git rebase, but keep author, committer & dates

git-fixbase

#!/usr/bin/env bash

set -eu
set -o pipefail

usage() {
	local me="$( basename "$0" | sed 's/^git-//' )"

	cat <<EOF
git $me [target]

	Does an interactive git rebase onto [target], but tries to keep the same
	author, committer and author- and commit-dates as on the original commits
	before the rebase.

	This should give you somewhat the same UX as with a normal git rebase, the
	todo list will just be a bit longer and inject \`exec\`s which after each
	\`pick\` which reset the author & committer data back to the original.
EOF
}

genTodoList() {
	local pick='pick %h %s'
	local envAuthor="GIT_AUTHOR_NAME='%an' GIT_AUTHOR_EMAIL='%ae' GIT_AUTHOR_DATE='format:iso8601:%aI'"
	local envCommitter="GIT_COMMITTER_NAME='%cn' GIT_COMMITTER_EMAIL='%ce' GIT_COMMITTER_DATE='format:iso8601:%cI'"
	local amend="exec ${envAuthor} ${envCommitter} git commit --quiet --amend --reset-author --no-edit"

	git log \
		--reverse \
		--format="${pick}%n${amend}%n" \
		"${1}..HEAD"
}

genEditor() {
	cat <<'EOF' | sed -e "s@__TODO__@${1}@g" -e "s@__EDITOR__@${2}@g"
#!/usr/bin/env bash

set -eu
set -o pipefail

sed 's/^/# /g' "$1" > '__TODO__.org'

{
	cat '__TODO__'
	echo
	echo '# ----[ original todo list ]----'
	cat '__TODO__.org'
} > "$1"

exec '__EDITOR__' "$@"
EOF
}

findDefaultEditor() {
	[ -n "${GIT_EDITOR:-}" ] && { echo "$GIT_EDITOR" ; return ; }
	[ -n "${EDITOR:-}" ] && { echo "$EDITOR" ; return ; }
	[ -n "${VISUAL:-}" ] && { echo "$VISUAL" ; return ; }

	command -v sensible-editor >/dev/null && { echo sensible-editor ; return ; }

	echo 'vi'
}

main() {
	if [ "$#" -ne 1 ]
	then
		usage >&2
		exit 1
	fi

	local tempDir todo todoEditor defaultEditor target

	tempDir="$( mktemp -d )"
	# shellcheck disable=SC2064
	trap "rm -rf -- '$tempDir'" EXIT

	todo="${tempDir}/todo"
	todoEditor="${tempDir}/editor"
	defaultEditor="$( findDefaultEditor )"
	target="$1"

	genTodoList "$target" > "$todo"
	genEditor "$todo" "$defaultEditor" > "$todoEditor"
	chmod +x "$todoEditor"

	GIT_EDITOR="$todoEditor" git rebase --interactive "${target}"
}

main "$@"