Managing Users & Groups in Linux – A Complete Guide

📖 Introduction

Managing Users & Groups in Linux: Linux is a multi-user operating system, meaning multiple users can access the system simultaneously. Proper user and group management ensures security, organized access control, and streamlined administration.

This guide covers:
✅ Creating, modifying, and deleting users in Linux
✅ Managing groups for better access control
✅ Assigning sudo privileges
✅ Best practices for secure user management

1️⃣ Understanding Linux Users & Groups

Before diving into commands, let’s understand how Linux handles users and groups.

🔹 User Types in Linux

1️⃣ Root User – The superuser with full system control (UID 0).
2️⃣ System Users – Used by system services and daemons (UIDs < 1000).
3️⃣ Regular Users – Created for human users (UIDs ≥ 1000).

🔹 Groups in Linux

Primary Group: Assigned when a user is created.
Supplementary Groups: Additional groups a user can be a part of.

📌 Command to View Users & Groups

cat /etc/passwd    # View users  
cat /etc/group     # View groups

2️⃣ Creating and Managing Users in Linux

🔹 Add a New User

To create a new user with a default home directory:

sudo useradd -m alice

Set a password:

sudo passwd alice

📌 Explanation:

-m creates a home directory (/home/alice).
passwd assigns a password.

🔹 Modify an Existing User

Change username:

sudo usermod -l newalice alice

Change home directory:

sudo usermod -d /newhome/alice -m alice

Force password reset on next login:

sudo passwd --expire alice

🔹 Delete a User

sudo userdel alice

To remove the home directory as well:

sudo userdel -r alice

3️⃣ Managing Groups in Linux

🔹 Create a New Group

sudo groupadd developers

🔹 Add a User to a Group

sudo usermod -aG developers alice

📌 Best Practice: Always use -aG to append a user to a group instead of overwriting existing groups.

🔹 View User’s Groups

groups alice
id alice

🔹 Remove a User from a Group

sudo gpasswd -d alice developers

4️⃣ Granting Sudo Privileges

🔹 Add a User to the Sudoers Group

For Ubuntu/Debian:

sudo usermod -aG sudo alice

For CentOS/RHEL:

sudo usermod -aG wheel alice

🔹 Test Sudo Access

sudo whoami

5️⃣ Best Practices for Secure User Management

✅ Use Strong Password Policies

Force password expiration every 90 days:

sudo chage -M 90 alice

Lock inactive accounts:

sudo usermod -L alice

✅ Restrict Root Access

Disable root login over SSH:

sudo nano /etc/ssh/sshd_config

Set: PermitRootLogin no
Restart SSH:

sudo systemctl restart sshd

✅ Monitor User Activity
Check recent logins:

last

Monitor failed login attempts:

cat /var/log/auth.log | grep "Failed password"

🔹 Summary

🔹 Learned how to create, modify, and delete users in Linux.
🔹 Explored group management for structured access control.
🔹 Discovered sudo privileges and security best practices.

💬 Have questions? Drop them in the comments below!

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