Explain the principle of access control system operation in Linux.

In Linux, the access control system is based on a model of user and group permissions. Each file and directory has three sets of permissions — for the owner (user), group (group), and others (others). Permissions include read (r), write (w), and execute (x).

Permissions are set using bits and can be viewed with the command ls -l.

In addition to basic permissions, there are extended mechanisms:

• ACL (Access Control Lists) — allow for more fine-grained permissions for individual users and groups.
• SELinux/AppArmor — mandatory access control systems that impose additional security policies.

An example of basic permissions:

-rwxr-xr-- 1 user group 1234 May 10 12:00 file.txt

Here, the owner has rwx, the group has r-x, and others have r--.

Thus, access to resources is controlled through permissions, owners, groups, and additional security policies.