Git-hook implemented as bash-script that adds a string to message based on the filetype

hook.sh

#!/usr/bin/env bash
# Simple git hook that appends/prepends a commit message with a tag

set -o errexit
set -o errtrace
set -o nounset
set -o pipefail

declare -r tag="#docu"
declare -r filefilter="\.(html|adoc)$"
declare -r commit_message_file="${1:-}"
declare -r commit_source="${2:-}"

if [ -n "$commit_message_file" ] && [ -n "$commit_source" ]; then
  while IFS= read -r line || [ -n "$line" ]; do
    if [[ "$line" =~ $filefilter ]]; then
      readonly commit_message=$(<"$commit_message_file")

      case "$commit_source" in
      # message was given (via -m or -F), append the tag at the end
      message)
        echo "$commit_message $tag" >"$commit_message_file"
        ;;
      merge) ;;
      template) ;;
      # prepend the tag to the message
      *)
        echo "$tag $commit_message" >"$commit_message_file"
        ;;
      esac

      break # Needs to run only once
    fi
  done < <(git diff --name-only --staged)
fi