Linux Permissions

🔐 Linux Permissions – Mini Study Guide

This topic is SUPER important kasi this controls who can do what in a Linux system.
If Linux is a house 🏠, permissions decide who can enter, modify, or break stuff.

I. Permission Groups (WHO?)

According to page 1, there are three permission groups.

⚠ Most dangerous? Others (All Users) — kasi that’s everyone.

🔥 Quick Summary

O-G-O
Owner
Group
Others

Memory trick: “Only Good Owners”

II. Permission Types (WHAT can they do?)

There are 3 main types:

🧠 Real-Life Example

If a file is like a Netflix account:

• r → You can watch

• w → You can edit profile

• x → You can actually run the app

🔥 Quick Summary

RWX = Read, Write, Execute

Memory trick:
“Real Writers Execute”

III. Permission Commands

From page 1–5, these are key commands:

💡 Important Ones for Exams

• chmod → change permissions

• chown → change owner

• umask → default permissions

Think:
chmod = change mode
chown = change owner

IV. File Types (Seen in ls -l)

On page 1, file types are shown by first character.

🧠 Example

If you run:

ls -l

And you see:

drwxr-xr-x

That d means directory.

V. Octal Notation (chmod Numbers)

On page 2, octal notation is explained.

Each permission has a number value:

🧠 How It Works

r = 4

w = 2

x = 1

Add them:

rw- = 4 + 2 = 6

r-x = 4 + 1 = 5

rwx = 4 + 2 + 1 = 7

🔥 Common Permission Values (Files)

From page 2:

⚠ Exam Favorite: 755

755 = rwxr-xr-x

Owner → full

Others → read & execute

Common for programs.

🔥 Quick Summary

Think of 3 digits:

[Owner][Group][Others]

Example:

755

7 → Owner

5 → Group

5 → Others

Memory trick:

“Seven is boss, Five can see and run.”

VI. Directory Permission Values

Directories behave differently.

For directories:

• r → list files

• w → create/delete files

• x → enter directory

Example

755 for directory:

Owner → full access

Others → can list files but NOT create/delete

VII. Symbolic Notation (u, g, o)

From page 2–3.

Examples (Page 3)

🧠 How to Read It

Format:

WHO  OPERATOR  PERMISSION

Example:

u + x

Owner add execute

🔥 Quick Summary

Symbolic = letters

Octal = numbers

Memory trick:

UGOA = You Get Ownership Access

VIII. umask Command

This sets default permissions for new files.

Page 3–4 shows binary examples like:

• umask 0002

• umask 0022

• umask 0000

Important Idea

umask subtracts permissions from default.

Example:

umask 0022

Removes write from group and others.

⚡ Common One

umask 022 → results in 755 for directories and 644 for files.

IX. su vs sudo

From page 4.

su

Switch user (usually root).

Example:

su -

You become superuser.

sudo

Run a single command as superuser.

Example:

sudo chmod 600 file.txt

Better security practice.

🔥 Quick Difference

su → full session switch

sudo → one command only

Memory trick:

su = switch user

sudo = super do

X. chown and chgrp

From page 4–5.

chown

Change file ownership.

Examples:

chgrp

Change group ownership only.

Example:

sudo chgrp admins file.txt

XI. passwd

Changes user password.

Example:

passwd
sudo passwd user

🔥 FINAL RECAP (Exam Weapon Mode)

1⃣ Permissions have:

• 3 groups (Owner, Group, Others)

• 3 types (r, w, x)

2⃣ chmod uses:

• Octal (755)

• Symbolic (u+x)

3⃣ 755 = most common

4⃣ 777 = dangerous

5⃣ 700 / 600 = private

6⃣ su vs sudo:

• su = full switch

• sudo = one command

7⃣ chown = change owner

8⃣ chgrp = change group

9⃣ umask = default permissions

🧠 MASTER MEMORY TRICK

Think:

Linux Security = 3-3-3 Rule

3 Groups

3 Permissions

3 Digits

And remember:

RWX = 4-2-1