Clap - 6th of August

cargo add clap -F derive

Clap is useful, if you want to write a CLI program, that accepts arguments and options.

Clap offers two ways of defining arguments and options for your program:

• Using the derive feature of clap: reference
• Using the builder method, which is more flexible and allows for more complex configurations: reference

For simplicity, we will look at the derive feature of clap.

How to use Clap

For a simple program, you define a struct, and it's fields become the arguments and options of your program. If you use the derive feature of clap, you can use the Parser derive macro to automatically generate the clap parser for your struct and you can instantly access the arguments and options.

An example from their docs:

use clap::Parser;

/// Simple program to greet a person
#[derive(Parser, Debug)]
#[command(version, about, long_about = None)]
struct Args {
    /// Name of the person to greet
    #[arg(short, long)]
    name: String,

    /// Number of times to greet
    #[arg(short, long, default_value_t = 1)]
    count: u8,
}

fn main() {
    let args = Args::parse();

    for _ in 0..args.count {
        println!("Hello {}!", args.name);
    }
}

If you now run this program with no arguments, Args::parse() will panic, as it expects an argument with --name, which is not an optional, therefore required argument.

But how can you give args to your Rust program:

• Via your binary (usually in your target directory):

./<you_binary_name> --name Bernd

• Via cargo

cargo run -- --name Bernd

• Installing your program (will be installed in ~/.cargo/bin, which is in your PATH) - then use it:

cargo install --path .
<your-binary-name> --name Bernd

Try it out

Generate a new rust project:

cargo new clap_project

Add clap as a dependency:

cargo add clap -F derive

Play around with it :). You can have a look at the clap examples.