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First of all, when confused read the FAQ:
It's not unthinkable that a question is suitable for more than one sites, in which case read their FAQs a second time and post in the one that feels a bit more suitable. If it's a good question, you shouldn't worry much about asking it at the wrong site, we can migrate questions automatically. If it's a bad question, well...
Now Unix and Linux and AskUbuntu overlap a little bit more than any two other sites would, and I think your question would be equally appropriate in either one. Personally I'd go with Unix and Linux, because I don't see anything Ubuntu specific in your question, but that doesn't necessarily mean it's off topic on AskUbuntu. In fact, it's already getting good answers.