Managing Users and Groups

Managing Users and Groups in Linux

1. Introduction

Linux is a multi-user operating system, which means multiple users can work on the system at the same time.
To maintain security, organization, and controlled access, Linux uses users and groups.

👉 User management controls who can access the system.
👉 Group management controls what resources users can access collectively.

2. Concept of Users in Linux

Types of Users

User Type	Description
Root user	Superuser with full privileges (UID = 0)
System users	Used by services (UID < 1000)
Normal users	Regular login users (UID ≥ 1000)

Each user is identified by:

Username
UID (User ID)
Home directory
Login shell

3. User Account Files (Exam Important)

File	Purpose
`/etc/passwd`	User account details
`/etc/shadow`	Encrypted passwords
`/etc/group`	Group information
`/etc/gshadow`	Group passwords

Example entry in /etc/passwd:

user:x:1001:1001:/home/user:/bin/bash

4. Managing Users in Linux

4.1 Creating a User

useradd username

Create user with home directory:

useradd -m username

4.2 Setting Password

passwd username

4.3 Modifying User Account

usermod -l newname oldname
usermod -aG groupname username

4.4 Deleting a User

userdel username
userdel -r username   # remove home directory

5. Concept of Groups in Linux

A group is a collection of users that share common permissions.

Types of Groups

Primary group – Default group of user
Secondary group – Additional groups

📌 Each user belongs to one primary group and multiple secondary groups.

6. Managing Groups

6.1 Creating a Group

groupadd groupname

6.2 Adding User to Group

usermod -aG groupname username

6.3 Changing Primary Group

usermod -g groupname username

6.4 Deleting a Group

groupdel groupname

7. Viewing User and Group Information

Command	Purpose
`id username`	User & group IDs
`groups username`	User groups
`whoami`	Current user
`who`	Logged-in users

8. File Ownership and Groups

Files are owned by:

User
Group

Change ownership:

chown user:group file.txt

Permissions depend on user, group, and others.

9. User Privileges & `sudo`

Only root can manage users/groups
sudo allows limited admin access

Example:

sudo useradd student

10. Real-World Example (Server Scenario)

Create users for employees
Assign groups like developers, admins
Control access to /var/www
Use sudo for admin tasks

11. Advantages of User & Group Management

Strong security
Organized access control
Efficient resource sharing
Prevents unauthorized access

12. Summary Table

Task	Command
Add user	`useradd`
Delete user	`userdel`
Modify user	`usermod`
Add group	`groupadd`
Delete group	`groupdel`
Add user to group	`usermod -aG`

13. Conclusion

Managing users and groups is a fundamental security mechanism in Linux. By properly organizing users into groups and assigning permissions, Linux ensures safe, controlled, and efficient multi-user operation, especially in server environments.