I noticed by mistake one day this undocumented shorthand for chown:chown user. filenameWhich is the shorthand for:chown user:user filenameThe latter was a real pain to type when running it often, so the shorthand came in very handy. I tried looking for its documentation in the docs but never found a hint about that hidden feature. Is this a POSIX compliant shorthand? Is it chown that parses it differently or a bash expansion of some sort? I'm using Debian 8.4 but I've been using that for many years already. Is it debian-implementation specific? 
Undocumented chown user. shorthand
debian;chown
Thanks to @JeffSchaller for the answer.Some older scripts may still use . in place of the : separator.  POSIX 1003.1-2001 (see Standards conformance) does not require support  for that, but for backward compatibility GNU chown supports . so  long as no ambiguity results. New scripts should avoid the use of .  because it is not portable, and because it has undesirable results if  the entire owner.group happens to identify a user whose name  contains ..This behavior is specific to GNU coreutils' chown command, and is not a function of bash. Being a GNU utility, it is also not Debian-specific.