Iteratively backmerges pull requests in sequence using the GitHub and Git CLIs

github-cascade-backmerge.sh

#!/bin/bash

# For when you have a train of pull requests lined up like A -> B -> C -> D 
# and you make a change to base D that you now want backmerged to C then B then A.

set -e

# ----------------------------------------------------------------------------

# List the pull request number (e.g. 42) in order that the backmerges
# should occur, not necessarily numerically.
PULL_REQUESTS=(
  42 # https://github.com/your-org/your-repo/pull/42
  43 # https://github.com/your-org/your-repo/pull/43
  40 # https://github.com/your-org/your-repo/pull/40
)

# ----------------------------------------------------------------------------

# $1 = message to log
function log_info() {
  # https://stackoverflow.com/a/5947802/470818
  local LIGHT_BLUE='\033[0;34m'
  local NC='\033[0m' # No Color

  echo "" >&2
  echo -e "${LIGHT_BLUE}> ${1} ${NC}" >&2
  echo "" >&2
}

# $1 = message to log
function log_warn() {
  # https://stackoverflow.com/a/5947802/470818
  local YELLOW='\033[0;33m'
  local NC='\033[0m' # No Color

  echo "" >&2
  echo -e "${YELLOW}> ${1}${NC}" >&2
  echo "" >&2
}

# Gets the pull request number from the given url or current git repo.
# $1 = pull request url (optional)
function get_pr_id() {
  local pr_url="${1}"

  gh pr view ${pr_url} --json number | jq -r '.number'
}

# Checks out a pull request into the current git repo.
# $1 = pull request url or number (required)
function git_checkout() {
  local pr_id="$(get_pr_id ${1})"
  local pr_title=$(get_pr_title ${1})

  log_info "checking out pull request -- ${pr_title} -- #${pr_id}"

  gh pr checkout ${pr_id}
}

# Get the base branch name of the currently checked out git repository.
# $1 = pull request url or number (optional)
function get_base_ref() {
  local pr_id="${1}"
  gh pr view ${pr_id} --json baseRefName | jq -r '.baseRefName'
}

# Get the title of the pull request.
# $1 = pull request url or number (optional)
function get_pr_title() {
  local pr_id="${1}"
  gh pr view ${pr_id} --json title | jq -r '.title'
}

# $1 = pull request url or number (optional)
function backmerge_base_ref() {
  local pr_id="${1}"
  local base_ref="$(get_base_ref ${pr_id})"

  log_info "backmerging branch ${base_ref}"

  git merge "origin/${base_ref}" --no-edit
}

function push_merged_changes() {
  log_info "pushing merged changes to ${get_head_ref}"

  git push
}

function git_fetch() {
  git fetch --all --force --prune --prune-tags --no-tags
}

function await_user_to_continue() {
  # https://stackoverflow.com/a/5947802/470818
  local YELLOW='\033[0;33m'
  local NC='\033[0m' # No Color

  # https://twitter.com/douglascayers/status/1628175887786029056
  log_warn "[ press <enter> to push changes ]"
  read -p ''
}

# ----------------------------------------------------------------------------

git_fetch

for pr_url in "${PULL_REQUESTS[@]}"; do
  git_checkout ${pr_url}

  merge_result=$(backmerge_base_ref)

  if [ "${merge_result}" != "Already up to date." ]; then
    await_user_to_continue
    push_merged_changes
  fi
done