use clap::{App, Arg}; fn main() { // There are two ways in which to get a default value, one is to use claps Arg::default_value // method, and the other is to use Rust's built in Option::unwrap_or method. // // I'll demo both here. // // First, we'll use clap's Arg::default_value with an "INPUT" file. let matches = App::new("myapp") .about("does awesome things") .arg( Arg::with_name("INPUT") .about("The input file to use") // Note, we don't need to specify // anything like, "Defaults to..." // because clap will automatically // generate that for us, and place // it in the help text .default_value("input.txt") .index(1), ) // Next we'll use the Option::unwrap_or method on this "CONFIG" option .arg( Arg::with_name("CONFIG") // Note that we have to manaully include some verbage to the user // telling them what the default will be. .about("The config file to use (default is \"config.json\")") .short('c') .takes_value(true), ) .get_matches(); // It's safe to call unwrap because the value with either be what the user input at runtime // or "input.txt" let input = matches.value_of("INPUT").unwrap(); // Using Option::unwrap_or we get the same affect, but without the added help text injection let config_file = matches.value_of("CONFIG").unwrap_or("config.json"); println!("The input file is: {}", input); println!("The config file is: {}", config_file); }