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Lesson 01 - Understanding Linux Users and Groups

Estimated time: 45–60 minutes

Prerequisites: - Module 00 – Introduction to Linux - Module 01 – Command Line Fundamentals - Module 02 – Linux Filesystem Administration

Difficulty: Beginner

Hands-on exercises: Yes

Overview

Linux is a multi-user operating system, designed from the ground up to allow many users to work on the same system safely and securely.

Every file, process and service belongs to a user and, usually, to a group. Understanding how users and groups work is fundamental to administering Linux systems, whether you're managing a single Raspberry Pi or hundreds of cloud servers.

In this lesson you'll learn how Linux identifies users, how groups simplify permission management, and how to inspect account information from the command line.

Learning Objectives

By the end of this lesson you will be able to:

  • Explain why Linux uses users and groups.
  • Distinguish between users and groups.
  • Understand User IDs (UIDs) and Group IDs (GIDs).
  • Identify system accounts and regular user accounts.
  • View information about the current user.
  • Examine user and group information stored by Linux.

Why Linux Uses Users

Linux assumes that multiple people may use the same computer at different times—or even simultaneously.

Each user has their own:

  • Home directory
  • Files
  • Settings
  • Password
  • Permissions

This separation helps keep data secure and prevents users from interfering with each other's work.

For example:

/home
├── alice
├── bob
└── james

Each user has their own workspace inside the /home directory.

What is a User?

A user represents an individual account on the system.

Every user has:

  • A username
  • A password (or another authentication method)
  • A unique User ID (UID)
  • A primary group
  • A home directory
  • A default login shell

For example:

Username: james
UID:      1000
Home:     /home/james
Shell:    /bin/bash

Linux uses the UID internally rather than the username to identify users.

What is a Group?

A group is a collection of users.

Groups simplify permission management by allowing multiple users to share access to files and directories.

For example:

developers
├── alice
├── bob
└── james

Instead of granting permissions to each user individually, permissions can be assigned to the group.

Primary and Secondary Groups

Every Linux user belongs to at least one group.

There are two types of group membership:

Primary Group

Every user has one primary group.

Example:

User: james
Primary group: james

Files created by the user normally belong to this group.

Secondary Groups

A user may also belong to additional groups.

Example:

User: james

Groups:
- james
- docker
- sudo

Secondary groups provide additional permissions without changing the primary group.

User IDs (UID)

Every user has a unique numerical identifier called a User ID (UID).

Typical ranges are:

UID Range Purpose
0 Root user
1–999 System and service accounts (distribution dependent)
1000+ Regular user accounts

The root user always has a UID of 0.

Group IDs (GID)

Groups also have unique numerical identifiers called Group IDs (GIDs).

Linux uses GIDs internally in the same way it uses UIDs for users.

The Root User

The root account is the Linux administrator.

Root can:

  • Access every file
  • Modify system configuration
  • Install software
  • Create and remove users
  • Change permissions
  • Start and stop services

Because root has unrestricted access, administrators normally use sudo rather than logging in directly as root.

System Accounts

Not every account belongs to a person.

Linux creates many accounts for running services.

Examples include:

www-data
daemon
nobody
systemd-network

These accounts improve security by allowing services to run with only the permissions they require.

Viewing Your Current User

Use the whoami command to display the current user.

whoami

Example:

james

Displaying User Information

The id command displays detailed information about the current user.

id

Example:

uid=1000(james)
gid=1000(james)
groups=1000(james),27(sudo),998(docker)

This shows:

  • User ID
  • Primary Group ID
  • Group memberships

Displaying Logged-in Users

To see who is currently logged in:

who

Example:

james    pts/0    09:45
admin    pts/1    10:12

Viewing Group Membership

To display the groups for the current user:

groups

Example:

james sudo docker

Where User Information is Stored

Linux stores account information in text files.

Common files include:

File Purpose
/etc/passwd User account information
/etc/group Group information
/etc/shadow Encrypted password information

You will explore these files in the next lesson.

Best Practices

✔ Use standard user accounts for everyday work.

✔ Use sudo only when administrative privileges are required.

✔ Assign permissions using groups rather than individual users whenever possible.

✔ Avoid logging in directly as the root user.

✔ Regularly review user accounts and group memberships.

Summary

In this lesson you learned:

  • Why Linux uses users and groups.
  • The difference between users and groups.
  • What UIDs and GIDs are.
  • The role of the root account.
  • The purpose of system accounts.
  • How to identify the current user and group memberships.

In the next lesson, you'll learn how to create, modify and manage user accounts and groups using common Linux administration commands.