Ok() is handling error message instead of Err() in match expression

Question

I execute commands:

use std::process::Command;

pub fn search(query: &str, flag: &str) -> Vec<String> {
    let command = format!("xdotool search --onlyvisible {} {}", flag, query);
    let output = Command::new("sh").arg("-c").arg(command).output();

    match output {
        Ok(o) => {
            println!("Success message: {:?}", o);

            String::from_utf8_lossy(&o.stdout)
                .lines()
                .map(|s| s.to_owned())
                .collect()
        }
        Err(err) => {
            eprintln!(
                "Error message: {:?}",
                String::from_utf8_lossy(&err.to_string().as_bytes())
                    .trim()
                    .to_owned()
            );
            Vec::new()
        }
    }
}

If I use a wrong flag, I get an error message, but it's being handled of Ok() in the search() function:

Success message: Output { status: ExitStatus(unix_wait_status(256)), stdout: "", stderr: "search: unrecognized ...

Why is this? And how to make Err() handle the error message instead?

Answer 1

Err returned from output() means spawning the process failed. It means nothing about the success/failure of the process itself. You need to check the ExitStatus:

if !o.status.success() {
    // Probably print stderr and bail.
}