Skill: CLI Guideline

Modern CLI design principles based on clig.dev — human-first design while maintaining composability.

Core Philosophy

1. Human-first design — CLIs are for humans, not just scripts
2. Simple parts that work together — Composable via pipes, stdin/stdout, exit codes
3. Consistency — Follow established conventions (flags, env vars, behavior)
4. Say just enough — Not too verbose, not too silent
5. Ease of discovery — Help, examples, suggestions
6. Conversation as norm — Trial-and-error is expected; guide the user
7. Robustness — Handle errors gracefully, feel solid
8. Empathy — Delight users, exceed expectations

Essential Rules

I/O Streams

Stream	Purpose	Example
stdout	Primary output, data, machine-readable	Results, JSON, piped data
stderr	Messages, logs, progress, errors	Status, warnings, spinners
Exit 0	Success
Exit ≠0	Failure (map codes to failure modes)

Help

• -h and --help show full help (exit 0, ignore other flags)
• No args + required args → concise help (description + 1-2 examples + "use --help")
• Lead with examples, not option lists
• Include support path (URL, issue tracker)
• Suggest likely fixes on typos ("Did you mean...?")
• If expecting stdin and it's a TTY, show help immediately (don't hang)

Output

• Human-readable by default; detect TTY for formatting
• --plain for stable, line-based output (scripts, grep/awk)
• --json for structured output
• Brief success messages; verbose only when state changes
• Suggest next commands in workflows
• Color: intentional (not decorative); disable via NO_COLOR, TERM=dumb, --no-color
• No animations/spinners when stdout is not TTY
• No debug noise by default; use --debug or DEBUG=1

Errors

• Rewrite for humans: what happened + how to fix
• High signal-to-noise; group similar errors
• Important info at the end (eye drawn there)
• Unexpected errors → debug path + bug report instructions

Arguments & Flags

• Prefer flags over positional args
• Full-length flags for everything (--help, not just -h)
• One-letter flags for common options only
• Standard names: --help, --version, --dry-run, --verbose, --json, --force, --output, --quiet, --debug, --no-input
• Sensible defaults (right thing for most users)
• Prompt for missing input (TTY only); never require prompts
• Confirm dangerous actions; support --dry-run
• - for stdin/stdout when flag takes a file
• Order-independent flags/subcommands when possible
• Never accept secrets via flags (use --password-file or stdin)

Interactivity

• Prompt only when stdin is TTY
• Support --no-input to disable all prompts
• Password prompts: don't echo
• Ctrl-C always exits quickly

Subcommands

• Consistent flags across subcommands
• Consistent naming pattern (noun verb or verb noun)
• Avoid ambiguous names (update vs upgrade)

Robustness

• Validate input early; fail fast with clear message
• Responsive < 100ms; show progress for long ops
• Network calls timeout (configurable)
• Recoverable on rerun; crash-only design
• Handle misuse (scripts, bad networks, concurrent instances)

Future-Proofing

• Treat interfaces as contracts (flags, env vars, config, output)
• Additive changes preferred
• Warn before breaking changes; deprecate gracefully
• No catch-all subcommand (blocks future commands)
• No arbitrary abbreviations of subcommands

Signals

• Ctrl-C: exit immediately, acknowledge, timeout cleanup
• Second Ctrl-C: force stop (document behavior)

Configuration

• Precedence: flags > env > project config > user config > system config
• Follow XDG spec (~/.config/...)
• Never silently edit other programs' config

Environment Variables

• Names: UPPERCASE_WITH_UNDERSCORES (no leading digit)
• Single-line values preferred
• Respect: NO_COLOR, FORCE_COLOR, DEBUG, EDITOR, HTTP_PROXY, PAGER, HOME, TMPDIR
• Read .env for project context (not as full config)
• Never read secrets from env vars (use files/stdin)

Naming

• Simple, memorable, lowercase
• Dashes only if needed; short but not cryptic
• Easy to type

Distribution

• Single binary when possible
• Easy uninstall (document it)

Analytics

• No telemetry without consent (opt-in preferred)
• Transparent collection; consider alternatives

Quick Reference

For detailed stress-testing checklist, see references/checklist.md.

For complete guidelines with examples, see references/full-guidelines.md.

Language-Specific Notes

Python

• Use argparse, click, or typer
• Return exit codes from main(); use raise SystemExit(main())
• No tracebacks for expected errors; --debug for unexpected
• Handle Ctrl-C cleanly (exit 130, no traceback)
• Separate TTY detection for stdout vs stderr

TypeScript/Node

• Use commander, yargs, oclif, or node:util parseArgs
• Separate stdout/stderr; handle async failures
• --debug for stack traces
• Avoid premature process.exit(); prefer process.exitCode

Bash

• Shebang matches features (#!/usr/bin/env bash for Bash-only)
• Robust flag parsing; handle quotes/whitespace
• trap for cleanup; mktemp for temp files
• Lint with shellcheck
• use skill: shellck