Permission Bits and Access Control Lists

Let's discuss how files are shared between different users, and how permission bits work.

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The abstraction of a process provided two central virtualizations: of the CPU and of memory. Each of these gave the illusion to a process that it had its own private CPU and its own private memory. In reality, the OS underneath used various techniques to share limited physical resources among competing entities in a safe and secure manner.

The file system also presents a virtual view of a disk, transforming it from a bunch of raw blocks into much more user-friendly files and directories, as described within this chapter. However, the abstraction is notably different from that of the CPU and memory, in that files are commonly shared among different users and processes and are not (always) private. Thus, a more comprehensive set of mechanisms for enabling various degrees of sharing are usually present within file systems.

Permission bits

The first form of such mechanisms is the classic UNIX permission bits. To see permissions for a file foo.txt, just type:

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