Show commit diff between expected and current submodule

git-submodule-diff.sh

#!/usr/bin/env bash

#****f* git/submodule_diff
# NAME
#   git_submodule_diff - show commit diff between expected and current submodule
# DESCRIPTION
#   When changing version of a superproject, submodules will often show up as
#   being different. Often the right answer is to run "git submodule update".
#   However, sometimes this is not the right answer. It is tedious to switch to
#   a submodule, look through the logs for the SHAs in question, and understand
#   what is really different. 
#
#   This script will easily "git log" the commits in each version so that you
#   can get a quick understanding of what is different in the submodule.
# INPUTS
#   $1 - path to submodule
#   [$2...] - Additional arguments to be sent to git log
# EXAMPLE
#   git_submodule_diff path_of_submodule --oneline
# AUTHOR
#   Andy Neff
#***

set -eu

if !(($#)); then
  echo "usage: git submodule-diff <submodule path> [<log options>...]"
  exit 2
fi

sm_path="$1"
shift 1

submodule_diff="$(git diff --no-color --word-diff=porcelain "${sm_path}")"

current_sha=$(grep -E '^\+[a-f0-9]+' <<< "${submodule_diff}")
expected_sha=$(grep -E '^\-[a-f0-9]+' <<< "${submodule_diff}")
current_sha=${current_sha#+}
expected_sha=${expected_sha#-}

cd "${sm_path}"

echo
echo "Showing commits in current (${current_sha:0:7}) but not in expected (${expected_sha:0:7})"
echo -------------------------------------------
echo
git log --decorate --graph ${@+"${@}"} ${expected_sha}..${current_sha}

echo
echo "Showing commits in expected (${expected_sha:0:7}) but not in current (${current_sha:0:7})"
echo -------------------------------------------
echo
git log --decorate --graph ${@+"${@}"} ${current_sha}..${expected_sha}