title: Why & How fish does not support POSIX?
body: <p>I have heard about fish that it's a friendly and out-of-box shell but also it doesn't support POSIX standard. 
  On the other hand I read about POSIX standard (and also I tested it on my Fedora, It's amazing and out-of-box shell now I want to change my default shell to fish).</p>
  
  <p>But the matter that I opened this question for is: I misunderstood about relation between fish and POSIX standard, what do you mean about fish does NOT support POSIX exactly? &amp; How? (Should I change my bash to fish?). </p>
  
  <p><em>Please explain it simple 'cause I'm a little newbie, thanks.</em></p>