use clap::{app_from_crate, arg}; fn main() { let matches = app_from_crate!() .arg( arg!() .help("Network port to use") .validator(|s| s.parse::()), ) .get_matches(); // Note, it's safe to call unwrap() because the arg is required let port: usize = matches .value_of_t("PORT") .expect("'PORT' is required and parsing will fail if its missing"); println!("PORT = {}", port); }