title: linux shell test `-d` on empty argument
body: <p>Here is the code,</p>
  
  <pre><code>x=
  if [ -d $x ]; then 
      echo "it's a dir"
  else
      echo "not a dir"
  fi
  </code></pre>
  
  <p>The above code gives me <code>"it's a dir"</code>, why? <code>$x</code> is empty, isn't it?</p>