Overview¶

Linux provides commands to manage users and groups, which are essential for:

Access control
Security management
Multi-user environments
System administration

This guide covers core commands for creating, modifying, and deleting users and groups.

useradd — Create New User¶

Description¶

useradd is used to create a new user account or update default user settings.

Syntax¶

useradd -m -c "Full Name" -s /bin/bash username

Options Explained¶

-m → creates home directory (e.g., /home/username)
-c → adds comment (usually full name)
-s → sets default login shell

Example¶

useradd -m -c "Subrat Samantaray" -s /bin/bash subrat

Explanation

Creates user subrat
Assigns /home/subrat as home directory
Sets default shell to Bash

Set Password for User¶

passwd subrat

Explanation
Assigns a password to the new user.

userdel — Delete User¶

Description¶

userdel removes a user account from the system.

Syntax¶

userdel -r username

Example¶

userdel -r subrat

Explanation

Deletes user subrat
-r removes home directory and user files

Group Management¶

View Group Information¶

cat /etc/group

Explanation
Displays all groups available on the system.

groupadd — Create New Group¶

groupadd groupname

Example¶

groupadd developers

Explanation
Creates a new group named developers.

usermod — Modify User¶

Description¶

usermod is used to modify user account properties.

Add User to Group¶

usermod -aG group username

Example¶

usermod -aG developers subrat

Explanation

Adds user subrat to developers group
-a → append (important to avoid removing existing groups)
-G → supplementary groups

Change Primary Group¶

usermod -g group username

Example¶

usermod -g developers subrat

Explanation
Sets developers as the primary group.

Multiple Groups Assignment¶

usermod -aG group1,group2 username

visudo — Edit Sudoers File Safely¶

Description¶

visudo is used to edit the /etc/sudoers file securely.

Usage¶

sudo visudo

Features¶

Prevents simultaneous edits
Checks syntax before saving
Avoids breaking sudo configuration

Example: Grant Sudo Access¶

Inside visudo:

username ALL=(ALL:ALL) ALL

Explanation
Grants full administrative privileges to the user.

Practical Workflow¶

Create User and Assign Group¶

useradd -m -c "Test User" -s /bin/bash testuser
passwd testuser
groupadd pentest
usermod -aG pentest testuser

Delete User Completely¶

userdel -r testuser

Important Notes¶

Root privileges are required for most commands
Always use -a with -G to avoid overwriting groups
Be careful when editing sudoers file
Verify changes using:

id username

Summary Table¶

Command	Purpose
`useradd`	Create user
`userdel`	Delete user
`groupadd`	Create group
`usermod`	Modify user
`cat /etc/group`	View groups
`visudo`	Edit sudo permissions

Conclusion¶

User and group management is critical for:

System security
Access control
Multi-user environments

Mastering these commands ensures proper control over user privileges and system resources in Linux.