title: What does !varname do in bash?
body: <p>I have this snippet of bash:</p>
  <p><a href="https://i.stack.imgur.com/SQxJ5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SQxJ5.png" alt="enter image description here" /></a></p>
  <p>I'd understand the snippet if the exclamation mark didn't exist, as it's a simple check of whether <code>varname</code> is empty.</p>
  <p>What the heck is <code>!varname</code>?</p>