Linux (Ubuntu) = Unix (NOT IMPORTANT - NO RUSH)

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http://www.unix.com – I'm learning off Linux (Ubuntu) right now. I want to move up to Unix, but I don't want to rush like I did when it came to Windows --> to Linux. What is the best Unix OS that fits in pretty well with Ubuntu. In other words is there kind of an equal Linux with Unix? Also what do I need to really know about Unix? Would it be good to do Virtual Box in Linux (Ubuntu) to play around with Unix when starting out? I'm trying to break my need of a GUI, but I would still build a GUI upon Unix to understand it. I'm just not quite sure on this. I really want an understanding Unix before (HowTos)