chown changes the user and/or group ownership of each given file
as specified by the first non-option argument as follows: if only a user name
(or numeric user ID) is given, that user is made the owner of each given file,
and the files' group is not changed. If the user name is followed by a colon
and a group name (or numeric group ID), with no spaces between them, the group
ownership of the files is changed as well.
The GNU version allows a dot instead of a colon (following BSD). [This was not
allowed by POSIX since a dot is a valid character in a user name.] If a colon
or dot but no group name follows the user name, that user is made the owner of
the files and the group of the files is changed to that user's login group. If
the colon or dot and group are given, but the user name is omitted, only the
group of the files is changed; in this case, chown performs the same
function as chgrp.