Best practices for building CLI applications across languages. Covers CLI design principles (Unix philosophy, command structure, subcommands vs flags), argument parsing (required/optional args, flags, environment variables, config files, precedence), user interface (help text, version info, progress indicators, color output, interactive prompts), output formatting (human-readable vs machine-readable JSON/YAML, exit codes), error handling (clear messages, suggestions, debug mode), cross-platform considerations (paths, line endings, terminal capabilities), testing strategies (integration tests, output verification, exit codes), documentation (README, man pages, built-in help), and language-specific libraries. Activate when working with CLI applications, command-line tools, argument parsing, CLI utilities, argument handling, commands, subcommands, CLI frameworks, or building command-line interfaces.