name: bash-expert description: Expert Bash help for writing, debugging, and optimizing safe, portable shell one-liners and scripts on macOS/Linux. Use when users ask for Bash or POSIX sh commands, script reviews, ShellCheck fixes, quoting/pipeline issues, process/file automation, or converting requirements into shell scripts.

Bash Expert

Overview

• Write safe Bash scripts and one-liners with correct quoting, piping, and error handling.
• Produce portable code for macOS/Linux; call out GNU vs BSD differences and POSIX-only paths when needed.
• Debug issues (tracing, ShellCheck) and propose tests/dry-runs before destructive actions.

Quick Start

1. Confirm shell/OS/permissions. Note macOS ships Bash 3.2 (no associative arrays); prefer Bash 4+ when available.
2. Choose shebang: /usr/bin/env bash for Bash 4+, /bin/sh for POSIX-only requirements.
3. Default safety (non-interactive):

   set -euo pipefail
   IFS=$'\n\t'
   shopt -s nullglob
   

   Avoid strict mode when partial failures are expected.
4. Quote everything ("${var}"), prefer printf over echo, and avoid word-splitting in loops.

Workflow for Script Requests

1. Clarify goal, inputs, side effects, and scale (file counts, data size).
2. Identify tools (find, grep, sed, awk, jq, tar, rsync, etc.) and portability constraints.
3. Scaffold with logging and cleanup:

   #!/usr/bin/env bash
   set -euo pipefail
   IFS=$'\n\t'
   trap 'cleanup' EXIT
   cleanup(){ :; }
   log(){ printf '%s\n' "$*" >&2; }
   die(){ log "error: $*"; exit 1; }
   

4. Add argument parsing (getopts), sanity checks, and dry-run mode before mutating data.
5. Use safe loops (while IFS= read -r line; do ...; done < file) and null-delimited traversal (find ... -print0 | while IFS= read -r -d '' path; do ...; done).
6. Validate (bash -n, shellcheck -x), run with representative inputs, then present final command plus usage notes/rollback guidance.

One-Liners & Pipelines

• Keep pipelines fail-fast with set -o pipefail or bash -o pipefail -c 'cmd'.
• Prefer process substitution over temp files when possible: diff <(cmd1) <(cmd2).
• Use xargs -0 -r with find -print0; avoid for f in $(...).
• Set LC_ALL=C for deterministic greps/sorts; specify PATH explicitly for cron/CI runs.

Portability & Performance

• Bash-only features: arrays, [[ ]], brace expansion, ** globstar. Offer POSIX alternatives when /bin/sh is requested.
• BSD vs GNU flags: sed -E vs -r, date vs gdate, stat vs gstat; note fallbacks for macOS.
• Prefer built-ins where possible; for large data sets, use awk/perl instead of large Bash loops.

Debugging & Validation

• Trace with set -x (or bash -x script.sh); use PS4='+ ${BASH_SOURCE}:${LINENO}: ' for context.
• shellcheck script.sh for linting; address quoting and word-splitting warnings first.
• Log intermediates with printf '%s\n' "msg: $var"; disable tracing after debug.
• Provide dry-run flags and confirmation prompts before destructive actions.

References

See references/bash_patterns.md for ready-to-use snippets: script skeletons, getopts template, safe find/xargs, mktemp + traps, heredocs, array handling, and bats test scaffolding. Load it only when deeper examples are needed.