user-full-name

user-full-name is a built-in function in `C source code'.

(user-full-name &optional UID)

Return the full name of the user logged in, as a string.
If the full name corresponding to Emacs's userid is not known,
return "unknown".

If optional argument UID is an integer or float, return the full name
of the user with that uid, or nil if there is no such user.
If UID is a string, return the full name of the user with that login
name, or nil if there is no such user.