File permissions!

In Linux and other UNIX-based operating systems, every file is associated with a user who is the owner. Every file is also associated with a group (a subset of all users) which has an interest in the file and certain rights, or permissions: read, write, and execute.

  • chown – Used to change user ownership of a file or directory
  • chgrp – Used to change group ownership
  • chmod – Used to change the permissions on the file, which can be done separately for owner, group and the rest of the world (often named as other)

Files have three kinds of permissions: read (r), write (w), execute (x). These are generally represented as in rwx. These permissions affect three groups of owners: user/owner (u), group (g), and others (o).

As a result, you have the following three groups of three permissions:

rwx: rwx: rwx u: g: o

There are a number of different ways to use chmod.u stands for user (owner), o stands for other (world), and g stands for group.

This kind of syntax can be difficult to type and remember, so one often uses a shorthand which lets you set all the permissions in one step. This is done with a simple algorithm, and a single digit suffices to specify all three permission bits for each entity. This digit is the sum of:

  1. 4 if read permission is desired
  2. 2 if write permission is desired
  3. 1 if execute permission is desired.

Thus, 7 means read/write/execute, 6 means read/write, and 5 means read/execute.

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