File permissions in Unix-like systems control who can read, write, or execute files and directories, forming the foundation of system security and access control. The POSIX permission model defines three classes (owner, group, others) and three basic operations (read, write, execute), but modern Linux extends this with ACLs for granular control, special permissions for privilege escalation, and mandatory access control systems like SELinux and AppArmor. Understanding how octal and symbolic notation work together, how umask sets defaults, and when to use extended attributes versus capabilities is essential for maintaining secure, properly isolated systems—especially in multi-user environments where effective permissions must be calculated across multiple layers.

---