If you'd like to wait for a process::Child to finish, you must call Child::wait, which will return a process::ExitStatus.

use std::process::Command;

fn main() {
    let mut child = Command::new("sleep").arg("5").spawn().unwrap();
    let _result = child.wait().unwrap();

    println!("reached end of main");
$ rustc wait.rs && ./wait
# `wait` keeps running for 5 seconds until the `sleep 5` command finishes
reached end of main