title: How to copy files to a remote machine through a jumping machine by scp command
body: <p>say that the target machine is <code>user@B.com</code> and the only place to login is <code>user@A.com</code>(usually called jumping machine)<br>
  and how can I copy local files to machine <strong>B</strong> using the <em>scp</em> command<br>
  I've tried <code>scp files user@B.com</code> but nothing happens,it only told me sort of <em>Operation Time Out</em>  </p>