This is called brace expansion: <a href="http:&#x2F;&#x2F;wiki.bash-hackers.org&#x2F;syntax&#x2F;expansion&#x2F;brace" rel="nofollow">http:&#x2F;&#x2F;wiki.bash-hackers.org&#x2F;syntax&#x2F;expansion&#x2F;brace</a><p>Very useful stuff. E.g. <i>mv nginx.conf{,.old}</i>