git hook to check c++ formatting

pre-commit

#!/bin/bash
#
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.

prompt_confirm() {
  while true; do
    read -r -n 1 -p "${1:-Continue?} [y/n]: " REPLY
    case $REPLY in
      [yY]) echo ; return 0 ;;
      [nN]) echo ; return 1 ;;
      *) printf " \033[31m %s \n\033[0m" "invalid input"
    esac 
  done  
}

if git rev-parse --verify HEAD >/dev/null 2>&1
then
	against=HEAD
else
	# Initial commit: diff against an empty tree object
	against=$(git hash-object -t tree /dev/null)
fi

# Redirect output to stderr and input from console
exec 1>&2 < /dev/tty

set -a unformatted_files

FORMATTER=clang-format-7

while IFS= read -r -d '' fn; do
  diff=$(diff <(git show :"$fn") <(git show :"$fn" | ${FORMATTER}))
  if [[ ! -z "$diff" ]]; then
  #if git show :${fn} | ${FORMATTER} -output-replacements-xml | grep -q '</replacement>'; then
    unformatted_files+=("$fn")
    echo
    echo Need to format ${fn}:
    echo "$diff"
  fi
done < <(git diff --cached --name-only --diff-filter=ACMR -z $against -- '*.[ch]pp')

if [ ${#unformatted_files[@]} -ne 0 ]; then
  echo
  echo "There are unformatted files:"
  printf '%s\n' "${unformatted_files[@]}"

  unstaged_changes_in_unformatted_files=$(comm -12 <(git diff --name-only) <(printf '%s\n' "${unformatted_files[@]}"))
  if [[ ! -z $unstaged_changes_in_unformatted_files ]]; then
    echo
    echo Can not run formatting because there are unstaged changes in unformatted files:
    echo "$unstaged_changes_in_unformatted_files"
    echo
    exit 1
  fi

  format_command="${FORMATTER} -i ${unformatted_files[@]}"
  echo
  if prompt_confirm "Apply formatting?"; then
    echo Applying formatting:
    echo $format_command
    $format_command
    git add ${unformatted_files[@]}
    if git diff --cached --name-only '*.[ch]pp' | sed 's/^/:/' | xargs git show | ${FORMATTER} -output-replacements-xml | grep -q '</replacement>'; then
      echo Aw, there are still some unformatted files. Please check manually.
      exit 1
    fi
  else
    echo You can still apply it manually with:
    echo $format_command
    exit 1
  fi
fi

# If there are whitespace errors, print the offending file names and fail.
exec git diff-index --check --cached $against --