTS:
#   - The branch is presumed to follow a gitflow style («type/ticket-id-and-other-things»)
#   - The ticket identifier is presumed to be of the form: uppercase letters
#     followed by a dash, and then a number (e.g ABC-1234, F-9, JIRA-3823787483)
#
# HOW TO USE:
#   - Copy this file in any .git/hooks directory you want it to be applied
#   - Commit away !
#
# EXAMPLES:
# Here are some branch samples and the computed ticket identifier that will be added
# in the commit message footer:
# 	« foo/bar-1 » -> bar-1
# 	« feature/ABC-1234-rest-of-the-branch-name » -> ABC-1234
# 	« bugfix/XYZ-456 » -> XYZ-456
# 	« ABC-1234 » -> <nothing added>
# 	« my-awesome-branch » -> <nothing added>
TICKET=$(git symbolic-ref HEAD | rev | cut -d/ -f1 | rev | grep -o -E "[A-Z]+-[0-9]+")
if [[ -n "${TICKET}" ]]; then
	grep -qe "^${TICKET}$" ${1}
	SHOULD_ADD_TICKET=$?
	if [[ ${SHOULD_ADD_TICKET} != 0 ]]; then
		echo -e "\n${TICKET}" >> ${1}
	fi
fi