useradd Command in Linux

Creating user accounts is a fundamental responsibility of a Linux system administrator, enabling secure access control and proper permission management. Linux provides powerful tools to create and manage users efficiently, especially in multi-user and server environments.

User accounts are commonly created to:

• Provide system access to new users such as developers or team members
• Create service accounts for applications like web servers or databases
• Isolate tasks and improve system security through permission separation
• Manage system resources by assigning users specific roles

Let us consider a command to add a user named test_user to the system. This command creates a new user account without additional customization.

Command:

sudo useradd test_user

Output:

Working with the useradd Command

Syntax:

useradd [options] [user_name]

• useradd – Command used to create a new user account
• options – Optional flags to customize the user (UID, home directory, shell, etc.)
• username – Name of the user account to be created

1. Create a User with a Custom Home Directory

This creates a new user and assigns a specific directory as the user’s home directory.

Command:

sudo useradd -d /home/test_user test_user

Output:

• User test_user is created
• Home directory is set to /home/test_user (directory may need to be created manually unless -m is used)

2. Create a User with a Specific User ID (UID)

This creates a user with a custom user ID instead of an automatically assigned one.

Command:

sudo useradd -u 1234 test_user

Output:

• User test_user is created with UID 1234
• UID is recorded in /etc/passwd

3. Create a User with a Specific Group ID (GID)

This assigns the user to an existing group using its group ID.

Command:

sudo useradd -g 1000 test_user

Output:

• User test_user is created
• Primary group is set to GID 1000

4. Create a User Without a Home Directory

This creates a user account without creating a home directory.

Command:

sudo useradd -M test_user

Output:

• User test_user is created
• No home directory is assigned or created

5. Create a User with an Account Expiry Date

This creates a user account that automatically expires on a specific date.

Command:

sudo useradd -e 2020-05-30 test_user

Output:

• User test_user is created
• Account expires on 30 May 2020
• After expiry, login is disabled

6. Create a User with a Comment or Description

This adds descriptive information (such as full name or role) to the user account.

Command:

sudo useradd -c "This is a test user" test_user

Output:

• User test_user is created
• Comment field is stored in /etc/passwd

7. Create a User with a Specific Login Shell

This sets a custom default shell for the user.

Command:

sudo useradd -s /bin/sh test_user

Output:

• User test_user is created
• Default login shell is set to /bin/sh

8. Display Help for the useradd Command

This displays all available options and usage information for useradd.

Command:

sudo useradd --help

Output:

• Displays a list of all useradd options
• Shows syntax and flag descriptions

Files Modified by useradd

When you run useradd, it's the command simply edits a set of system files to register the new user.

• /etc/passwd: Stores user information like the username, UID, GID, home directory path, and default shell.
• /etc/shadow: Stores the user's encrypted password. This is why useradd alone can't set a password.
• /etc/group: Stores information about user groups.
• /etc/gshadow: Stores secure information for groups.
• /home/[username]: (If created) This is the user's personal directory. The initial files in it are copied from /etc/skel.

adduser vs useradd

• adduser is an interactive, user-friendly tool commonly used on Debian/Ubuntu systems. It creates the user, home directory, password, and prompts for additional details.
• useradd is a low-level, non-interactive command designed for scripting and automation, commonly used on RHEL, CentOS, and Fedora systems.