Linux/Unix: User & Group Management – useradd, usermod, groups, id Explained

Introduction – Why Learn User & Group Management in Linux?

User and group management is at the core of Linux system administration. It controls who can access the system, what they can do, and how resources are shared. With tools like useradd, usermod, groups, and id, you can easily manage users, assign roles, and control file permissions.

In this guide, you’ll learn:

• How to create, modify, and manage users and groups
• How to check group memberships and user identity
• Practical examples with command outputs

1. useradd – Add a New User

What is useradd?

useradd is the standard command to create new user accounts.

Syntax:

sudo useradd [options] username

Common Options:

Example 1: Create a new user with home and bash shell

sudo useradd -m -s /bin/bash alice

Output:
Creates /home/alice and assigns bash as the default shell.

Example 2: Add user to sudo group

sudo useradd -m -G sudo bob

2. usermod – Modify Existing User

What is usermod?

usermod is used to change user attributes such as group membership, home directory, or login shell.

Syntax:

sudo usermod [options] username

Common Tasks:

👩‍👧 3. groups – Show Group Memberships

What is groups?

Displays the groups a user belongs to.

Syntax:

groups [username]

Example:

groups alice

Output:

alice : alice sudo developers

Helpful to confirm if a user has sudo or team-specific access.

4. id – Display User and Group IDs

What is id?

Shows the user ID (uid), primary group ID (gid), and all groups a user belongs to.

Syntax:

id [username]

Example:

id bob

Output:

uid=1001(bob) gid=1001(bob) groups=1001(bob),27(sudo)

Use this to programmatically check user identity in scripts.

Bonus: Related Commands

Summary – Recap & Next Steps

Managing users and groups is foundational for Linux security, multi-user access, and resource permissions. These tools help you enforce access policies, add team members, and automate user setup.

Key Takeaways:

• Use useradd -m -s /bin/bash to create new users with default shell and home.
• Use usermod -aG to assign users to groups without overwriting existing ones.
• Use groups or id to confirm access roles and identities.

FAQs

How do I add a user to multiple groups?
Use:

sudo usermod -aG group1,group2 username

What’s the difference between useradd and adduser?
useradd is low-level and universal. adduser is a Debian-friendly interactive script that uses useradd underneath.

Can I change a username?
Yes:

sudo usermod -l newname oldname

How do I remove a user and their home directory?
Use:

sudo userdel -r username

How do I temporarily disable a user account?
Use:

sudo usermod -L username