call shell run pipeline failed in rust's Command.output

Question

I'm trying to get result from call shell and grep the result. But it failed, in shell, it works.

use std::process::Command;

fn main() {
    let result = Command::new("sh")
        .arg("-c")
        .arg("last")                // by this line it works
        // .arg("last | grep 'still logged in'")  // by this line, it will return 256 code
        .output()
        .expect("'last' command failed to start");
    println!("{:?}", result);
}

Answer 1

https://doc.rust-lang.org/stable/std/process/struct.ExitStatus.html

An ExitStatus represents every possible disposition of a process. On Unix this is the wait status . It is not simply an exit status (a value passed to exit ).

See I used wait(&status) and the value of status is 256, why? . grep returns 1 as no match was found, and this becomes an exist status of 256 .

If you want the exit code, call output.status.code() . This correctly returns Some(1) .