2.4 Permission Management

In Linux, every file and directory is owned by a User and a Group, and has permissions for User (u), Group (g), and Others (o).

Viewing Permissions

Run ls -l:

-rwxr-xr--  1 alice developers  2048 Jan 01 12:00 script.sh

• -: File type (- = file, d = directory).
• rwx: User permissions (Read, Write, Execute).
• r-x: Group permissions (Read, no Write, Execute).
• r--: Others permissions (Read only).
• alice: Owner.
• developers: Owning Group.

Changing Permissions (chmod)

Symbolic Mode

Easy to read.

• chmod u+x script.sh: Add execute for user.
• chmod g-w file.txt: Remove write for group.
• chmod o=r file.txt: Set others to read only.

Octal Mode

Professional shortcut.

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

Common patterns:

• 755 (rwxr-xr-x): Standard for scripts/programs. User full, others read/exec.
• 644 (rw-r--r--): Standard for data files.
• 600 (rw-------): Secure (SSH keys). Only owner can read/write.
• 777: Everyone can do everything. Avoid this!

  Danger

  Avoid 777 permissions in production as it allows anyone to modify or execute the file.

Changing Ownership (chown)

Only root can change ownership of files.

Caution

Changing file ownership can break applications if not done carefully.

# Change owner to bob
sudo chown bob file.txt

# Change owner to bob and group to staff
sudo chown bob:staff file.txt

```bash
# Recursive (folder)
sudo chown -R bob:staff /var/www/html

Special Permissions

Beyond standard read/write/execute, there are three special permissions:

Permission	Code	Function on File	Function on Directory
SUID (Set User ID)	s (4)	Run as the file owner (e.g., passwd).	N/A
SGID (Set Group ID)	s (2)	Run as the group owner.	New files inherit the group.
Sticky Bit	t (1)	N/A	Only owner can delete their own files (e.g., /tmp).

Setting Special Permissions

• Symbolic:
  • chmod u+s file (SUID)
  • chmod g+s dir (SGID)
  • chmod +t dir (Sticky)
• Octal (Fourth digit prefixed):
  • chmod 4755 file (SUID)
  • chmod 2755 dir (SGID)
  • chmod 1777 dir (Sticky)

Default Permissions (umask)

When you create a file or directory, it gets default permissions determined by the umask.

• Standard umask: 0022 (or 022).
• Calculation:
  • Files start at 666. 666 - 022 = 644 (rw-r--r--).
  • Directories start at 777. 777 - 022 = 755 (rwxr-xr-x).
• Changing: umask 027 (results in strict permissions).