Clap: improving the help with value_name

---

By default the help (generated by providing -h) includes the name of the field in angle brackets. This is usually a repetition of the parameter name in upper case letters. In many cases this gives good indication of the value the program is expecting, but sometimes you might want to give a better hint. In such cases you can set the string that will appear inside the angle brackets using the value_name parameter.

• In this example the input field has the default: INPUT while the out field was set to FILENAME by the value_field.

examples/clap/value-name/src/main.rs

use clap::Parser;

#[derive(Parser, Debug)]
struct Cli {
    #[arg(long)]
    input: String,

    #[arg(long, value_name = "FILENAME")]
    output: String,
}

fn main() {
    let args = Cli::parse();
    println!("{args:?}");
}

Usage: value-name --input <INPUT> --output <FILENAME>

Options:
      --input <INPUT>
      --output <FILENAME>
  -h, --help               Print help