Want to really understand how Linux handles users and groups? This hands-on guide not only gives you commands — it breaks them down flag by flag, so you know exactly what you're doing.

Grab your terminal and follow along. Ready? Let’s get started. 🚀

📁 Important Files — Know What Powers User Management 

/etc/passwd   # User info: username:x:UID:GID:comment:home:shell
/etc/shadow   # Has (hashed) passwords
/etc/group    # Lists all groups

💡 Try:

cat /etc/passwd | head -n 3
sudo cat /etc/shadow | head -n 3
cat /etc/group | head -n 3

🛠️ Create a User with useradd 

useradd [OPTIONS] username

🔑 Common Flags:

  • -m → Create a home directory (e.g., /home/username)

  • -d /custom/path → Set a custom home directory

  • -s /bin/bash → Set user’s login shell

  • -c "comment" → Add a comment (e.g., full name or role)

  • -G group1,group2 → Add to secondary groups (must already exist)

  • -g group → Set primary group (must already exist)

📌 Practical Example:

sudo useradd -m -d /home/john -c "C++ Developer" -s /bin/bash -G sudo,adm john

Explanation:

  • -m: Create /home/john

  • -d /home/john: Use this as the home directory

  • -c "C++ Developer": Store this comment

  • -s /bin/bash: Set Bash as login shell

  • -G sudo,adm: Add to secondary groups

➡️ Now set a password:

sudo passwd john

🔄 Modify a User with usermod 

usermod [OPTIONS] username

Useful Flags:

  • -aG group1,group2 → Append user to new secondary groups

  • -s /new/shell → Change default shell

  • -d /new/home → Change home dir (-m to move files)

  • -c → Change comment

🔧 Example:

sudo usermod -aG docker,developers john

Explanation:

  • -a: Append (don’t remove existing groups!)

  • -G: Add to docker and developers groups

❌ Delete a User with userdel 

sudo userdel [OPTIONS] username

Flags:

  • -r → Remove user’s home directory and mail spool Example: 
sudo userdel -r john

Result: Deletes john + /home/john

👥 Group Management with groupadd, groupdel

➕ Add a Group:

sudo groupadd devops

❌ Delete a Group:

sudo groupdel devops

🔍 View All Groups:

cat /etc/group

🔍 Check Group Membership 

groups           # For current user
groups username  # For another user
id username      # UID, GID, group list

🧑‍💼 Make a User Admin (Sudo Access)

On Ubuntu:
sudo usermod -aG sudo john

On CentOS:
sudo usermod -aG wheel john

🕵️ Monitor Logged-In Users 

who -H       # Show who is logged in
w            # Who is logged in + what they’re doing
uptime       # Show load, users, system uptime
id           # Show UID, GID, and groups
whoami       # Show your effective user ID

📜 Login History 

last             # Recent logins
last -u username # Logins by a specific user

✍️ Final Words

Linux user & group management is must-know stuff for sysadmins, DevOps engineers, and anyone using Linux daily.