git-spinoff

#!/bin/sh
set -eu

#
# Inspired by Magit's super useful `magit-branch-spinoff` command.
# See also https://magit.vc/manual/magit/Branch-Commands.html
#

usage () {
    echo "usage: git spinoff [-h] <new-name> [<base>]" >&2
    echo >&2
    echo "Creates and checks out a new branch starting at and tracking the" >&2
    echo "current branch.  That branch in turn is reset to the last commit it" >&2
    echo "shares with its upstream.  If the current branch has no upstream or no" >&2
    echo "unpushed commits, then the new branch is created anyway and the" >&2
    echo "previously current branch is not touched." >&2
    echo >&2
    echo "This is useful to create a feature branch after work has already" >&2
    echo "began on the old branch (likely but not necessarily \"main\")." >&2
    echo >&2
    echo "Options:" >&2
    echo "-h    Show this help" >&2
}

while getopts h flag; do
    case "$flag" in
        h) usage; exit 2;;
    esac
done
shift $(($OPTIND - 1))

if [ $# -lt 1 ] || [ $# -gt 2 ]; then
  usage
  exit 2
fi

new_name="$1"
rawbase="${2:-}"
if [ -z "$rawbase" ]; then
    base="$(git current-branch)"
else
    base="$rawbase"
fi

base_sha="$(git sha -s "$base")"

#
# NOTE:
# The flag -B is the transactional equivalent of
#     $ git branch -f <branch> [<start point>]
#     $ git checkout <branch>
#
git checkout -q --track -B "$new_name" "$base"

rtb="$(git remote-tracking-branch "$base")"
if [ -n "$rtb" ]; then
    merge_base="$(git merge-base "$base" "$rtb")"
    git branch -vf "$base" "$merge_base"
fi

if [ "$(git sha -s "$base")" != "$base_sha" ]; then
    echo "$base reset to $(git sha -s "$base") (was $base_sha)"
else
    echo "$base not touched"
fi