UNIX/POSIX Standards CLI Best Practices

Comprehensive guidelines for building command-line tools that follow UNIX conventions, designed for AI agents and LLMs. Contains 44 rules across 8 categories, prioritized by impact from critical (argument handling, exit codes, output streams) to incremental (configuration and environment).

When to Apply

Reference these guidelines when:

• Writing new CLI tools in any language
• Parsing command-line arguments and flags
• Deciding what goes to stdout vs stderr
• Choosing appropriate exit codes
• Handling signals like SIGINT and SIGTERM

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Argument & Flag Design	CRITICAL	args-
2	Exit Codes	CRITICAL	exit-
3	Output Streams	CRITICAL	output-
4	Error Handling	HIGH	error-
5	I/O & Composition	HIGH	io-
6	Help & Documentation	MEDIUM-HIGH	help-
7	Signals & Robustness	MEDIUM	signal-
8	Configuration & Environment	MEDIUM	config-

Quick Reference

1. Argument & Flag Design (CRITICAL)

• args-use-getopt - Use standard argument parsing libraries
• args-provide-long-options - Provide long options for all short options
• args-support-double-dash - Support double-dash to terminate options
• args-require-help-version - Implement --help and --version options
• args-prefer-flags-over-positional - Prefer flags over positional arguments
• args-use-standard-flag-names - Use standard flag names
• args-never-read-secrets-from-flags - Never read secrets from command-line flags
• args-support-option-bundling - Support option bundling

2. Exit Codes (CRITICAL)

• exit-zero-for-success - Return zero for success only
• exit-use-standard-codes - Use standard exit codes
• exit-signal-codes - Use 128+N for signal termination
• exit-partial-success - Handle partial success consistently
• exit-distinguish-error-types - Distinguish error types with different exit codes

3. Output Streams (CRITICAL)

• output-stdout-for-data - Write data to stdout only
• output-stderr-for-errors - Write errors and diagnostics to stderr
• output-detect-tty - Detect TTY for human-oriented output
• output-provide-machine-format - Provide machine-readable output format
• output-line-based-text - Use line-based output for text streams
• output-respect-no-color - Respect NO_COLOR environment variable

4. Error Handling (HIGH)

• error-include-program-name - Include program name in error messages
• error-actionable-messages - Make error messages actionable
• error-use-strerror - Use strerror for system errors
• error-avoid-stack-traces - Avoid stack traces in user-facing errors
• error-validate-early - Validate input early and fail fast

5. I/O & Composition (HIGH)

• io-support-stdin - Support reading from stdin
• io-write-to-stdout - Write output to stdout by default
• io-be-stateless - Design stateless operations
• io-handle-binary-safely - Handle binary data safely
• io-atomic-writes - Use atomic file writes
• io-handle-multiple-files - Handle multiple input files consistently

6. Help & Documentation (MEDIUM-HIGH)

• help-show-usage-on-error - Show brief usage on argument errors
• help-structure-help-output - Structure help output consistently
• help-show-defaults - Show default values in help
• help-include-examples - Include practical examples in help
• help-version-format - Format version output correctly

7. Signals & Robustness (MEDIUM)

• signal-handle-sigint - Handle SIGINT gracefully
• signal-handle-sigterm - Handle SIGTERM for clean shutdown
• signal-handle-sigpipe - Handle SIGPIPE for broken pipes
• signal-cleanup-on-second-interrupt - Skip cleanup on second interrupt

8. Configuration & Environment (MEDIUM)

• config-follow-xdg - Follow XDG Base Directory Specification
• config-precedence-order - Apply configuration in correct precedence order
• config-env-naming - Use consistent environment variable naming
• config-never-store-secrets - Never store secrets in config files or environment
• config-respect-standard-vars - Respect standard environment variables

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Reference Files

File	Description
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information