name: cli-building
description: Build command-line interfaces with async-first design, composable commands, and proper output formatting. Use when creating CLI tools, commands, or interactive terminal applications.

CLI Building

Guidelines for building command-line interfaces with modern patterns and best practices.

When to Use

• Creating new CLI tools or commands
• Building interactive terminal applications
• Adding commands to existing projects
• Implementing command-line interfaces
• Working with CLI frameworks

Core Principles

• Async-first: All I/O operations should be async/await, avoid blocking operations
• Composable commands: Commands should be modular and reusable, use command composition
• Strategy pattern: Use strategy pattern for branching workflows or task-based commands
• Output formatting: Proper formatting with unicode symbols and color support

Framework Selection

TypeScript/JavaScript

stricli (@bloomberg/stricli, recommended for modern async-first CLIs):
- Built for async/await from ground up
- Type-safe command definitions with full type inference
- Lazy loading for startup performance
- Zero dependencies

oclif (alternative):
- Mature framework with extensive features
- Plugin system
- Good for complex CLIs

Python

cyclopts (recommended for async-first):
- Modern async-first CLI framework
- Type-safe with excellent async support
- Clean API design

typer (when fully async support available):
- Based on Python type hints
- Clean and intuitive
- Good for simple to medium complexity CLIs

Command Architecture

Composable Commands

Design commands as reusable modules:
- Shared command utilities
- Command middleware
- Reusable command modules
- Command composition patterns

Strategy Pattern

Use strategy pattern for:
- Workflow branching
- Task-based commands
- Dynamic command routing
- Conditional command execution

Output Formatting

• No emojis: Do not use emojis unless explicitly directed
• Unicode symbols: Use unicode symbols (✓, ✗, →, ⚠) for status indicators
• Color support: Use color libraries, never hardcoded ANSI codes
• NO_COLOR: Always respect NO_COLOR environment variable
• Formatting: Use formatting for better readability (bold, dim, etc.)

Async Patterns

Async-First Design

All I/O should be async:
- File operations: use async file APIs
- Network requests: use async HTTP clients
- Process execution: use async process APIs
- Database operations: use async database clients

Error Handling

Handle async errors properly:
- Use try/catch with await
- Handle promise rejections
- Provide clear error messages
- Exit with appropriate codes

Command Structure

Basic Command

// stricli example
import { createCli } from '@bloomberg/stricli';

async function myCommand() {
  // Async implementation
}

const cli = createCli({
  name: 'my-cli',
  commands: {
    'my-command': myCommand
  }
});

cli.run();

# cyclopts example
from cyclopts import App

app = App()

@app.default
async def my_command():
    # Async implementation
    pass

if __name__ == '__main__':
    app()

Best Practices

1. Async by default: All operations should be async
2. Composable design: Build reusable command modules
3. Strategy pattern: Use for workflow branching
4. Proper formatting: Unicode symbols and color with NO_COLOR support
5. Error handling: Clear error messages and exit codes
6. Type safety: Use TypeScript types or Python type hints
7. Testing: Test commands in isolation

References

For detailed guidance, see:
- references/async-patterns.md - Async/await best practices
- references/composable-commands.md - Command composition patterns
- references/strategy-pattern.md - Strategy pattern for workflows
- references/output-formatting.md - Output formatting guidelines
- references/frameworks.md - Framework comparisons and selection