4.2. POSIX Permissions
As previously mentioned, basic POSIX permissions have been with OS X since its inception. Using POSIX, you can apply access restrictions at three different classes: that of the owner, of the group, and of everyone else. A file references its owner or group based upon an ID that is stored directly with the file. In order to determine the actual owner or group of a file, the OS cross-references the files user and group id against its Directory Services database.
For instance, every OS X system out of the box has a local user "admin," created with user id 501, and the default group "staff," which has an ID of 20. Therefore, any file that is owned by the user id 501 and the group id 20 is owned by the user "admin" and the ...
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