clap/examples/tutorial_builder/04_02_validate.md

$ 04_02_validate --help
A simple to use, efficient, and full-featured Command Line Argument Parser

Usage: 04_02_validate[EXE] <PORT>

Arguments:
  <PORT>  Network port to use

Options:
  -h, --help     Print help
  -V, --version  Print version

$ 04_02_validate 22
PORT = 22

$ 04_02_validate foobar
? failed
error: invalid value 'foobar' for '<PORT>': `foobar` isn't a port number

For more information try '--help'

$ 04_02_validate 0
? failed
error: invalid value '0' for '<PORT>': port not in range 1-65535

For more information try '--help'