Linux File Permissions

Every file and directory in Linux has permissions assigned to three categories:

1. User (Owner): A user is the owner of the file. By default, the person who created a file becomes its owner.
2. Group: A user- group can contain multiple users. All users belonging to a group will have the same Linux group permissions access to the file.
3. Others: Any other user who has access to a file.

Each category has three types of permissions:

• r (Read) → Can view the file contents
• w (Write) → Can modify the file
• x (Execute) → Can run the file (if it's a script or program)

Trick to Remember Permission Numbers

Linux uses a 3-digit numeric system to represent permissions. The digits come from adding these values:

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

Each digit represents the sum of allowed permissions for User, Group, and Others.

Examples:

Permission	Numeric Code	Explanation
---	0	No permissions (0)
--x	1	Execute only (1)
-w-	2	Write only (2)
-wx	3	Write + Execute (2+1)
r--	4	Read only (4)
r-x	5	Read + Execute (4+1)
rw-	6	Read + Write (4+2)
rwx	7	Read + Write + Execute (4+2+1)

Example Breakdown: -rw-r--r--

-rw-r--r-- 1 foyez staff 1156 Feb 25 14:06 Makefile

• - → This is a file (if it were a directory, it would be d)
• rw- (User) → Read & Write (4+2 = 6)
• r-- (Group) → Read only (4)
• r-- (Others) → Read only (4)

How to Change Permissions?

Use the chmod command:

chmod 755 file.sh

This sets:

• User: rwx (7)
• Group: r-x (5)
• Others: r-x (5)