title: Expand environment variables in path
body: <p>I want to expand environment variables in R like I can do this in Python:</p>
  
  <pre class="lang-py prettyprint-override"><code>os.path.expandvars('$WHATEVER/hello/world')
  </code></pre>
  
  <p>However, the same does not work with <code>path.expand()</code></p>
  
  <pre><code>path.expand('$WHATEVER/hello/world')
  </code></pre>
  
  <p>How can I do it anyway?</p>