How to get the system user with User ID

Hi,

my problem is i want to get the real system user from UserID.

I want to know which user is from which Position.

Example:

"WF-MM-4" is the user and "50012181" is a Position in a organisation.

I want to get the user from "50012181" in ABAP CODE. Not in Transaction.

I hope you can help me!