title: grep from the beginning of a file (grep -f )
body: <p>I use <code>commands | grep -f - file</code> to extract piped content from the file.<br>
  However I want to extract only if the matching string is in the beginning of the line. Usually I use <code>grep '^string'</code>, but it doesn't work with <code>grep -f</code>.  </p>
  
  <pre><code>grep -f '^-' file
  grep: ^-: No such file or directory
  </code></pre>
  
  <p>How can I use <code>grep -f -</code> and <code>grep '^'</code> together?  </p>