Zsh script to prune locale branches already merged into the default branch

git-prune-branches.zsh

#!/usr/bin/env zsh
# Prune local Git branches that are already merged into the default base branch.
# The script skips the base branch, the current branch, and branches with related stashes,
# then asks for confirmation before deleting each eligible branch.

prune_local_branches() {
    local version="1.0.0"
    
    case "${1:-}" in
        -h|--help)
            cat <<EOF
Usage: git-prune-branches [OPTIONS]

Prune local Git branches that are already merged into the default base branch.
The script skips the base branch, the current branch, and branches with related
stashes, then asks for confirmation before deleting each eligible branch.

Options:
  -h, --help      Show this help message
  -v, --version   Show version number
EOF
            return 0
            ;;
        -v|--version)
            echo "git-prune-branches version $version"
            return 0
            ;;
        "")
            ;;
        *)
            echo "Error: unknown option '$1'" >&2
            return 1
            ;;
    esac

    command -v git >/dev/null 2>&1 || { echo "Error: git is not installed."; return 1; }
    git rev-parse --is-inside-work-tree >/dev/null 2>&1 || { echo "Error: not a git repository."; return 1; }

    local base current branch ans stash_count s
    local -a removed skipped

    base=$(git symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/null)
    base=${base#origin/}
    if [[ -z "$base" ]]; then
        if git show-ref --verify --quiet refs/heads/main; then
            base=main
        elif git show-ref --verify --quiet refs/heads/master; then
            base=master
        else
            echo "Error: could not determine main branch."
            return 1
        fi
    fi

    current=$(git branch --show-current)

    while IFS= read -r branch; do
        [[ -z "$branch" ]] && continue

        if [[ "$branch" == "$base" ]]; then
            skipped+=("$branch: protected base branch")
            continue
        fi

        if [[ "$branch" == "$current" ]]; then
            skipped+=("$branch: current branch")
            continue
        fi

        stash_count=0
        while IFS= read -r s; do
            [[ "$s" == "WIP on $branch:"* || "$s" == "On $branch:"* ]] && ((stash_count++))
        done < <(git stash list --format='%gs')

        if (( stash_count > 0 )); then
            skipped+=("$branch: has $stash_count related stash entries")
            continue
        fi

        if ! git merge-base --is-ancestor "$branch" "$base" >/dev/null 2>&1; then
            skipped+=("$branch: not merged into $base")
            continue
        fi

        printf "Delete branch '%s'? [y/N] " "$branch"
        read -r ans
        if [[ "$ans" == [yY] ]]; then
            if git branch -d "$branch" >/dev/null 2>&1; then
                removed+=("$branch")
            else
                skipped+=("$branch: deletion failed")
            fi
        else
            skipped+=("$branch: user declined")
        fi
    done < <(git for-each-ref --format='%(refname:short)' refs/heads)

    if (( ${#removed[@]} == 0 )); then
        echo "no branch pruned"
    fi

    if (( ${#removed[@]} )); then
        echo "Removed branches:"
        printf ' - %s\n' "${removed[@]}"
    fi

    if (( ${#skipped[@]} )); then
        echo "Not removed:"
        printf ' - %s\n' "${skipped[@]}"
    fi
}

prune_local_branches