useradd Command Guide: Create New User Accounts

Overview of useradd

`useradd` command performs actions such as adding user account information to system files like `/etc/passwd`, `/etc/shadow`, `/etc/group`, `/etc/gshadow`, creating the user's home directory, and copying default configuration files (skeleton files). All of these actions require administrator privileges (`sudo`).

Main Roles of useradd

`useradd` command is primarily used for the following purposes:

Key Use Cases

Registering New Users: Adds new regular users to the system, allowing them to log in and access files.
Creating Service Accounts: Creates non-login accounts for specific applications or services to run.
Security Management: Grants each user a unique account to control system access and enhance accountability.

Key Components When Creating User Accounts

When a user account is created, the following key components are set:

Key Account Components

Username: A unique identifier for the account, used during login.
User ID (UID): A numeric ID that the system uses to identify the user, recorded in `/etc/passwd`.
Primary Group: The main group that is automatically assigned when the user is created.
Supplementary Groups: Additional groups that the user may belong to.
Home Directory: The space where the user's personal files are stored, typically in the format `/home/username`.
Login Shell: The default shell program that will be executed when the user logs in. (e.g., `/bin/bash`, `/bin/sh`, `/bin/false`)
Password: Used for user authentication, set separately using the `passwd` command.

📂 Default Configuration Files (Skeleton Directory)

/etc/skel

When a new user account is created, the user's home directory is populated with predefined default configuration files (e.g., `.bashrc`, `.profile`). These files are usually located in the `/etc/skel` directory, referred to as the skeleton directory.

Key useradd Command Options

`useradd` command provides options to set various account attributes, such as the user's home directory, shell, groups, and expiration date.

1. Basic Account Settings

2. Group and UID/GID Settings

3. Account Expiration and Others

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Usage Examples

Learn how to create new user accounts and set attributes through various examples of the `useradd` command.

Create New User Account (Default Settings)

sudo useradd testuser

Creates a new user account named `testuser`. The home directory and password for this account have not been set yet.

Create New User Account with Home Directory

sudo useradd -m devuser
sudo passwd devuser

Creates a new user account named `devuser`, creates the home directory `/home/devuser`, and copies the default configuration files from `/etc/skel`. After creating the account, you must set the password using the `passwd` command to log in.

Create User with Specific Home Directory and Shell

sudo useradd -m -d /var/www/webuser -s /bin/false webuser

Creates an account named `webuser`, sets the home directory to `/var/www/webuser`, and the default shell to `/bin/false` (no login). Suitable for web service accounts.

Add User to Multiple Groups

sudo useradd -m -G sudo,www-data -c "Administrator for Web" adminuser
sudo passwd adminuser

Creates an `adminuser` account, adds it to the `sudo` group and `www-data` group, and also sets a user description.

Create User Account with Expiration Date

sudo useradd -m -e 2025-12-31 tempuser
sudo passwd tempuser

Creates a `tempuser` account, creates the home directory, and sets it to expire on December 31, 2025. Useful for temporary worker accounts.

Check Current useradd Defaults

useradd -D

Checks the defaults (primary group, home directory prefix, shell, etc.) that will apply when creating a new user.