Clap

Clap (Rust) - Production CLI Patterns

Overview

Clap provides declarative command-line parsing with strong help output, validation, and subcommand support. Use it to build CLIs with predictable UX and testable execution paths.

Quick Start

Minimal CLI

✅ Correct: derive Parser

use clap::Parser;

#[derive(Parser, Debug)]
#[command(name = "mytool", version, about = "Example CLI")]
struct Args {
    /// Enable verbose output
    #[arg(long)]
    verbose: bool,

    /// Input file path
    #[arg(value_name = "FILE")]
    input: String,
}

fn main() {
    let args = Args::parse();
    if args.verbose {
        eprintln!("verbose enabled");
    }
    println!("input={}", args.input);
}

❌ Wrong: parse multiple times

fn main() {
    let _a = Args::parse();
    let _b = Args::parse(); // duplicate parsing and inconsistent behavior
}

Subcommands (real tools)

Model multi-mode CLIs with subcommands and shared global flags.

✅ Correct: global flags + subcommands

use clap::{Parser, Subcommand, ValueEnum};

#[derive(Parser, Debug)]
struct Args {
    #[arg(long, global = true)]
    verbose: bool,