java怎么编写 sync; echo 1 > /proc/sys/vm/drop_caches 执行Linux清除缓存命令

Question

想手动清除linux系统的缓存，调用Process来执行：

    //sync; echo 1 > /proc/sys/vm/drop_caches 
      ProcessBuilder pb0 = new ProcessBuilder("sync",";","echo","1",">","/proc/sys/vm/drop_caches");
        try {
            Process ps0 = pb0.start();//Starting a new process.
            InputStream is = ps0.getInputStream();
            BufferedReader br = new BufferedReader(new InputStreamReader(is));  
            String line = null;  
            while( (line = br.readLine())!=null){
                System.out.println(line);
            }
            ps0.waitFor();//Causes the current thread to wait, if necessary, until the process represented by this Process object has terminated.
            ps0.destroy();//Kills the subprocess.
        } catch (Exception e) {
            e.printStackTrace();
        }
        
        可发现运行起来，通过free -m来查看，并没有真正的清除linux系统的缓存。
        
        要怎么才能真正实现清除缓存呢？