title: File executable only for users in a certain group in bash shell
body: <p>I just want to know how to make a file executable only for certain group.</p>
  
  <p>I have a file <code>/bin/ping</code> with this permissions</p>
  
  <pre><code>rwxr-xr-x root root 34248 oct 14 2010 /bin/ping
  </code></pre>
  
  <p>How do I make it only executable for users of a group called "friends", for example?</p>