Linux File Permissions Explained: Users, Groups, and chmod Basics

In Linux, every user has a unique UID and belongs to at least one group, with the kernel using these groups to enforce file permissions. Each file has three permission categories — owner user, owner group, and others — each assigned read, write, and execute rights represented in octal notation. Permissions are expressed as three-digit values, such as 755, where each digit is the sum of read (4), write (2), and execute (1) for each category. Commands like chmod, chown, and chgrp allow modification of permissions and ownership, though most require root access or specific group membership to run. The root user bypasses standard permission checks and retains read and write access even on files with 000 permissions, though it still cannot execute such files without explicitly granting execute permission.