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