Clean up old Git branches

git-gc-branches

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n'

## Deletes branches (both local and remote) that have been merged upstream.

function usage () {
    echo "Usage:"
    echo
    echo "    $(basename "${BASH_ARGV0}") [-h] [-b DEFAULT_BRANCH] [-u UPSTREAM] [-o ORIGIN]"
    echo
    echo "  DEFAULT_BRANCH serves multiple purposes. On UPSTREAM, it’s the branch we’re"
    echo "                 checking to see if we’re merged. On ORIGIN and locally, it’s"
    echo "                 a branch we want to keep even if it’s merged. It defaults to"
    echo "                `git config init.defaultBranch`."
    echo
    echo "  UPSTREAM is the name of the remote that we expect to be merged into. It"
    echo "           defaults to “upstream”."
    echo
    echo "  ORIGIN is the name of a fork that we have push access to, so we can delete"
    echo "         merged branches from it. If it is not provided, we delete local"
    echo "         branches instead."
}

## TODO:
## • report branches that could have been deleted but weren’t because a worktree
##   is tracking them.
## • ensure we don’t delete an `origin_remote` branch that still has a local
##   branch with unmerged changes tracking it (and vice-versa).

init_default_branch="$(git config init.defaultBranch)"
upstream_default_branch="${init_default_branch}"
origin_default_branch="${init_default_branch}"
upstream_remote="upstream"

while getopts "hb:u:o:" option; do
    case "${option}" in
        h)
            usage
            exit 0
            ;;
        b)
            upstream_default_branch="${OPTARG}"
            origin_default_branch="${OPTARG}"
            ;;
        u)
            upstream_remote="${OPTARG}"
            ;;
        o)
            origin_remote="${OPTARG}"
            ;;
        ?)
            usage
            exit 1
            ;;
    esac
done
if [[ -v "${OPTIND}" ]]; then
    usage
    exit 1
fi

merged_against="${upstream_remote}/${upstream_default_branch}"

## This function sets the variable `proceed`.
function get_permission () {
    if [[ -v origin_remote ]]; then
        where="on ${origin_remote}"
    else
        where="locally"
    fi

    echo "The following branches are about to be deleted (${where}):"
    for i in "${branches[@]}"; do
        echo "• ${i}"
    done
    read -p "Do you want to proceed? (y/N) " proceed
}

git fetch --quiet "${upstream_remote}" "${upstream_default_branch}"

if [[ -v origin_remote ]]; then
    ## remote branches
    git fetch --prune "${origin_remote}"
    set +e # `read` returns `1` here for some reason
    read -rd '' -a branches <<<"$( \
        git branch --remotes --list "${origin_remote}/*" --merged "${merged_against}" \
        | grep -v "${origin_remote}/${origin_default_branch}" \
        | sed "s#^[[:space:]]\+${origin_remote}/##")"
    set -e
else
    ## local branches
    set +e # `read` returns `1` here for some reason
    read -rd '' -a branches <<<"$( \
        git branch --merged "${merged_against}" \
        | grep -v '[*+] ' \
        | grep -v "  ${origin_default_branch}" \
        | sed "s#^[[:space:]]\+##")"
    set -e
fi

if [[ "${#branches[@]}" -eq 0 ]]; then
    echo "There are no branches to delete."
else
    get_permission

    if [[ "${proceed}" =~ [Yy] ]]; then
        if [[ -v origin_remote ]]; then
            refspecs=()
            for i in "${branches[@]}"; do
                refspecs+=(":${i}")
            done
            echo "Deleting ${#refspecs[@]} branch(es) on ${origin_remote}."
            git push "${origin_remote}" "${refspecs[@]}"
        else
            echo "Deleting ${#branches[@]} local branch(es)."
            git branch --delete "${branches[@]}"
        fi
    else
        echo "Canceling deletions."
    fi
fi