fix(ralph-loop): crash on macOS (bash 3.2) with "mem_limit_prefix[@]: unbound variable"

[opensource-SantanderAI]

ralph-loop crashes on macOS (bash 3.2) with mem_limit_prefix[@]: unbound variable

Environment

• OS: macOS (arm64)
• Shell: GNU bash 3.2.57 (macOS default; resolved via #!/usr/bin/env bash when no newer bash is on PATH)
• Config: RALPH_MEMORY_MAX=8G, RALPH_TOOL=codex

Symptom

On the first iteration the loop aborts with exit code 1:

Warning: RALPH_MEMORY_MAX is set (8G) but systemd-run --user scopes are unavailable; running the agent without a RAM limit.
ralph-loop: line 308: mem_limit_prefix[@]: unbound variable

Root cause

Three factors combine:

1. set -u is active (line 22).
2. macOS has no systemd, so build_mem_limit_prefix() leaves mem_limit_prefix=() empty (warning branch).
3. On bash < 4.4 (macOS ships 3.2), expanding an empty array with "${arr[@]}" under set -u is treated as an unbound variable. On bash >= 4.4 this expands to nothing without error, so it does not reproduce on Linux.

Affected: the four tool invocations that prefix the array — codex (308), claude (322), gemini (346), devin (357).

Isolated repro

/bin/bash -uc \arr=(); echo before; "${arr[@]}"; echo after'
# bash: arr[@]: unbound variable

Fix

Use the safe empty-array idiom, compatible with bash 3.2+: ${arr[@]:+"${arr[@]}"} (expands to nothing when empty; to the quoted elements otherwise). Applied to all four run_tool() invocations.

Verification

• bash -n ralph-loop.sh OK under bash 3.2.57.
• RALPH_MEMORY_MAX=8G ralph-loop 1 plan/plan.md no longer crashes; the informative one-shot warning remains and the agent starts without a RAM limit.
• No Linux regression: when mem_limit_prefix is populated (systemd available), ${arr[@]:+"${arr[@]}"} expands identically to "${arr[@]}".
• Added a bats regression guard that fails if any bare "${mem_limit_prefix[@]}" expansion is reintroduced.

Notes

The systemd --user RAM limit never applies on macOS; the warning is already a one-shot (mem_limit_warned). Bumping the shebang / requiring bash >= 4.4 was considered but the :+ fix is less intrusive and keeps compatibility with the factory macOS bash.