public void command_line()  {
		while (true) {
			System.out.println("Please input your cmd(input help for info):");
			@SuppressWarnings("resource")
			Scanner sc = new Scanner(System.in);
			String cmd = sc.nextLine();
			switch (cmd) {  //5
			case "upload":  
				upload_cmd();
				continue;
			case "download":
				download_cmd();
				continue;
			case "delete":
				delete_cmd();
				continue;
			case "ls":
				client.list_files();
				continue;
			case "help":
				System.out.println("There are 5 command: upload, download, delete, ls(all files), exits(exit the command)");
				continue;
			case "exits":
				client.NameNodeSerial();
				System.exit(0);				
			default:
				System.out.println("Wrong command.");
				continue;
			}
		}
	}