git-rewind-rebase

#!/bin/bash
set -e

SCRIPT_FILENAME=$(basename "${BASH_SOURCE[0]}")
ARG_COMMIT=

usage() {
    echo -n "\
USAGE:
    $SCRIPT_FILENAME [OPTIONS] <commit>

While in an interactive git-rebase, this allows to go back to a previous
commit without losing the changes already done so far.

In other words, it allows the user to do \"multiple passes\" on the
revision range being updated in the same rebase section (i.e. no need to
finish this rebase and start a new one just because of a missed
changed).

This is achieved by hard-reseting to the commit passed as argument, and
prepending the range <commit>..HEAD to the git-rebase's todo file.

OPTIONS:
    -h
        Show this help message.
"
}

argerr() {
	echo "invalid argument: $*" >&2
	echo >&2
	usage >&2
	exit 1
}

parse_args() {
    while (( $# > 0 )); do
        case "$1" in
        -h)
            usage
            exit 0
            ;;
        -*)
            argerr "Unknown option $1."
            ;;
        *)
            if [[ -n $ARG_COMMIT ]]; then
                argerr "Commit specified more than once."
            fi

            ARG_COMMIT=$1
            ;;
        esac
        shift
    done

    if [[ -z $ARG_COMMIT ]]; then
        argerr "Missing commit to rebase onto."
    fi
}

generate_new_todo() {
    local git_dir
    local todo
    local new_todo

    git_dir=$(git rev-parse --git-dir)
    todo="$git_dir/rebase-merge/git-rebase-todo"

    if [ ! -f "$todo" ]; then
        echo "Not in an interactive rebase?" > /dev/stderr
        exit 1
    fi

    new_todo=$(mktemp --tmpdir git-rewind-rebase.XXXXXXXXXX)

    git log --reverse "$ARG_COMMIT..HEAD" --format="pick %H %s" > "$new_todo"

    # insert a break so we are back to the previously stopped place
    echo "x echo \"rewind-rebase previously stopped here\"" >> "$new_todo"
    echo "break" >> "$new_todo"

    cat "$todo" >> "$new_todo"

    # replace todo
    mv "$new_todo" "$todo"
}

parse_args "$@"
generate_new_todo
git reset --hard "$ARG_COMMIT"