Is there any straightforward way for Clap to display help when no command is provided?

I'm using the Clap crate for parsing command line parameters. I've defined a subcommand ls that should list files. Clap also defines a help subcommand that displays information about the application and its usage.

If no command is provided, nothing gets displayed at all, but I want the app to display help in that case.

I've tried this code, which looks pretty straightforward, but it doesn't work:

extern crate clap; use clap::{App, SubCommand}; fn main() { let mut app = App::new("myapp") .version("0.0.1") .about("My first CLI APP") .subcommand(SubCommand::with_name("ls").about("List anything")); let matches = app.get_matches(); if let Some(cmd) = matches.subcommand_name() { match cmd { "ls" => println!("List something here"), _ => eprintln!("unknown command"), } } else { app.print_long_help(); } } 

I get an error that app is used after move:

error[E0382]: use of moved value: `app` --> src/main.rs:18:9 | 10 | let matches = app.get_matches(); | --- value moved here ... 18 | app.print_long_help(); | ^^^ value used here after move | = note: move occurs because `app` has type `clap::App<'_, '_>`, which does not implement the `Copy` trait 

Reading through the documentation of Clap, I've found that the clap::ArgMatches that's returned in get_matches() has a method usage that returns the string for usage part, but, unfortunately, only this part and nothing else.

2 Answers

Use clap::AppSettings::ArgRequiredElseHelp:

App::new("myprog") .setting(AppSettings::ArgRequiredElseHelp) 

See also:

• Method call on clap::App moving ownership more than once

You can also use Command::arg_required_else_help as a bool on the command itself.

Command::new("rule").arg_required_else_help(true)