Permissions in Linux

Special Permissions

Special permissions add extra behavior beyond normal rwx rights:

setuid: Executable runs as the file owner.
setgid: Executable runs as the file’s group; directories enforce group inheritance.
sticky bit: In a directory, only root, the file owner, or the directory owner can delete/rename files—even if others have 777.

Symbolic vs Numeric Notation

Symbolic: u+s, g+s, +t
Numeric: First digit in chmod represents special bits:
- 4 = setuid
- 2 = setgid
- 1 = sticky bit
Combine them by addition:
- chmod 4755 file → setuid + rwxr-xr-x
- chmod 2755 dir → setgid + rwxr-xr-x
- chmod 1755 dir → sticky + rwxr-xr-x
- chmod 6755 file → setuid + setgid + rwxr-xr-x

Permission Table

Special Bit	Effect	Symbolic	Numeric	Example Command	Example `ls -l` Output
setuid	Executable runs with owner’s UID	`u+s`	`4000`	`chmod 4755 /usr/bin/sudo`	`---s--x--x. 1 root root ... /usr/bin/sudo`
setgid	Executable runs with file’s GID; directories enforce group inheritance	`g+s`	`2000`	`chmod 2555 /usr/bin/wall`	`-r-xr-sr-x. 1 root tty ... /usr/bin/wall`
sticky bit	In directory: only root, file owner, or dir owner can delete/rename files	`+t`	`1000`	`chmod 1777 /tmp`	`drwxrwxrwt. 11 root root ... /tmp`

✅ Quick Visual for Numeric Notation

chmod [special][owner][group][others]
          ^       ^      ^      ^
          |       |      |      |
          |       |      |      └─ others permissions (rwx)
          |       |      └──────── group permissions (rwx)
          |       └─────────────── owner permissions (rwx)
          └─────────────────────── special bits (4=setuid, 2=setgid, 1=sticky)

✅ Key Points

Sticky bit only affects deletion/rename, not read/write/execute.
s replaces x in user/group execute position when setuid/setgid is active.
t replaces x in others execute position when sticky bit is active.