pre-commit hook for checking source files are clang-formatted, giving command to format any unformatted files

pre-commit.hook

#!/bin/bash
#
# Check that all source files follow a fixed format.
# Called by "git commit" with no arguments.  
# Commit is blocked if any file does not follow format and outputs
# command to format them.
# Enable this by copying this file into .git/hooks/pre-commit

ROOT=$(git rev-parse --show-toplevel)
FORMATTER="clang-format"
files=$(git diff --cached --name-only -z | tr '\0' '\n')

formats_to_format="\.(cpp|hpp|c|h)$"
files_to_format=$(echo "$files" | grep -E $formats_to_format)

bad_files=""
for file in $files_to_format ; do
	out=$($FORMATTER $file)
	DIFF=$(diff <(echo "$out") $file)
	if [ "$DIFF" != "" ]
	then
		bad_files="$bad_files \"$ROOT/$file\""
	fi
done

if [ "$bad_files" != "" ]
then
	echo "Some files are not formatted correctly; format with:
$FORMATTER -i $bad_files"
	exit 1
fi

exit 0