title: Cannot change the permission to 600 in iMac
body: <p>I just found that I cannot change the permissions settings in my iMac. The group and other permissions are always the same as the user permissions, so when I type:</p>
  
  <pre><code>chmod 600 a.txt
  </code></pre>
  
  <p>the permissions will become </p>
  
  <pre><code>-rw-rw-rw-  1 chaol1  owner           0 Oct 18 19:43 a.txt
  </code></pre>
  
  <p>If I use - to delete the permissions of group and other, it will not delete them.</p>