我写了这个脚本，但不起作用：

package com.mkyong.android;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;
import java.io.IOException;
import com.example.toast.R;

public class MainActivity extends Activity {

private Button button;

public void onCreate(Bundle savedInstanceState) {
    final Runtime runtime = Runtime.getRuntime();
    try {
        runtime.exec("su");
    }
    catch (IOException e) {
        e.printStackTrace();
    }
    super.onCreate(savedInstanceState);
    setContentView(R.layout.tab1);

    button = (Button) findViewById(R.id.button1);
    button.setOnClickListener(new OnClickListener() {
        @SuppressLint("SdCardPath")
        @Override
        public void onClick(View arg0) {
            final Runtime runtime = Runtime.getRuntime();
            try {
                runtime.exec("echo 3 > /proc/sys/vm/drop_caches");
                Toast.makeText(MainActivity.this, "Script lanciato con `successo, memoria svuotata.", Toast.LENGTH_LONG).show();`
            }
            catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}
}

不释放RAM内存：（但通过终端模拟器去。.如果我尝试更改命令，例如，使一个目录与mkdir去，去甚至一个文件txt的写作。.怎么了？