ralph-loop: setup script crashes on macOS due to bash 3.2 empty array bug

[rockydev]

Bug

/ralph-loop fails immediately with:

setup-ralph-loop.sh: line 113: PROMPT_PARTS[*]: unbound variable

Root Cause

The script uses #!/bin/bash which on macOS resolves to /bin/bash (bash 3.2.57). Bash 3.2 has a known bug where expanding an empty array (${array[*]}) with set -u active triggers an "unbound variable" error, even when the array was properly initialized.

Line 6 sets set -euo pipefail, and line 113 does PROMPT="${PROMPT_PARTS[*]}". When no positional arguments have been collected into PROMPT_PARTS, the expansion fails before reaching the empty-prompt validation on line 116.

Fix

Either (or both):

1. Use #!/usr/bin/env bash instead of #!/bin/bash so users with a modern bash (e.g. via Homebrew) get it picked up automatically.

2. Use a safe expansion on line 113:

   PROMPT="${PROMPT_PARTS[*]:-}"

   This provides an empty default, avoiding the unbound variable error on bash 3.2.

Environment

• macOS (Darwin 25.3.0, Apple Silicon)
• /bin/bash = GNU bash 3.2.57
• /opt/homebrew/bin/bash = GNU bash 5.3.9 (not used due to #!/bin/bash)