Mastering Linux File Permissions: The Ultimate Guide.

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-- -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.

Apr 15, 2025 - 10:30

Mastering Linux File Permissions: The Ultimate Guide.

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--

-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.