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$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 the Mathematica book: Section 2.13.5.
See also: $HomeDirectory, Environment.
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