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

args-

exit-

output-

error-

io-

help-

signal-

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