Covers safe defaults (set -euo pipefail, TRACE guard, main wrapper),
executable permission checks, and shellcheck usage.

Assisted-by: Claude Opus 4.6 via pi

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+---
+name: bash
+description: Use when writing, reviewing, or debugging shell scripts — covers safe defaults, shellcheck, executable permissions, and scripting patterns
+---
+
+# Bash scripting
+
+## New scripts
+
+Start with safe defaults:
+
+```bash
+#!/usr/bin/env bash
+
+set -euo pipefail
+if [[ "${TRACE-0}" == "1" ]]; then
+    set -o xtrace
+fi
+
+main() {
+}
+
+main "$@"
+```
+
+`set -euo pipefail` exits on errors, undefined variables, and pipe
+failures. The `TRACE` guard provides opt-in tracing via `TRACE=1
+./script.sh` without cluttering normal output.
+
+Wrapping logic in `main()` and calling `main "$@"` at the bottom
+prevents partial execution if the script is truncated during
+download or write.
+
+## Executable permissions
+
+Scripts meant to be run directly need the executable bit (`chmod +x`).
+Check permissions before committing — a missing executable bit is a
+common oversight that breaks scripts silently when deployed.
+
+Non-executable shell files are appropriate for files meant to be
+sourced (`. script.sh` or `source script.sh`), not run directly.
+
+## Shellcheck
+
+Run [shellcheck](https://www.shellcheck.net/) on all shell scripts
+before committing.
+
+```bash
+shellcheck path/to/script.sh
+```
+
+Fix all warnings. If a warning is genuinely inapplicable, disable it
+inline with a directive comment on the line above:
+
+```bash
+# shellcheck disable=SC2059
+printf "$fmt" "$arg"
+```
+
+Don't disable warnings globally or at the file level unless there's a
+compelling reason.
+
+---
+
+*This is a self-improving skill — see the `self-improving-skills` skill.*