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,

    #[arg(long, global = true, env = "MYTOOL_CONFIG")]
    config: Option<String>,

    #[command(subcommand)]
    cmd: Command,
}

#[derive(Subcommand, Debug)]
enum Command {
    Serve { #[arg(long, default_value_t = 3000)] port: u16 },
    Migrate { #[arg(long, value_enum, default_value_t = Mode::Up)] mode: Mode },
}

#[derive(Copy, Clone, Debug, ValueEnum)]
enum Mode { Up, Down }

fn main() {
    let args = Args::parse();
    match args.cmd {
        Command::Serve { port } => println!("serve on {}", port),
        Command::Migrate { mode } => println!("migrate: {:?}", mode),
    }
}

Config layering (CLI + env + config file)

Prefer explicit precedence:

1. CLI flags
2. Environment variables
3. Config file
4. Defaults

✅ Correct: merge config with CLI overrides

use clap::Parser;
use serde::Deserialize;

#[derive(Parser, Debug)]
struct Args {
    #[arg(long, env = "APP_PORT")]
    port: Option<u16>,
}

#[derive(Deserialize)]
struct FileConfig {
    port: Option<u16>,
}

fn effective_port(args: &Args, file: &FileConfig) -> u16 {
    args.port.or(file.port).unwrap_or(3000)
}

Exit codes and error handling

Map failures to stable exit codes. Return Result from command handlers and centralize printing.

✅ Correct: command returns Result

use std::process::ExitCode;

fn main() -> ExitCode {
    match run() {
        Ok(()) => ExitCode::SUCCESS,
        Err(e) => {
            eprintln!("{e}");
            ExitCode::from(1)
        }
    }
}

fn run() -> Result<(), String> {
    Ok(())
}

Testing (assert_cmd)

Test the binary surface (arguments, output, exit codes) without coupling to internals.

✅ Correct: integration test

use assert_cmd::Command;

#[test]
fn shows_help() {
    Command::cargo_bin("mytool")
        .unwrap()
        .arg("--help")
        .assert()
        .success();
}

Shell completions (optional)

Generate completions for Bash/Zsh/Fish.

✅ Correct: emit completions

use clap::{CommandFactory, Parser};
use clap_complete::{generate, shells::Zsh};
use std::io;

fn print_zsh_completions() {
    let mut cmd = super::Args::command();
    generate(Zsh, &mut cmd, "mytool", &mut io::stdout());
}

Anti-Patterns

• Parse arguments in library code; parse once in main and pass a typed config down.
• Hide failures behind unwrap; return stable exit codes and structured errors.
• Overload one command with flags; use subcommands for distinct modes.

Resources

• Clap: https://docs.rs/clap
• assert_cmd: https://docs.rs/assert_cmd
• clap_complete: https://docs.rs/clap_complete