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:

organvm --verbose registry update --organ IV a-i--skills

Framework Selection

Building with Typer (Recommended for Python)

Basic Command

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

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

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

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

@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

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

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

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

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

# 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

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

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