Unix - Creating Users in Unix Using useradd?

In Linux and Unix-like systems, user management is a fundamental administrative task. The useradd command is used to create new user accounts and define their default settings such as home directory, shell, and group membership.

Basic User Creation

To create a new user with default settings, use:

sudo useradd username

This creates an account but does not set a password. You must assign one separately:

sudo passwd username

Common `useradd` Options

The useradd command provides several options to customize user creation:

Create a home directory:

sudo useradd -m username

Specify a custom home directory:

sudo useradd -d /home/customdir username

Set a default shell:

sudo useradd -s /bin/bash username

Add user to a supplementary group:

sudo useradd -G wheel,developers username

Set a specific user ID (UID):

sudo useradd -u 1050 username

Understanding What `useradd` Does Internally

When a user is created, Linux updates several system files:

/etc/passwd – user account details
/etc/shadow – encrypted password information
/etc/group – group memberships
/etc/gshadow – secure group data

Default settings are controlled by:

/etc/default/useradd
/etc/login.defs

Verifying the User

To confirm user creation:

id username

getent passwd username

Best Practices

Always create a home directory using -m
Assign strong passwords immediately
Use groups instead of direct permission changes
Avoid using the root account for regular tasks

In summary, useradd is a powerful and flexible command for creating and managing user accounts in Linux, giving administrators fine-grained control over system access and security.