Taken from https://github.com/cockroachdb/cockroach/blob/8badea4c8e0d48216b207de4a2846d177f0c0e08/githooks/prepare-commit-msg

prepare-commit-msg

#!/usr/bin/env bash
#
# Prepare the commit message by adding a release note.

require_justification=0
set -euo pipefail

if [[ "${2-}" = "message" ]]; then
  # This git command is non-interactive so it will not filter out any comments
  # we add. There is nothing more for us to do.
  exit 0
fi

give_up() {
  echo "prepare-commit-msg: $@" >&2
  exit 0  # exit with successful status to allow the commit to proceed
}

if ! test -e "$1" -o -z "$1"; then
	give_up "$1: commit message file does not exist or is empty"
fi

# Git can be configured to use any character as the comment indicator. See the
# core.commentChar Git option. We can deduce what comment character is in effect
# by looking for text that we know will be preceded by the comment character.
if ! cchar=$(grep "^. Please enter the commit message for your changes." "$1" | head -c1); then
  give_up "unable to determine comment char"
fi

if ! tempfile=$(mktemp); then
  give_up "failed to create temporary file"
fi
trap "rm -f $tempfile" EXIT

sed_script=''

# If the setting 'cockroachdb.disable-commit-template' is set and
# true, all the template recommendations will be commented out as
# opposed to be filled in.
tchar=''
tdisable=$(git config --bool --get cockroachdb.disable-commit-template || echo false)
if test x"$tdisable" != xfalse; then
	tchar="$cchar "
fi

if [ "$require_justification" = 1 ]; then
  # Add an explicit "Release justification: None" if no release justification was specified.
  if ! grep -q '^Release justification' "$1"; then
  	sed_script+="/$cchar Please enter the commit message for your changes./i\\
\\
${tchar}Release justification:
;
"
  fi
fi

# Add a commit message template if the commit message is empty.
if ! grep -q -E -v "^$cchar|^$" "$1"; then
	sed_script+="/^$/i\\
${tchar}<pkg>: <short description - lowercase, no final period>\\
\\
${tchar}<what was there before: Previously, ...>\\
${tchar}<why it needed to change: This was inadequate because ...>\\
${tchar}<what you did about it: To address this, this patch ...>
;
"
	if test x"$tchar" = x; then
		sed_script+="/^$/i\\
$cchar Note: to disable this commit template, run: git config --global --add cockroachdb.disable-commit-template true
;
"
	fi
fi

# Add an explicit "Release note: None" if no release note was specified.
if ! grep -q '^Release note' "$1"; then
	sed_script+="/$cchar Please enter the commit message for your changes./i\\
$cchar Use 'Release note: None' if there is no user-visible change.\\
${tchar}Release note (<category, see below>): <what> <show> <why>\\

;
"
fi

# Inject commit message recommendations into the commit message help text.
sed_script+="/$cchar.*an empty message aborts the commit./a\\
$cchar\\
$cchar Commit message recommendation:\\
$cchar\\
$cchar     ---\\
$cchar     <pkg>: <short description>\\
"

if [ "$require_justification" = 1 ]; then
  sed_script+="$cchar\\
$cchar     Release justification: <release justification>\\
"
fi
sed_script+="$cchar\\
$cchar     <what was there before: Previously, ...>\\
$cchar     <why it needed to change: This was inadequate because ...>\\
$cchar     <what you did about it: To address this, this patch ...>\\
$cchar\\
$cchar     Release note (<category>): <what> <show> <why>\\
$cchar     ---\\
$cchar\\
$cchar Wrap long lines! 72 columns is best.\\
$cchar See also: https://wiki.crdb.io/wiki/spaces/CRDB/pages/73072807/Git+Commit+Messages\\
$cchar\\
"

if [ "$require_justification" = 1 ]; then
  sed_script+="$cchar\\
$cchar Categories for release justification:\\
$cchar     - non-production code changes\\
$cchar     - bug fixes and low-risk updates to new functionality\\
$cchar     - fixes for high-priority or high-severity bugs in existing functionality\\
$cchar     - low risk, high benefit changes to existing functionality\\
$cchar\\
"
fi

sed_script+="$cchar The release note must be present if your commit has user-facing\\
$cchar or backward-incompatible changes. Use 'Release note: None' otherwise.\\
$cchar\\
$cchar Things to keep in mind for release notes:\\
$cchar   - past tense (this was changed...) or present tense (now possible to...)\\
$cchar   - what has changed: narrow down the product area / feature\\
$cchar     Note: for bug fixes, indicate since when the bug was present\\
$cchar   - show what changed: how a user can see the change for themselves\\
$cchar     Note: for bug fixes, show the symptom(s) to recognize the bug\\
$cchar   - why the change: who does this impact, how and why should they care\\
$cchar\\
$cchar See also: https://wiki.crdb.io/wiki/spaces/CRDB/pages/73072807/Release+notes\\
$cchar\\
$cchar Example release notes:\\
$cchar\\
$cchar   Release note (sql change): The IMPLEMENT statement was extended\\
$cchar   to support the new STEP clause. This can be used to\\
$cchar   implement more gradually, as often required by teams of two\\
$cchar   or more.\\
$cchar\\
$cchar   Release note (bug fix): The system.replication_stats report no longer\\
$cchar   erroneously considers some ranges belonging to table partitions to be\\
$cchar   over-replicated. This bug was present since version 19.2.\\
$cchar\\
$cchar Categories for release notes:\\
$cchar   - cli change\\
$cchar   - ops change\\
$cchar   - sql change\\
$cchar   - api change\\
$cchar   - ui change\\
$cchar   - security update\\
$cchar   - general change (e.g., change of required Go version)\\
$cchar   - build change (e.g., compatibility with older CPUs)\\
$cchar   - enterprise change (e.g., change to backup/restore)\\
$cchar   - backwards-incompatible change\\
$cchar   - performance improvement\\
$cchar   - bug fix
;
"

if ! sed "$sed_script" "$1" > "$tempfile"; then
  give_up "unable to inject commit message recommendations"
fi

if ! mv "$tempfile" "$1"; then
  give_up "failed overwriting commit message file"
fi