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Understanding Unix file permissions

Every file on Linux and macOS has three permission sets — for the owner, the group and everyone else — each combining read (4), write (2) and execute (1). Octal notation sums these: 7 means full access, 5 means read and execute, and 644 gives the owner write access while everyone else can only read. Directories also need the execute bit to be entered. The special setuid, setgid and sticky bits add a fourth leading digit and control advanced behaviour such as shared folders.