title: What does "!S:~%I%,1!"=="" mean?
body: <p>I found some sample code but I am unable to get what this <code>if</code> condition means:</p>
  
  <pre><code>set /p sourceDB=Enter Source DB: %=%    
  set S=%sourceDB%
  set I=0
  set L=-1
  :l ----- Forget about this line
  if "!S:~%I%,1!"=="" goto ld
  if "!S:~%I%,1!"=="/" set K=%I%
  if "!S:~%I%,1!"=="@" set Z=%I%
  if "!S:~%I%,1!"==":" set Y=%I%
  set /a I+=1
  goto l
  </code></pre>