How do I get a Rust Command to actually output to stdout?

Question

I am trying to use Command::process to spawn some processes and to get started I want to check I can get a simple command like echo to work correctly. I want to take some user input and then spawn a process to echo this back to stdout, so that I can verify that I get a response like:

> echo this!
echo this!    <-- my echoed text

Currently, I am trying to do this like this:

use std::io;
use std::process::Command;

fn main() {
    let mut some_input = String::new();
    io::stdin()
        .read_line(&mut some_input)
        .unwrap();

    let command_example = Command::new("echo")
        .arg(&some_input)
        .output()
        .unwrap();

    let expected_output = format!("{}\n", &some_input);

    assert_eq!(expected_output.as_bytes(), command_example.stdout.as_slice());
}

The assert block is passing, so I can see that the output of the command ends up in the struct's stout buffer. However, when I run this in the terminal, there is no output, the program just ends without printing anything back to stdout. What am I doing wrong that this code doesn't result in something being printed to the stdout?

Answer 1

From the documentation:

Executes the command as a child process, waiting for it to finish and collecting all of its output.

Thus, you obtain all the standard output of the process in your command_example variable. Nothing appears in the terminal, because this was collected.

Alternatively, instead of .output(), you can use .status() in order to run the process without capturing its standard output; it will then show up in the terminal.