stdout_read.rs

use std::process::{ Command, Stdio };
use std::io::{ Read, BufReader, Error, ErrorKind };

fn main() -> Result<(), Error> {
    let external_process_name = "..."; // some process name that prints:
    // line 1
    // .. waits for 2 seconds ..
    // line 2
    // .. waits for 2 seconds ..
    // line 3
  
    println!("SPAWNING");

    let mut child = Command::new(external_process_name)
        .stdout(Stdio::piped())
        .spawn()?;

    println!("SPAWNED");

    let stdout = child.stdout.take().ok_or_else(|| Error::new(ErrorKind::Other, "Coudln't capture stdout"))?;

    let mut reader = BufReader::new(stdout);
    let mut buf = Vec::with_capacity(100);

    reader.read(&mut buf).expect("Can't read");
    println!("Read 1 before .wait: {:?}", buf); // expecting it to wait for the first line and print 'line 1' in bytes

    reader.read(&mut buf).expect("Can't read");
    println!("Read 2 before .wait: {:?}", buf); // expecting it to wait for the second line and print 'line 2' in bytes

    child.wait()?;

    reader.read(&mut buf).expect("Can't read");
    println!("Read after .wait {:?}", buf);
        
    println!("DONE");

    Ok(())
}

// Expected output:
// SPAWNING
// SPAWNED
// Read 1 before .wait: 'line 1' (in bytes for simplicity)
// Read 2 before .wait: 'line 2' (in bytes for simplicity)
// Read after .wait 'line 3' (in bytes for simplicity)
// DONE

// Received output:
// SPAWNING
// SPAWNED
// Read 1 before .wait: []
// Read 2 before .wait: []
// Read after .wait []
// DONE