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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: can anyone explain the rationale for why a single user can own a file? Isn't having group/other permissions enough? With your answer, I'd be happy if you reference to discussions I can read on this topic.

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closed as not constructive by Ƭᴇcʜιᴇ007, bwDraco, Indrek, Dave, Renan Oct 8 '12 at 14:17

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Surely this is sort of self evident. If I create a file on a multiuser machine, which I want kept private, then I want to be able to specify that in the permissions. This is especially important with user specific data such as private keys.

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It also specifies your own relationship to the file: you can make a file read only if you don't want to change it accidentally (although this doesn't prevent deletion). Or chmod 000 to make it unreadable (mostly useless, except for debugging) – pjc50 Oct 8 '12 at 13:55

This is especially important if there are isn’t a specific group for each user (as there is today on, say, Debian, where a group userx is created when userx is created). Furthermore, it allows to give access to both a specific user, who may then give access to another group of which he is not necessarily a member without having to add himself to that group, something that can only be done by root.

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