Pre-commit git hook to optimize assets upon commit

pre-commit

#!/bin/sh

if git rev-parse --verify HEAD >/dev/null 2>&1
then
	against=HEAD
else
	# Initial commit: diff against an empty tree object
	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
fi

# Redirect output to stderr.
exec 1>&2

# Check for changes to png or jpg assets
if test $(git diff --cached --name-only --diff-filter=ACMR $against | grep "\.\(png\|jpg\)$" | wc -l | tr -d ' ') != 0
then
	cat <<\EOF
============================= Optimizing assets =============================

EOF
	# TODO: Check if pngcrush is installed. If not, display a warning
	# Optimize pngs
	exec git diff --cached --name-only --diff-filter=ACMR $against | grep "\.png$" | xargs -I % pngcrush -q -brute -ow -noforce %
	exec git diff --cached --name-only --diff-filter=ACMR $against | grep "\.png$" | xargs -I % git add %

	# TODO: Check if jpegoptim is installed. If not, display a warning
	# Optimize jpgs
	exec git diff --cached --name-only --diff-filter=ACMR $against | grep "\.jpg$" | xargs -I % jpegoptim %
	exec git diff --cached --name-only --diff-filter=ACMR $against | grep "\.jpg$" | xargs -I % git add %
	cat <<\EOF

======================== Asset optimization complete ========================
EOF
fi

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