Linux permissions control who can read, write, and execute files.
Permission model:
- User (u): File owner
- Group (g): Group owner
- Other (o): Everyone else
- Modes: Read (), Write (), Execute ()
Commands:
chmod 755 file: rwxr-xr-xchown user:group file: Change ownership
Special permissions:
- SUID: Execute as file owner (e.g.,
passwd) - SGID: Execute as group owner
- Sticky bit: Only owner can delete files in directory (e.g.,
/tmp)
Interview question: "Why does /usr/bin/passwd have SUID set?" It needs root privileges to modify /etc/shadow.