How to pipe string into a Rust app parsing args with Clap?

Question

I'm Brand new to Rust and I have been writing some practice apps. I am trying to accept command-line arguments using Clap. The code below takes a string and a number and prints them back out, as such:

$ cargo run "this is a test" -n11
this is a test
11

This works fine, but I want to be able to pipe input in place of the string like this:

$ echo "this is a test" | cargo run -- -n11
this is a test
11

Trying this yields:

error: The following required arguments were not provided:
    <INPUT>

USAGE:
    clap_sample --num <num> <INPUT>

For more information try --help

I can get around this using xargs like this:

$ echo "this is a test" | xargs -d '\n' cargo run -- -n11

Is there a better way to do this so that I can accept piped in strings while still using the -n option? Thanks in advance.

  use clap::{Arg, App}; 
  
  fn main() {
     let matches = App::new("Clap Sample")
         .arg(Arg::new("INPUT")
             .required(true)
             .index(1))
         .arg(Arg::new("num")
             .short('n')
             .long("num")
             .takes_value(true))
         .get_matches();
 
     let usr_string = matches.value_of("INPUT").unwrap();
     let key: u8 = matches.value_of("num").unwrap()
         .parse()
         .expect("NaN :(");
 
     println!("{}", usr_string);
     println!("{}", key);
 }

Bonus question: If I pipe a string in with xargs, I can have newlines in the string (with delimiter set to \0) and they are reflected in the output. If I pass it directly without echo and xargs, a literal '\n' shows in the output. Is there a way to have the newline represented when run directly?

`echo -n "this is a test" | xargs cargo run -- -n11` – Elliott Frisch Jan 18 '22 at 02:56 — Elliott Frisch, Jan 18 '22 at 02:56

Simson · Answer 1 · 2022-01-18T05:22:21.587

Your code is checking the command line for arguments, it is not reading standard input. To use xargs get move the input from the pipe to the command line is a good way of doing it.

echo -n "this is a test" | xargs cargo run -- -n11

The other option you have is to change your program so it reads from stdin if no user_string argument was given. Here is a good starting point to read stdin https://doc.rust-lang.org/std/io/struct.Stdin.html

You should also replace the unwrap() here:

 let key: u8 = matches.value_of("num").unwrap()

with a check if the argument was given as it was not .required(true) for instance with a

if let Some(key) = matches.value_of("num")

or maybe with a unwrap_or("0")

How to pipe string into a Rust app parsing args with Clap?

1 Answers1