String cmd = "crontab  -l";
          try
        {
             Process child = Runtime.getRuntime().exec(cmd);
             InputStream in = child.getInputStream();
             int c;
             while ((c = in.read()) != -1) {
              //写文件
             }
             in.close();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }