Transplant (rebase) a range of commits from one branch onto another

git-transplant.sh

#!/usr/bin/env bash

# Stop on undefined variables and errors
set -u
set -e

get_commit_parent() {
  local target_commit="$1";

  local _parent_commit=`git log --pretty=%P -n 1 "${target_commit}"`;
  local parent_count=`echo "${_parent_commit}" | wc | awk '{ print $2; }'`;

  if [ "${parent_count}" -ne "1" ]; then
    echo "'${target_commit}' must have only one parent (parents are '${parent_commit}')." >&2;
    exit 1;
  else
    export parent_commit="${_parent_commit}";
  fi
}

do_transplant() {
  local target_branch="$1";
  local parent_of_first_commit="$2";
  local last_commit="$3";

  set -x;

  git checkout "${target_branch}";

  # Checkout a new temporary branch at the current location
  git checkout -b tmp;

  # Move the target branch to the head of the new patchset
  git branch -f "${target_branch}" "${last_commit}";

  # Rebase the patchset onto tmp, the old location of the target branch
  git rebase \
    --onto tmp "${parent_of_first_commit}" "${target_branch}" \
    --committer-date-is-author-date;

  set +x;

  cleanup_transplant;
}

function abort_transplant() {
  local target_branch="$1";

  set -x;

  git rebase --abort;
  git checkout tmp;
  git branch -f "$target_branch" tmp;
  git checkout "$target_branch";

  set +x;

  cleanup_transplant;
}

function cleanup_transplant() {
  set -x;

  # Delete temporary branch
  git branch -D tmp;

  set +x;
}

function echo_error() {
  message="$1";

  echo "$message" 1>&2;
}

function print_usage() {
  script_name=`basename "${0}"`;

  echo_error "GIT Transplant -- Used to transplant a range of commits from one branch (for";
  echo_error "example, 'develop') onto another (for example, a hotfix).";
  echo_error "";
  echo_error "This script requires GIT and can only be run inside a non-bare GIT repo.";
  echo_error "";
  echo_error "Pass --abort as the fourth parameter to abort a transplant that is ";
  echo_error "in-progress, rolling branches back to their original state.";
  echo_error "";
  echo_error "Pass --cleanup as the fourth parameter to cleanup after a transplant ";
  echo_error "that you manually completed successfully after being interrupted by ";
  echo_error "merge conflicts.";
  echo_error "";
  echo_error "Usage: ${script_name} <target branch> <first commit hash (inclusive)> \\";
  echo_error "       <last commit hash (inclusive)> [--abort|cleanup]";
}

if [[ ($# -ne 3) && (($# -ne 4) || (($# -eq 4) && ($4 != '--abort') && \
      ($4 != '--cleanup'))) ]]; then
  print_usage;
  exit 1;
else
  if [[ $# -eq 4 ]]; then
    if [ $4 == '--abort' ]; then
      abort_transplant "$1";

    elif [[ $4 == '--cleanup' ]]; then
      cleanup_transplant;
    fi;
  else
    export parent_commit="";

    get_commit_parent "$2" && do_transplant "$1" "${parent_commit}" "$3";
  fi;
fi;