java 执行 terminal 命令无效

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here
    try{            
        Runtime r=Runtime.getRuntime();
        String[] cmd={"ls","~/Desktop"};            
        Process p=r.exec(cmd);
        //p.wait();
        p.waitFor();
        InputStreamReader ir=new InputStreamReader(p.getInputStream());
        BufferedReader br=new BufferedReader(ir);
        String line=null;
        while ((line=br.readLine())!=null){
            System.out.println(line);
        }
    }
    catch (IOException e){
        e.printStackTrace();
    }
    catch(InterruptedException e){
        e.printStackTrace();
    }
}