title: Unix tr command to convert lower case to upper AND upper to lower case
body: <p>So I was searching around and using the command <code>tr</code> you can convert from lower case to upper case and vice versa. But is there a way to do this both at once?</p>
  
  <p>So:</p>
  
  <pre><code>$ tr '[:upper:]' '[:lower:]' or  $ tr A-Z a-z
  </code></pre>
  
  <p>Will turn <em>"Hello World ABC"</em> to <em>"hello world abc"</em>, but what I want is <em>"hELLO wORLD abc"</em>.</p>