UNIX File Access Control
Inodes · Permission Bits · Special Bits · Scenarios
In UNIX, every file has an inode: a small record on disk that stores ownership, timestamps, and 12 permission bits. Nine bits control read, write, and execute access for three categories (owner, group, others) and three special bits add behaviours like running a program with elevated privileges (SetUID) or protecting a shared directory from accidental deletion (Sticky). Use the builder to experiment, then test yourself with the scenario challenges.
The highlighted fields are directly involved in access control. When a process tries to access a file, the kernel compares the process’s UID/GID against the inode to decide whether the owner, group, or others permissions apply.
Permission Bit Builder
Click any bit to toggle it on or off. The ls -l output, octal value, and
chmod command update in real time.
Examples
Read each situation and set the permission bits accordingly. Hit Check to see if you got it right, or Reveal to see the answer and the hint.
Cheatsheet
Add the active values per group: r=4, w=2, x=1.
Example: rwx = 7, r-x = 5, r-- = 4.
The octal is written as [special][owner][group][others], e.g. 0755.