👤 Users and Permissions¶

Understanding how Linux manages users, groups, ownership and access control.

Table of Contents¶

Why Users Exist
Users
Groups
Root User
Sudo
Authentication vs Authorization
Ownership
Permission Model
Read, Write and Execute
Viewing Permissions
chmod
Numeric Permissions
Symbolic Permissions
chown
Useful User Commands
Useful Group Commands
Security Principles
Mental Model

1️⃣ Why Users Exist¶

Linux was designed as a multi-user operating system.

Without users and permissions:

Everyone could access everything

which would be a major security risk.

Users allow Linux to identify:

who is performing an action
who owns a file
who can access a resource

2️⃣ Users¶

A user represents an identity inside Linux.

Examples:

sara
student
admin

Users can:

own files
run processes
belong to groups
access resources

Every action performed on Linux is associated with a user.

Viewing Current User¶

whoami

Displays the current logged-in user.

Example:

sara

Viewing User Information¶

id

Displays:

User ID (UID)
Group ID (GID)
Group memberships

Example:

uid=1000(sara)
gid=1000(sara)
groups=1000(sara),27(sudo)

3️⃣ Groups¶

Groups allow permissions to be assigned to multiple users simultaneously.

Instead of managing:

Sara
John
Maria

individually, Linux can use:

developers

and assign permissions to the group.

Viewing Groups¶

groups

Displays groups associated with the current user.

Creating Groups¶

groupadd developers

Creates a new group.

Adding Users to Groups¶

usermod -aG developers sara

Adds a user to a group.

Why Groups Matter¶

Groups simplify permission management.

Instead of assigning permissions individually, permissions can be assigned once to a group.

4️⃣ Root User¶

Root is the administrator account.

Root has unrestricted access.

Root can:

install software
create users
remove users
modify system configuration
change permissions

Think of root as:

The owner of the entire system

5️⃣ Sudo¶

Instead of logging directly as root, Linux commonly uses:

sudo command

Example:

sudo apt update

Sudo allows:

Temporary administrator privileges

Benefits:

accountability
logging
better security

6️⃣ Authentication vs Authorization¶

These concepts are different.

Authentication¶

Answers:

Who are you?

Examples:

Passwords
SSH Keys

Authorization¶

Answers:

What are you allowed to do?

Examples:

Read files
Modify files
Install software

Authentication identifies.

Authorization grants permissions.

7️⃣ Ownership¶

Every file has:

an owner
a group owner

Example:

Owner: sara
Group: developers

Ownership determines who controls the resource.

Viewing Ownership¶

ls -l

Example:

-rw-r--r-- 1 sara developers file.txt

8️⃣ Permission Model¶

Linux uses three permission categories.

Owner
Group
Others

Each category receives different permissions.

9️⃣ Read, Write and Execute¶

Linux permissions are built around three actions.

Read (r)¶

Allows viewing content.

Examples:

open files
view directories

Write (w)¶

Allows modifications.

Examples:

edit files
create files
delete files

Execute (x)¶

Allows execution.

Examples:

run programs
run shell scripts

🔟 Viewing Permissions¶

Example:

-rwxr-xr--

Breakdown:

rwx | r-x | r--

Owner | Group | Others

Meaning:

Owner:

Read
Write
Execute

Group:

Read
Execute

Others:

Read

1️⃣1️⃣ chmod¶

chmod means:

Change Mode

Used to modify permissions.

Example:

chmod 755 script.sh

Changes file permissions.

Making a script executable:

chmod +x script.sh

One of the most common uses of chmod.

Removing write permission:

chmod -w file.txt

1️⃣2️⃣ Numeric Permissions¶

Linux can represent permissions numerically.

Common values:

Relationship:

r = 4
w = 2
x = 1

Examples:

7 = rwx
6 = rw-
5 = r-x
4 = r--

Example:

chmod 644 file.txt

1️⃣3️⃣ Symbolic Permissions¶

Instead of numbers, permissions can be modified symbolically.

Examples:

chmod +x script.sh
chmod -w file.txt
chmod g+w file.txt

Symbolic mode focuses on:

Adding or removing permissions

1️⃣4️⃣ chown¶

chown means:

Change Owner

Used to transfer ownership.

Example:

chown sara file.txt

Changes the owner.

Changing owner and group:

chown sara:developers file.txt

Ownership and permissions work together.

1️⃣5️⃣ Useful User Commands¶

Display current user:

whoami

Display user information:

id

Create a user:

useradd username

Delete a user:

userdel username

1️⃣6️⃣ Useful Group Commands¶

Display groups:

groups

Create group:

groupadd developers

Modify group membership:

usermod -aG developers sara

Delete group:

groupdel developers

1️⃣7️⃣ Security Principles¶

Linux permissions follow important security concepts.

Least Privilege¶

Give users only the permissions they need.

Separation of Responsibilities¶

Different users perform different tasks.

Access Control¶

Protect resources according to their importance.

Permissions are one of Linux's primary security mechanisms.

1️⃣8️⃣ Mental Model¶

Imagine an office building.

Users are:

Employees

Groups are:

Departments

Files are:

Documents

Permissions are:

Access badges

Root is:

The building owner

Final Mental Image

Users
    ↓
Groups
    ↓
Permissions
    ↓
Security

Linux uses this hierarchy to control who can access what.