diff-git-commits

#!/bin/bash -eu
#
# Diff two comits or patches and colorize the results
#

function out()
{
	local rc=$?

	trap - INT TERM EXIT HUP

	rm -f "${TMP1}" "${TMP2}"

	if [ "${rc}" -ne 0 ] ; then
		echo "Script failed" >&2
	fi

	exit "${rc}"
}

function usage()
{
cat <<EOF
Usage: diff-git-commits [-h] [--no-colors] [-w NUM] COMMIT [COMMIT]

Show the differences between two commits side-by-side and color-highlight the
differences. If only one commit is provided, the script tries to parse the
first commit message for the hash of the second commit.

Positional arguments:
  COMMIT           A git commit hash or a patch file.

Optional arguments:
  -h, --help       Show this help text and exit.
  --no-colors
  -w, --width NUM  Output at most NUM print columns. If not specified, uses the
                   width of the current terminal (tput cols).
EOF
}

diff=colordiff
width=$(tput cols)
commit1=
commit2=

while [ $# -gt 0 ] ; do
	case "${1}" in
		-h|--help)
			usage
			exit
			;;
		--no-colors)
			diff=diff
			;;
		-w|--width)
			width=${2}
			shift
			;;
		*)
			if [ -z "${commit1}" ] ; then
				commit1=${1}
			elif [ -z "${commit2}" ] ; then
				commit2=${1}
			else
				echo "Invalid argument: ${1}" >&2
				exit 1
			fi
			;;
	esac
	shift
done

if [ -z "${commit1}" ] ; then
	usage
	exit 1
fi

TMP1=
TMP2=
trap out EXIT INT TERM HUP

if [ -e "${commit1}" ] ; then
	patch1=${commit1}
else
	TMP1=$(mktemp)
	git show "${commit1}" > "${TMP1}"
	patch1=${TMP1}
fi

if [ -z "${commit2}" ] ; then
	show_opts=()
	commit2=$(grep -P '(cherry picked|(back|forward )ported) from commit [0-9a-f]{40}' \
				   "${patch1}" | tail -1 | grep -oP '[0-9a-f]{40}' || true)
fi

if [ -z "${commit2}" ] ; then
	show_opts=("-R")
	commit2=$(grep -P 'This reverts commit [0-9a-f]{40}' \
				   "${patch1}" | grep -oP '[0-9a-f]{40}' || true)
fi

if [ -z "${commit2}" ] ; then
	echo "Unable to find second commit hash in first commit message" >&2
	exit 1
fi

if [ -e "${commit2}" ] ; then
	patch2=${commit2}
else
	TMP2=$(mktemp)
	git show "${show_opts[@]}" "${commit2}" > "${TMP2}"
	patch2=${TMP2}
fi

"${diff}" -y -W "${width}" "${patch1}" "${patch2}" || true