git-reauthor

#!/bin/sh

### Change commit user (author) name and email for the last BACK commits
### (C) 2019-2024 by Jim Klimov

set -u
set -e
[ -n "${DEBUG-}" ] || set -xv
[ -n "${BACK-}" ] || BACK=2
PREV=1
[ -n "${COMMIT_USER_NAME-}" ] || COMMIT_USER_NAME="`git config user.name || git config --global user.name`"
[ -n "${COMMIT_USER_NAME-}" ] || { echo "COMMIT_USER_NAME or proper git config is required" >&2 ; exit 1; }
[ -n "${COMMIT_USER_EMAIL-}" ] || COMMIT_USER_EMAIL="`git config user.email || git config --global user.email`"
[ -n "${COMMIT_USER_EMAIL-}" ] || { echo "COMMIT_USER_EMAIL or proper git config is required" >&2 ; exit 1; }
[ "${SIGNOFF-}" = true ] || SIGNOFF=false
INITLOG="`git log -1`"

[ -n "${GSED-}" ] || {
	# Historically: for expression below, sed should not claim
	#     bad flag in substitute command: '}'
	# GIT_EDITOR="$GSED -e '0,/^pick /{s/^pick /edit /}' -i " git rebase -i HEAD~${PREV}
	# Currently a POSIX-portable expression is used,
	# but on a system with choices we might see some
	# "very other" implementation so prefer GNU anyway.
	(command -v gsed) && GSED=gsed || GSED=sed
}

ANEW="$COMMIT_USER_NAME <$COMMIT_USER_EMAIL>"

echo "Going back BACK=$BACK commits to change the commit authors to '$ANEW' ..."
[ -n "${COMMIT_USER_FILTER-}" ] && { echo "...matching regex '.*$COMMIT_USER_FILTER.*'" ; } \
|| { echo "You do not have COMMIT_USER_FILTER set, waiting a bit in case you need that" >&2 ; sleep 5; }

# Note that we must rewrite history starting from the newest
# commit, tracking back to the oldest, otherwise the edited
# timestamps tend to end up broken (reset to "today + offset").
# With the interactive rebase, the first line is the oldest
# commit of those selected (so fixing up the one PREV commits
# ago, one by one for different PREV values); watch out for
# merge-commits though!
while [ "$PREV" -le "$BACK" ] ; do
	GIT_EDITOR="$GSED -e '/^pick /{s/^pick /edit /;:p' -e 'n;bp' -e '}' < \$1 > \$1.tmp && mv \$1.tmp \$1 ; : " git rebase -i HEAD~${PREV} && \
	while sleep 0.1 ; do
		echo "===== START COMMIT PARSE:"
		AOLD="`git show -s --format='%an <%ae>'`"

		echo "=== AMENDING:"
		echo "=== OLD USER: $AOLD"
		DO_AMEND=true
		if [ -n "$COMMIT_USER_FILTER" ]; then
			if expr "$AOLD" : ".*$COMMIT_USER_FILTER.*" > /dev/null ; then
				echo "===== OLD USER matches the regex '$COMMIT_USER_FILTER'"
			else
				echo "===== OLD USER does not match the regex '$COMMIT_USER_FILTER', skipped"
				DO_AMEND=false
			fi
		fi

		if $DO_AMEND ; then
			echo "=== NEW USER: $ANEW"
			git commit --amend --no-edit --author "$ANEW"

			if $SIGNOFF ; then
				if git log -1 | grep "Signed-off-by: $ANEW" > /dev/null ; then
					echo "===== ALREADY Signed-off-by: $ANEW" # no-op
				else
					echo "===== NEW SIGNATURE: Signed-off-by: $ANEW"
					if [ -n "$COMMIT_USER_FILTER" ]; then (
						GIT_EDITOR="$GSED -e '/^Signed-off-by: '"$COMMIT_USER_FILTER"'\$/d' < \$1 > \$1.tmp && mv \$1.tmp \$1 ; : " \
						git commit --amend --author "$ANEW" -s
					) ; else
						git commit --amend --no-edit --author "$ANEW" -s
					fi
				fi
			fi
		fi

		echo "=== RESULT LOG:"
		git log -1

		echo "=== CONTINUE REBASE:"
		git rebase --continue || break

		git status | grep rebasing || { echo "+++ DONE" && break ; }

		echo "===== Next loop..."
		echo ""
	done
	PREV="`expr $PREV + 1`"
	echo "+++++++++++++++ NEXT CYCLE"
done

echo ""
echo ""
echo "DONE; if you want to 'git reset' to initial state, this was the starting point:"
echo "$INITLOG"