Building CLI Apps

Building CLI Applications

Overview

CLI apps are filters in a pipeline. They read input, transform it, write output. The Unix philosophy applies: do one thing well, compose with others.

When to Use CLI vs TUI vs GUI

digraph decision {
    rankdir=TB;
    "User interaction needed?" [shape=diamond];
    "Complex state/navigation?" [shape=diamond];
    "Scriptable/automatable?" [shape=diamond];
    "CLI" [shape=box, style=filled, fillcolor=lightblue];
    "TUI" [shape=box, style=filled, fillcolor=lightgreen];
    "GUI" [shape=box, style=filled, fillcolor=lightyellow];

    "User interaction needed?" -> "Complex state/navigation?" [label="yes"];
    "User interaction needed?" -> "Scriptable/automatable?" [label="no"];
    "Scriptable/automatable?" -> "CLI" [label="yes"];
    "Scriptable/automatable?" -> "GUI" [label="no"];
    "Complex state/navigation?" -> "TUI" [label="yes"];
    "Complex state/navigation?" -> "CLI" [label="no"];
}

Choose CLI when: Single operation, pipeable, scriptable, CI/CD, simple prompts Choose TUI when: Dashboard, multi-view navigation, real-time monitoring Choose GUI when: Non-technical users, complex visualizations, drag/drop

Quick Reference: Libraries by Language

Core Patterns

1. Streams: stdout vs stderr

stdout → Data/results (pipeable)
stderr → Progress, logs, errors (human feedback)

Python:

import sys
from rich.console import Console

console = Console(stderr=True)  # Progress/logs to stderr
output = Console()              # Results to stdout

console.print("[dim]Processing...[/]")  # → stderr
output.print_json(data=result)          # → stdout (pipeable)

TypeScript:

// Results to stdout
console.log(JSON.stringify(result));

// Progress to stderr
process.stderr.write('Processing...\n');

C#:

Console.WriteLine(result);           // stdout
Console.Error.WriteLine("Working..."); // stderr

2. Exit Codes

# Python
import sys
sys.exit(0)  # Success
sys.exit(1)  # Error

// TypeScript
process.exit(0);
process.exitCode = 1;  // Preferred - allows cleanup

// C#
Environment.Exit(0);
return 1;  // From Main

3. Configuration Hierarchy

Precedence (highest to lowest):

1. CLI arguments (--config value)
2. Environment variables (APP_CONFIG)
3. Config file (.apprc, config.json)
4. Defaults

# Python with typer
import typer
import os

def main(
    config: str = typer.Option(
        os.environ.get("APP_CONFIG", "default"),
        "--config", "-c"
    )
):
    pass

4. Subcommand Structure

mycli/
├── src/
│   ├── main.py          # Entry point, registers commands
│   ├── commands/
│   │   ├── __init__.py
│   │   ├── process.py   # mycli process <file>
│   │   └── config.py    # mycli config show|set
│   └── lib/             # Shared logic
└── tests/
    └── commands/
        └── test_process.py

Python with typer:

# main.py
import typer
from commands import process, config

app = typer.Typer()
app.add_typer(process.app, name="process")
app.add_typer(config.app, name="config")

if __name__ == "__main__":
    app()

TypeScript with commander:

// index.ts
import { Command } from 'commander';
import { processCommand } from './commands/process';
import { configCommand } from './commands/config';

const program = new Command();
program.addCommand(processCommand);
program.addCommand(configCommand);
program.parse();

C# with System.CommandLine:

var rootCommand = new RootCommand("My CLI");
rootCommand.AddCommand(ProcessCommand.Create());
rootCommand.AddCommand(ConfigCommand.Create());
await rootCommand.InvokeAsync(args);

5. Interactive vs Non-Interactive Mode

import sys
import typer
from rich.prompt import Confirm

def main(
    force: bool = typer.Option(False, "--force", "-f"),
    file: str = typer.Argument(...)
):
    # Check if running interactively
    is_interactive = sys.stdin.isatty()

    if not force and is_interactive:
        if not Confirm.ask(f"Delete {file}?"):
            raise typer.Abort()
    elif not force and not is_interactive:
        # Non-interactive without --force: fail safe
        typer.echo("Use --force in non-interactive mode", err=True)
        raise typer.Exit(1)

    # Proceed with operation
    delete_file(file)

6. Reading from stdin (Piped Input)

Support both file arguments and piped input (- convention):

import sys
import typer

@app.command()
def process(
    file: str = typer.Argument(..., help="Input file (or - for stdin)")
):
    if file == "-":
        content = sys.stdin.read()
    else:
        content = Path(file).read_text()
    # Process content...

import { createInterface } from 'readline';

async function readInput(file: string): Promise<string> {
    if (file === '-') {
        const lines: string[] = [];
        const rl = createInterface({ input: process.stdin });
        for await (const line of rl) lines.push(line);
        return lines.join('\n');
    }
    return fs.readFileSync(file, 'utf-8');
}

Usage: cat data.txt | mycli process - or echo "test" | mycli process -

7. Signal Handling

import signal
import sys

def handle_sigint(signum, frame):
    print("\nInterrupted, cleaning up...", file=sys.stderr)
    cleanup()
    sys.exit(130)

signal.signal(signal.SIGINT, handle_sigint)

process.on('SIGINT', () => {
    console.error('\nInterrupted, cleaning up...');
    cleanup();
    process.exit(130);
});

Console.CancelKeyPress += (sender, e) => {
    e.Cancel = true;  // Prevent immediate termination
    Console.Error.WriteLine("\nInterrupted, cleaning up...");
    Cleanup();
    Environment.Exit(130);
};

Anti-Patterns

Testing CLI Apps

Python with pytest:

from typer.testing import CliRunner
from myapp.main import app

runner = CliRunner()

def test_process_success():
    result = runner.invoke(app, ["process", "test.txt"])
    assert result.exit_code == 0
    assert "processed" in result.stdout

def test_process_missing_file():
    result = runner.invoke(app, ["process", "nonexistent.txt"])
    assert result.exit_code == 1
    assert "not found" in result.stderr

def test_piped_input(tmp_path):
    input_file = tmp_path / "input.txt"
    input_file.write_text("test data")
    result = runner.invoke(app, ["process", "-"], input="test data")
    assert result.exit_code == 0

TypeScript with Jest:

import { execSync } from 'child_process';

test('process command succeeds', () => {
    const result = execSync('npx ts-node src/index.ts process test.txt');
    expect(result.toString()).toContain('processed');
});

test('process command fails on missing file', () => {
    expect(() => {
        execSync('npx ts-node src/index.ts process nonexistent.txt');
    }).toThrow();
});

Help Text Best Practices

import typer

app = typer.Typer(
    help="Process loan notices with AI classification.",
    no_args_is_help=True,  # Show help if no args
)

@app.command()
def process(
    file: str = typer.Argument(..., help="Path to notice file (or - for stdin)"),
    output: str = typer.Option(None, "--output", "-o", help="Output file (default: stdout)"),
    format: str = typer.Option("json", "--format", "-f", help="Output format: json, csv, table"),
    verbose: bool = typer.Option(False, "--verbose", "-v", help="Show processing details"),
):
    """
    Process a loan notice through the classification pipeline.

    Examples:
        mycli process notice.pdf
        mycli process notice.pdf --format table
        cat notice.txt | mycli process - --output result.json
    """
    pass

Error Messages

Good error messages include:

1. What went wrong
2. Why it's a problem
3. How to fix it

# Bad
print("Error: invalid input")
sys.exit(1)

# Good
print(f"Error: File '{path}' is not a valid PDF.", file=sys.stderr)
print(f"Expected: PDF file with loan notice content", file=sys.stderr)
print(f"Try: mycli process --help for supported formats", file=sys.stderr)
sys.exit(1)

Distribution