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Is there a reason why 'owner' permissions exist? Aren't group permissions enough?

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linux-howto

http://unix.stackexchange.com – I think I rather understand how file permissions work in linux. However, I don't really understand why they are split into three levels and not into two. I'd like the following issues answered: Is this deliberate design or a patch? That is - was the owner/group permissions designed and created together with some rationale or did they come one after another to answer a need? Is there a scenario where the user/group/other scheme is useful but a group/owner scheme will not suffice? Answers to the first should quote either textbooks or official discussion boards. Use cases I have considered (HowTos)