title: Ruby If and Unless conditional explanation
body: <p>I want to ask for a little explanation about if and unless, I know the two of this is an opposite of each other. All of this time I have used both without no problem until now I find something weird. Let's check my code :</p>
  
  <pre><code>ausysdir = AUDIO_SYSTEM_FOLDER_NAME 
  
  Dir.mkdir(ausysdir) if ausysdir != "" &amp;&amp; !File.directory?(ausysdir)     # Number 1 
  Dir.mkdir(ausysdir) unless ausysdir == "" &amp;&amp; File.directory?(ausysdir)  # Number 2
  </code></pre>
  
  <p>Now in the directory I work the folder <strong>ausdyr</strong> is already exist. 
  But from the code above the the number one conditional which is <strong>if</strong> not return an error(this is what I expected, so great), But not with number 2 conditional which is <strong>unless</strong>, when I think that <strong>if</strong> and unless is an opposite, then my syntax above should be correct, am I right? Or my syntax is wrong? Or <strong>Unless</strong> have a different behaviour? Or do I missed something here? </p>
  
  <p>Thank you very much.</p>
  
  <p>EDIT : When I mean error, is an exception because the file is already exist.</p>