Clap basic

Parse command line arguments

This application describes the structure of its command-line interface usingclap's builder style. Thedocumentation gives two other possible ways toinstantiate an application.

In the builder style, each possible argument is described by anArgstruct. The string given toArg::new() is the internalname of the argument. Theshort andlong options control theflag the user will be expected to type; short flags look like-f and longflags look like--file.

Theget_one() method is used to get an argument's value.It returnsSome(&value) if the argument was supplied bythe user, elseNone.

The use ofPathBuf is to allow file paths which are legalin Linux and MacOS, but not in Rust UTF-8 strings. This isbest practice: one encounters such paths quite rarely inpractice, but when it happens it is really frustratingwithout this.

use std::path::PathBuf;use clap::{Arg, Command, builder::PathBufValueParser};fn main() {    let matches = Command::new("My Test Program")        .version("0.1.0")        .about("Teaches argument parsing")        .arg(Arg::new("file")                 .short('f')                 .long("file")                 .help("A cool file")                 .value_parser(PathBufValueParser::default()))        .arg(Arg::new("num")                 .short('n')                 .long("number")                 .help("Five less than your favorite number"))        .get_matches();    let default_file = PathBuf::from("input.txt");    let myfile: &PathBuf = matches.get_one("file").unwrap_or(&default_file);    println!("The file passed is: {}", myfile.display());    let num_str: Option<&String> = matches.get_one("num");    match num_str {        None => println!("No idea what your favorite number is."),        Some(s) => {            let parsed: Result<i32, _> = s.parse();            match parsed {                Ok(n) => println!("Your favorite number must be {}.", n + 5),                Err(_) => println!("That's not a number! {}", s),            }        }    }}

Usage information is generated byclap -h. The usage forthe example application looks like this.

Teaches argument parsingUsage: clap-cookbook [OPTIONS]Options:  -f, --file <file>   A cool file  -n, --number <num>  Five less than your favorite number  -h, --help          Print help  -V, --version       Print version

We can test the application by running a command like the following.

$ cargo run -- -f myfile.txt -n 251

The output is:

The file passed is: myfile.txtYour favorite number must be 256.