shell-scripting

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Shell Scripting

Shell scripting is the art of automating tasks through the Unix shell - combining built-in commands, control flow, and process management to build reliable CLI tools and automation workflows. This skill covers production-quality bash and zsh scripting: robust error handling, portable argument parsing, safe file operations, and the idioms that separate fragile one-liners from scripts that hold up in production.

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When to use this skill

Trigger this skill when the user:

• Asks to write or review a bash or zsh script
• Needs to parse command-line arguments or flags
• Wants to automate a CLI workflow or task runner
• Asks about exit codes, signal trapping, or error handling in shell
• Needs to process files, lines, or streams from the terminal
• Asks about here documents, process substitution, or subshells
• Wants a portable script that works across bash, zsh, and sh

Do NOT trigger this skill for:

• Python or Node.js CLI tools (shell is the wrong tool for complex logic)
• Scripts that require structured data parsing at scale (use a real language instead)

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Key principles

1. Always use set -euo pipefail - Start every non-trivial script with this. -e exits on error, -u treats unset variables as errors, -o pipefail catches failures in pipelines. Without this, silent failures hide bugs for weeks.
2. Quote everything - Always double-quote variable expansions: "$var", "$@", "${array[@]}". Unquoted variables break on whitespace and glob characters. The only exceptions are intentional word splitting and arithmetic contexts.
3. Check dependencies upfront - Verify required commands exist before the script runs. Fail fast at the top with a clear error, not halfway through a destructive operation.
4. Use functions for reuse and readability - Extract logic into named functions. Shell functions support local variables (local), can return exit codes, and make scripts testable. A main() function at the bottom with a guard is idiomatic.
5. Prefer shell built-ins over external commands - [[ ]] over [ ], ${var##*/} over basename, ${#str} over wc -c. Built-ins are faster, more portable, and avoid spawning subshells. Use printf over echo for reliable output formatting.