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attr has more to do with a how a file is treated regardless of who accesses it.
acl has more to do with WHO can access it and what access they have to the file. It is like permissions (see chmod) on steroids.
That is to say the first one focuses on attributes of the file irrespective of users and the second focuses on permissions for users. You can set attr and acl on the same file.
Right but acl, seems to be have the same concept only applied to 'ownership'. More like ownership oriented pre-defined attributes. Yet I don't see any utilities that will use the arbitrary attribute except a custom written backend that will know this attributes.
The names seem pretty self-explanatory IMHO. Extended attributes are an extension of normal attributes (read, write and execute) and have absolutely nothing to do with ACLs -- my favorite is immutable BTW... even root can't delete a file with that set.
The "attr" is set on the file so it doesn't matter what the utility thinks should be done therefore nothing has to be written in the application to deal with it - the filesystem itself deals with it. he file's attr determines it. There is no "permission" in "attr" as there is in "acl". In the latter your utility has to be aware of acls in order to properly interpret them. In the former your utility doesn't need to interpret anything because the filesystem will determine what to do based on the file's attr regardless of what the utility wants to do.