Noooo! You pointed to the elephant in the room!
Unix is basically a subset of SU and SF. Programming questions are typically closed as belonging on SO.
Ubuntu questions can be asked here or on AU, but with different expectations on the answers: AU answers tend to focus on the software and UI that's installed by default, and don't make any effort to apply to non-Ubuntu systems. Unix.SE answers tend to be more generic, as the answerer might know nothing about Ubuntu but suggest a generic method that works on all Linux distributions, say.
For Unix-related questions, Unix.SE bridges the SU-SF split. It's also a way to make unix questions easy to find by people who don't want to sort through the myriad unrelated questions on SU and SF (this is mostly an issue with SU's abudance of poorly-tagged questions).
I wonder if there is a place in the Stack Exchange ecosystem for a dumping ground for computer questions. SU would be it. Between Unix.SE, a yet-to-be created site for Windows, and a yet-to-be created site for computer hardware (IIRC there's a SE 1.0 site that never took on), almost all the topics of SU would be covered. It would be sad not to have a good place for “alternative” OSes, but SU doesn't attract experts on these topics anyway, because they represent a tiny proportion of questions and they're not easy to find. More problematic is the lack of a good place to discuss interactions between OSes (virtual machines, networking, emulation, ...).
Where to ask shell questions has been inconclusively debated before, e.g. Where questions about linux shell commands go? and I'm sure I've seen other questions. In practice people continue to ask on all three sites, four now with Unix.SE.
