title: What's the meaning of `unless -d`?
body: <p>Reading the code,I find this:</p>
  
  <pre><code>use File::Path;
  mkpath($hinitdir) unless -d $hinitdir;
  mkpath($hrestdir) unless -d $hrestdir;
  </code></pre>
  
  <p>So what's the meaning of the option <code>-d</code>?</p>