title: Change file permission in application
body: <p>I try to change file permission in application. Code is below:</p>
  
  <pre><code>Runtime.getRuntime().exec("chmod 777 /sdcard/test.txt");
  </code></pre>
  
  <p>This code NOT works in my application, but no error log.
  I also checked the shell tools under /system/bin, find chmod is under /system/bin, but some other info shown that chmod > toolbox. I am not clear about this. My application has used android:sharedUserId="android.uid.system".
  How to run this code or how to change permission of file? Thanks a lot. </p>