Linux Permission 101

Linux permissions are one of the first things you should understand if you want to work comfortably and safely on Linux systems.

This guide covers the essentials in a practical way.

1. Identity: Users vs. Groups

Linux is a multi-user operating system. To manage access, each file and directory is tied to:

• A user owner (the primary account that owns it)
• A group owner (a collection of users associated with it)

Why this matters:

• You can keep one primary owner while granting team access through a group.
• You can control security without constantly changing file ownership.

Switching Users with su

You can switch between user accounts with the su command.

su -l username

Using -l (login) is a best practice because it loads that user’s shell environment and starts you in their home directory.

2. Actions: Read, Write, and Execute

Linux permissions are built from three core actions:

• Read (r): View the contents of a file.
• Write (w): Modify a file (and, depending on context, remove or replace it).
• Execute (x): Run a file as a program or script.

These actions are applied separately to:

• Owner
• Group
• Others

3. The Math: Numeric Permissions

Linux shows permissions as letters (rwx), but it stores and applies them as numbers.

Permission	Numeric Value
Read (r)	4
Write (w)	2
Execute (x)	1
None (-)	0

How to Calculate

Permissions always come in three sets: Owner, Group, and Others.

Add the values in each set:

Example: rwx r-x r--

• Owner (rwx) = 4 + 2 + 1 = 7
• Group (r-x) = 4 + 0 + 1 = 5
• Others (r--) = 4 + 0 + 0 = 4

Result: 754

Common Permission Codes

• 777: Everyone can read, write, and execute. High security risk.
• 755: Owner can edit; others can read and execute.
• 644: Owner can edit; others can only read.
• 700: Only owner has access.

Why This Matters in Real Work

You apply numeric permissions with chmod (change mode):

chmod 750 secret.txt

This means:

• Owner gets 7 (read, write, execute)
• Group gets 5 (read, execute)
• Others get 0 (no access)

Quick Mental Model

• Start from least privilege.
• Give only the access needed.
• Avoid 777 unless you fully understand the risk and have a temporary reason.

Once permissions click, Linux administration gets much easier and much safer.