The Owner / Group / Other Model

Every Linux file has:

• An owner (user ID) — usually who created it.
• A group — the file's group ID.
• A set of permission bits for owner, group, and others.

Three permissions per category: read (r), write (w), execute (x).

Meaning per Object Type

For directories the x bit is critical — without it you cannot cd in even if you have r.

Reading ls -l

-rwxr-xr-- 1 alice dev 1024 Apr 30 12:00 script.sh

Decoding the first 10 characters:

The number after the type is the link count (1 here). Then owner (alice), group (dev), size, modification time, name.

Special Bits

Beyond rwx for u/g/o, three special bits exist:

• Setuid (s on owner x) — when an executable runs, the process's effective UID becomes the file's owner. /usr/bin/passwd is a classic setuid-root program.
• Setgid (s on group x) — sets effective GID; on directories, new files inherit the directory's group.
• Sticky bit (t on others x) — on directories, only the file owner (or root) can delete entries — used on /tmp.

Examining Permissions

ls -l is the daily tool. Other useful commands:

• stat file — full metadata in human form.
• namei -l /path/to/file — walks the path and prints permissions of each component.
• getfacl file — extended ACLs (when present).
• id — your UID/GID and supplementary groups.
• groups user — groups a user belongs to.

Default Permissions and umask

New files are created with mode (0666 & ~umask) and directories with (0777 & ~umask). With umask 022, files become 644 (rw-r--r--) and directories 755 (rwxr-xr-x). Show with umask; set with umask 077 (private).

Worked Example

Suppose /home/alice/private.txt shows -rw-------. User Bob runs cat /home/alice/private.txt:

1. The kernel checks the path components — Bob needs x on /, /home, /home/alice. If /home/alice is drwx------, Bob is denied at the directory.
2. Even if Bob could traverse, the file's mode forbids group/others reading.

So privacy comes from the combination of directory-traversal and file-mode permissions.

Summary

• Three triplets (u/g/o) × three bits (r/w/x) define basic permissions.
• Directory x bit is required to traverse, separate from r for listing.
• Special bits (setuid, setgid, sticky) cover privileged execution and shared directories.