developing-posix-shell-scripts skill

This skill covers any task involving writing, reviewing, or refactoring a POSIX-compliant shell script (/bin/sh).

Bash-specific features are forbidden. See common.md for the full list of bash-isms to avoid.

Step 1 — Classify the Script

Audit flags before classifying. Ask for each proposed flag: would a real caller ever pass a different value, or can it be a readonly constant? Beyond that, apply common sense — not everything that could vary should be a flag. Internal paths, fixed timeouts, log levels for non-CLI tools, and similar values are typically hardcoded in practice even if they could theoretically differ. When reviewing an existing script, re-classify from scratch; the current number of flags is not evidence of correct classification.

Simple — all of: < 50 lines of logic, 0–2 genuine flags, no structured logging, no -h/help output, no resource cleanup, not shared across systems/environments.

Complex — any of: ≥ 50 lines, 3+ genuine flags, structured logging, -h/help output, resource cleanup, shared across systems/environments.

When in doubt, prefer Simple.

Step 2 — Follow the Reference Document

Always load common.md first, then load the matching document:

• developing-simple-posix-shell-scripts.md — Load when classified as Simple.
• developing-complex-posix-shell-scripts.md — Load when classified as Complex. Also load reference-code-blocks.md to compose only the blocks the script actually needs.