Linux file permissions can feel cryptic at first, but once you break them down, they’re not only simple — they’re powerful. Whether you're building systems, writing scripts, or deploying code, understanding how to control file access is a must-have skill. This guide will walk you through everything you need to know.

Why File Permissions Matter

Linux is a multi-user system. Proper permissions:

  • Keep your system secure

  • Prevent accidental or malicious changes

  • Allow collaboration without compromising safety

Understanding rwx and User Classes
Permissions are shown as:

-rwxr-xr--

Breakdown:

  • First char: - (file) or d (directory)

  • Next 3: rwx for User (owner)

  • Next 3: r-x for Group

  • Last 3: r-- for Others

User Classes

  • u = User (owner)

  • g = Group

  • o = Others

  • a = All (u+g+o)

Permission Types

  • r = Read

  • w = Write

  • x = Execute

  • -= No permission

Viewing File Permissions

Using ls

ls -l /etc/passwd
-rw-r--r-- 1 root root 2871 Aug 22 14:43 /etc/passwd

Using stat

stat /etc/shadow
Access: (0640/-rw-r-----)  Uid: (0/root)   Gid: (42/shadow)

Modifying Permissions

Symbolic (Relative) Mode

chmod u+x file.txt        # Add execute for user
chmod g-w file.txt        # Remove write from group
chmod o-r file.txt        # Remove read from others
chmod a+r file.txt        # Add read for all

Octal (Absolute) Mode

User Group Other Octal Command Meaning
rwx rwx rwx 777 chmod 777 file.txt Full permissions to everyone
rwx r-x r-x 755 chmod 755 file.txt Owner: all, Others: read/exec
rw- r-- r-- 644 chmod 644 file.txt Owner: read/write, rest: read

Setting Permissions from Another File

chmod --reference=file1 file2

Recursive Permissions

chmod -R u+rw,o-rwx mydir/

Special Permissions

SUID (Set User ID)
Executes file with owner’s privileges.

chmod u+s file
chmod 4755 file

Example:

ls -l /usr/bin/umount
-rwsr-xr-x 1 root root 39144 /usr/bin/umount

SGID (Set Group ID)
Runs with group’s privileges, or maintains group ownership in directories.

chmod g+s dir
chmod 2750 dir

Sticky Bit
Only file owner can delete their files in shared directories.

chmod +t dir
chmod 1777 dir

Example:

ls -ld /tmp
drwxrwxrwt 10 root root 4096 /tmp

UMASK: Default Permissions

View Current UMASK

umask

Set New UMASK

umask 0022

How it Works
UMASK subtracts permissions from 666 (files) or 777 (dirs).

Ownership Commands

Change Owner

chown new_user file

Change Group

chgrp new_group file

Change Both

chown user:group file

Recursive Ownership Change

chown -R user:group dir

Bonus: File Attributes (Advanced Layer)

View Attributes

lsattr file

Change Attributes

sudo chattr +i file   # Make file immutable
sudo chattr -i file   # Make it editable again

Cheat Sheet Summary

  • chmod → change permissions

  • chown / chgrp → change ownership

  • umask → set default permissions

  • ls -l / stat → view permissions

  • +x, -w, a+r → symbolic changes

  • 644, 755, 777 → octal changes

  • SUID, SGID, Sticky Bit → special bits

Conclusion

Linux permissions are a superpower once you understand the logic. Master these commands, practice regularly, and you’ll never get caught off-guard by a “Permission denied” again.