Default value on the command line

• In this example we set the config_file variable to the parameter passed on the command line
• or to a default value that can be any string, including the empty string.

fn main() {
    let argv: Vec<String> = std::env::args().collect();

    let config_file = if argv.len() == 2 {
        &argv[1]
    } else {
        //""
        "config.yaml"
    };

    println!("'{}'", config_file);
}