title: Explanation of colon operator in ": ${foo=value}"
body: <p>I understand the colon operator in bash that acts like <a href="http://devmanual.gentoo.org/tools-reference/bash/index.html#multiple-selection" rel="noreferrer" title="Bash Conditionals">a null</a>, and I know it's used in <a href="http://www.faqs.org/docs/bashman/bashref_29.html" rel="noreferrer" title="Shell Parameter Expansion">parameter expansion</a>, as well as being used other ways, but can someone explain this: </p>
  
  <pre><code>: ${SOMETHING='value'}
  </code></pre>
  
  <p>From experimentation I know that this sets the environment variable <code>$SOMETHING</code> to <code>'value'</code> but why?</p>
  
  <p>"Just because it does" is a valid answer but then please point me to the documentation for it (which I can't seem to find) or a proper name for this usage would be useful. I'm hoping there's a more enlightening explanation though. </p>