cli-tool-design

What this skill does

# CLI Tool Design

Build command-line tools that are discoverable, composable, and pleasant to use.

## Design Principles

### The UNIX Philosophy Applied

1. **Do one thing well** — Each command has a clear, singular purpose
2. **Compose through pipes** — Support stdin/stdout for chaining
3. **Fail loudly** — Non-zero exit codes and stderr for errors
4. **Be predictable** — Consistent flags, consistent output format

### Command Structure

```
program [global-options] command [command-options] [arguments]
```

Example:
```bash
organvm --verbose registry update --organ IV a-i--skills
```

## Framework Selection

| Framework | Language | Best For |
|-----------|----------|----------|
| **Typer** | Python | Modern CLIs with type hints, auto-completion |
| **Click** | Python | Complex CLIs, plugins, nested groups |
| **argparse** | Python | Zero-dependency, stdlib-only |
| **clap** | Rust | High-performance, compiled CLIs |
| **cobra** | Go | Go microservice CLIs |

## Building with Typer (Recommended for Python)

### Basic Command

```python
import typer

app = typer.Typer(help="ORGANVM system management CLI")

@app.command()
def status(
    organ: str = typer.Argument(help="Organ number (I-VII or META)"),
    verbose: bool = typer.Option(False, "--verbose", "-v", help="Show detailed output"),
):
    """Show the status of an organ's repositories."""
    # Implementation here
```

### Subcommand Groups

```python
app = typer.Typer()
registry_app = typer.Typer(help="Registry operations")
app.add_typer(registry_app, name="registry")

@registry_app.command("update")
def registry_update(
    organ: str = typer.Argument(help="Target organ"),
    dry_run: bool = typer.Option(False, "--dry-run", "-n"),
):
    """Update registry entries for an organ."""
```

### Rich Output Integration

```python
from rich.console import Console
from rich.table import Table

console = Console(stderr=True)  # Status to stderr

def show_repos(repos: list[dict]):
    table = Table(title="Repositories")
    table.add_column("Name", style="cyan")
    table.add_column("Status", style="green")
    table.add_column("Tier")
    for repo in repos:
        table.add_row(repo["name"], repo["status"], repo["tier"])
    console.print(table)
```

## Argument Design

### Positional vs Optional

| Use Case | Type | Example |
|----------|------|---------|
| Required input | Positional | `program FILE` |
| Behavior modifier | Flag | `--verbose`, `--dry-run` |
| Configuration | Option | `--output FORMAT` |
| Multiple inputs | Variadic | `program FILE...` |

### Flag Conventions

```
-v, --verbose       Increase output verbosity
-q, --quiet         Suppress non-error output
-n, --dry-run       Show what would happen without doing it
-f, --force         Skip confirmation prompts
-o, --output FILE   Write output to FILE instead of stdout
    --json          Machine-readable JSON output
    --no-color      Disable colored output
```

### Boolean Flags with Negation

```python
@app.command()
def deploy(
    color: bool = typer.Option(True, "--color/--no-color"),
    interactive: bool = typer.Option(True, "--interactive/--no-interactive"),
):
```

## Output Design

### Human vs Machine Output

```python
import json
import sys

def output_results(results: list[dict], json_mode: bool = False):
    if json_mode:
        # Machine output to stdout
        json.dump(results, sys.stdout, indent=2)
    else:
        # Human output with formatting
        for r in results:
            console.print(f"[cyan]{r['name']}[/] — {r['status']}")
```

### Progress Indicators

```python
from rich.progress import track

for item in track(items, description="Processing..."):
    process(item)
```

### Stderr vs Stdout

- **stdout**: Data output (pipeable)
- **stderr**: Status messages, progress, errors

## Exit Codes

| Code | Meaning |
|------|---------|
| 0 | Success |
| 1 | General error |
| 2 | Usage error (bad arguments) |
| 64-78 | BSD sysexits conventions |
| 130 | Interrupted (Ctrl+C) |

```python
import sys

def main():
    try:
        result = run_command()
        if not result.success:
            console.print(f"[red]Error:[/] {result.error}", file=sys.stderr)
            raise SystemExit(1)
    except KeyboardInterrupt:
        raise SystemExit(130)
```

## Configuration Loading

Priority order (highest to lowest):
1. Command-line arguments
2. Environment variables
3. Project-level config file (`.tool.yaml`)
4. User-level config (`~/.config/tool/config.yaml`)
5. System defaults

```python
def get_config(cli_value: str | None = None) -> str:
    return (
        cli_value
        or os.environ.get("TOOL_CONFIG")
        or load_project_config()
        or load_user_config()
        or DEFAULT_VALUE
    )
```

## Shell Completion

### Typer Auto-Completion

```bash
# Generate completion script
my-cli --install-completion

# Or manually
_MY_CLI_COMPLETE=bash_source my-cli > ~/.my-cli-complete.bash
source ~/.my-cli-complete.bash
```

### Custom Completions

```python
def complete_organ(incomplete: str) -> list[str]:
    organs = ["I", "II", "III", "IV", "V", "VI", "VII", "META"]
    return [o for o in organs if o.startswith(incomplete.upper())]

@app.command()
def status(organ: str = typer.Argument(autocompletion=complete_organ)):
    ...
```

## Testing CLIs

```python
from typer.testing import CliRunner

runner = CliRunner()

def test_status_command():
    result = runner.invoke(app, ["status", "IV"])
    assert result.exit_code == 0
    assert "a-i--skills" in result.stdout

def test_invalid_organ():
    result = runner.invoke(app, ["status", "INVALID"])
    assert result.exit_code == 2
```

## Anti-Patterns

- **Requiring interactive input in scripts** — Always support `--yes` / `--no-interactive` flags
- **Mixing data and status on stdout** — Use stderr for progress and status messages
- **Inconsistent flag naming** — Pick a convention and stick to it across all subcommands
- **No help text** — Every command, argument, and option needs a help string
- **Swallowing errors** — Always exit with appropriate non-zero codes on failure