// whether process exsit
fn check_process(pidfile: String) -> Option<u32> {
    // read pid from pidfile
    let pid: u32 = read_pid(pidfile);
    if pid == 0 {
        None
    } else {
        let pid_str = pid.to_string();
        let args = vec!["-p", &pid_str, "-o", "pid="];
        let output = Command::new("ps")
            .args(args)
            .output()
            .expect("failed to execute ps -p");
        if output.status.success() {
            let otpt_str = String::from_utf8(output.stdout).unwrap();
            if otpt_str.contains(&pid.to_string()) {
                Some(pid)
            } else {
                None
            }
        } else {
            None
        }
    }
}