等待 Wait

如果你想等待 process::Child 完成，就必须调用 Child::wait，这会返回一个 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
reached end of main
# `wait` keeps running for 5 seconds
# `sleep 5` command ends, and then our `wait` program finishes