git prune-branches: Delete Merged and Squash-Merged Branches

#!/usr/bin/env bash
# Delete local branches that are already merged (including squash- and
# rebase-merged) into the default branch, so `git branch` only lists work
# still in flight.
#
# Usage: git prune-branches [-n] [-y] [--gone] [--no-fetch] [base]    (see -h)
set -euo pipefail

usage() {
  cat <<'EOF'
Usage: git prune-branches [options] [base]

Delete local branches already merged into <base>, including ones merged via
"Squash and merge" or "Rebase and merge". <base> defaults to the remote's
default branch.

Options:
  -n, --dry-run    List what would be deleted and exit
  -y, --yes        Skip the confirmation prompt (required when not on a TTY)
      --gone       Also delete unmerged branches whose upstream was deleted
      --no-fetch   Don't fetch (and prune) the remote first
  -h, --help       Show this help
EOF
}

die() {
  printf 'git prune-branches: %s\n' "$*" >&2
  exit 1
}

dry_run=false
assume_yes=false
include_gone=false
fetch=true
base=

while [ $# -gt 0 ]; do
  case $1 in
    -n | --dry-run) dry_run=true ;;
    -y | --yes) assume_yes=true ;;
    --gone) include_gone=true ;;
    --no-fetch) fetch=false ;;
    -h | --help) usage; exit 0 ;;
    -*) die "unknown option '$1' (see -h)" ;;
    *)
      [ -z "$base" ] || die "unexpected argument '$1' (see -h)"
      base=$1
      ;;
  esac
  shift
done

git rev-parse --is-inside-work-tree >/dev/null 2>&1 || die "not inside a Git work tree"
current=$(git symbolic-ref --quiet --short HEAD || true)

remote=$(git config --get "branch.${current:-main}.remote" || echo origin)
if $fetch && git remote get-url "$remote" >/dev/null 2>&1; then
  git fetch --prune --quiet "$remote" || die "fetch from '$remote' failed (use --no-fetch to skip)"
fi

# Compare against the remote's branch, since local main may be behind. <remote>/HEAD
# is only set by `git clone` (or `git remote set-head`), so fall back by name.
if [ -z "$base" ]; then
  base=$(git symbolic-ref --quiet --short "refs/remotes/$remote/HEAD" 2>/dev/null || true)
  for fallback in "refs/remotes/$remote/main" "refs/remotes/$remote/master" refs/heads/main refs/heads/master; do
    [ -n "$base" ] && break
    git rev-parse --quiet --verify "$fallback" >/dev/null && base=$(git rev-parse --abbrev-ref "$fallback")
  done
  [ -n "$base" ] || die "can't detect the default branch; pass it explicitly"
fi
git rev-parse --quiet --verify "$base^{commit}" >/dev/null || die "no such branch or commit '$base'"

# Never delete the base, long-lived branches, or anything checked out in a worktree.
protected=$'\n'"${base#"$remote"/}"$'\n'main$'\n'master$'\n'develop$'\n'
while read -r key ref; do
  [ "$key" = branch ] && protected+="${ref#refs/heads/}"$'\n'
done < <(git worktree list --porcelain)

merged=() squashed=() gone=()
while IFS='|' read -r branch track; do
  case $protected in *$'\n'"$branch"$'\n'*) continue ;; esac

  if git merge-base --is-ancestor "refs/heads/$branch" "$base"; then
    merged+=("$branch")
    continue
  fi

  # Rebase-merge check: every commit on the branch already has an equivalent
  # in base. (sed reads all the output, so pipefail can't trip on SIGPIPE.)
  if [ -z "$(git cherry "$base" "refs/heads/$branch" | sed -n '/^+/p')" ]; then
    squashed+=("$branch")
    continue
  fi

  # Squash-merge check: build a throwaway commit holding the branch's whole
  # diff, then ask `git cherry` whether an equivalent patch is already in base.
  fork=$(git merge-base "$base" "refs/heads/$branch" || true)
  if [ -n "$fork" ]; then
    probe=$(git commit-tree "refs/heads/$branch^{tree}" -p "$fork" -m probe)
    if [ "$(git cherry "$base" "$probe")" = "- $probe" ]; then
      squashed+=("$branch")
      continue
    fi
  fi

  [ "$track" = "[gone]" ] && gone+=("$branch")
done < <(git for-each-ref --format='%(refname:short)|%(upstream:track)' refs/heads/)

# The ${a[@]+...} form keeps empty arrays safe under `set -u` on bash 3.2 (macOS).
doomed=(${merged[@]+"${merged[@]}"} ${squashed[@]+"${squashed[@]}"})
$include_gone && doomed+=(${gone[@]+"${gone[@]}"})

list() {
  local label=$1; shift
  [ $# -eq 0 ] && return
  echo "$label:"
  printf '  %s\n' "$@"
}

list "Merged into $base" ${merged[@]+"${merged[@]}"}
list "Squash- or rebase-merged into $base" ${squashed[@]+"${squashed[@]}"}
if $include_gone; then
  list "Upstream deleted (unmerged work will be lost)" ${gone[@]+"${gone[@]}"}
elif [ ${#gone[@]} -gt 0 ]; then
  list "Kept — upstream deleted but not merged (use --gone to delete)" ${gone[@]+"${gone[@]}"}
fi

if [ ${#doomed[@]} -eq 0 ]; then
  echo "Nothing to prune."
  exit 0
fi
$dry_run && exit 0

if ! $assume_yes; then
  [ -t 0 ] || die "refusing to delete branches non-interactively without --yes"
  read -r -p "Delete ${#doomed[@]} branch(es)? [y/N] " reply
  case $reply in [yY] | [yY][eE][sS]) ;; *) die "aborted" ;; esac
fi

# -D is needed for squash- and rebase-merged branches; print each tip so any can be restored.
for branch in ${doomed[@]+"${doomed[@]}"}; do
  sha=$(git rev-parse --short "refs/heads/$branch")
  git branch --quiet -D "$branch"
  echo "Deleted $branch (restore: git branch $branch $sha)"
done

mkdir -p ~/.local/bin
# Save the script above as ~/.local/bin/git-prune-branches, then:
chmod +x ~/.local/bin/git-prune-branches

# Make sure ~/.local/bin is on your PATH (add this to ~/.bashrc or ~/.zshrc)
export PATH="$HOME/.local/bin:$PATH"

git prune-branches -h

git prune-branches -n          # Dry run: list what would go, delete nothing
git prune-branches             # Fetch, list, confirm, delete
git prune-branches --gone      # Also delete branches whose remote branch was deleted
git prune-branches develop     # Compare against a branch other than the default
git prune-branches --no-fetch  # Work offline from the last fetch

$ git prune-branches
Merged into origin/main:
  fix/header-overlap
  feat/rss-feed
Squash- or rebase-merged into origin/main:
  feat/search-index
Kept — upstream deleted but not merged (use --gone to delete):
  spike/view-transitions
Delete 3 branch(es)? [y/N] y
Deleted fix/header-overlap (restore: git branch fix/header-overlap c1b4b57)
Deleted feat/rss-feed (restore: git branch feat/rss-feed 0313a32)
Deleted feat/search-index (restore: git branch feat/search-index 84e14d9)