CLI Development Principles

Guidelines for building fast, intuitive, cross-platform CLI tools.

When to Invoke

• Designing CLI tool architecture
• Implementing command hierarchies and argument parsing
• Adding shell completions and interactive features
• Cross-platform distribution planning

Architecture

Command Hierarchy

app <command> [subcommand] [flags] [args]
app task create --priority high "Deploy fix"
app task list --status active --format json

Design Principles

1. Startup time < 100ms — lazy-load expensive dependencies.
2. Sensible defaults — zero-config for common cases.
3. Progressive disclosure — simple interface, advanced flags for power users.
4. Machine-readable output — --format json for scripting.
5. Exit codes — 0 = success, 1 = error, 2 = usage error.

Argument Parsing Libraries

Language	Library	Notes
Go	cobra + pflag	Most popular, auto-completions
Rust	clap (derive)	Type-safe, auto-help
Python	click or typer	Decorator-based, typer for type hints
Node.js	commander or yargs	Mature, well-documented

Error Handling

1. Helpful error messages — what went wrong, why, how to fix:

   Error: config file not found at ~/.myapp/config.yaml
   Hint: run 'myapp init' to create a default config
   

2. --verbose / --debug flags for diagnostic output.

3. Never show stack traces by default — only with --debug.

Shell Completions

Generate completions for bash, zsh, fish, PowerShell. Most CLI frameworks support this.

# Generate completions
myapp completion bash > /etc/bash_completion.d/myapp
myapp completion zsh > ~/.zsh/completions/_myapp

Cross-Platform

1. Path handling — use filepath.Join (Go), Path (Python), not string concat.
2. Line endings — handle \r\n on Windows.
3. Color support — detect terminal capabilities, respect NO_COLOR env var.
4. Unicode — test with non-ASCII filenames and input.

Distribution

Method	Best For
Go/Rust binary	Single binary, no runtime dependency
pip install / npm install -g	Language ecosystem users
Homebrew formula	macOS/Linux users
Docker image	Containerized environments
GitHub Releases	Universal, with checksums

Testing

For universal testing principles, see .agents/rules/testing-strategy.md. Below: language-specific patterns only.

1. Unit test command logic — separate from CLI framework.
2. Integration tests — run actual CLI commands, assert exit codes and output.
3. Golden file tests — snapshot expected output for complex commands.

Related

• Command Execution Principles @.agents/rules/command-execution-principles.md
• Error Handling Principles .agents/rules/error-handling-principles.md