$UserName gives the login name of the user who invoked the Mathematica kernel, as recorded by the operating system.
On Unix and similar operating systems, $UserName is derived from the UID associated with the Mathematica kernel process.
On operating systems where no login name can be found, $UserName is None.
See Section 2.14.5.
See also: $HomeDirectory, Environment, $UserBaseDirectory.
New in Version 3.