useradd (Add User)
The useradd command is used to create a new user. Usually, the useradd command comes with the -m option, which can create the home directory for the new user under the path of /home with the user name. To run this command, you need to run it with superuser privileges.
Create a new user with its home directory
Adding a user is a privilege of the superuser. When you run the command, you need to run it as root or with the sudo command. If you run the command as a normal user without sudo, you'll see a permission error like the one below.
useradd user_a
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.
To create a new user, switch to the superuser and run the useradd command. Use the -m option to create the home directory for the new user.
sudo su -
useradd -m user_a
Check the user data
The user data is recorded under the /etc/passwd file. To check the file, run the following command.
cat /etc/passwd
You can see that a new user is added at the bottom of the file.
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
:
:
systemd-coredump:x:999:999:systemd Core Dumper:/:/usr/sbin/nologin
ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash
lxd:x:998:100::/var/snap/lxd/common/lxd:/bin/false
user_a:x:1001:1001::/home/user_a:/bin/sh
In this file, the following information is covered (left to right):
- user name
- user password (x) (actual encrypted user passwords are saved in a different file named /etc/shadow)
- user ID
- primary group ID
- user comment or full name (it's blank in the example above)
- user's home directory
- login shell path
You can also use the grep command to see only the necessary part of the user data. For example, what we need to know is the user data which has a string with "user" in it. Run the following command to get the data.
grep user /etc/passwd
You'll get the result like the one below.
user_a:x:1001:1001::/home/user_a:/bin/sh
Options
There are several options for the useradd command. You can use these options to customize user data.
-u: specify user ID-g: specify primary group ID-c: specify user’s comment-s: specify login shell-d: specify home directory path
Practice (1): Create new users
For practice purposes, let's create three users: user_a, user_b, and user_c. First, switch to the superuser.
sudo su
Run the useradd commands three times.
useradd -m user_a
useradd -m user_b
useradd -m user_c
Check the user data with the grep command.
grep user /etc/passwd
You'll see that three user accounts have been created.
user_a:x:1001:1001::/home/user_a:/bin/sh
user_b:x:1002:1002::/home/user_b:/bin/sh
user_c:x:1003:1003::/home/user_c:/bin/sh
Practice (2): Switch to a new user
You can further switch to another user by running the su command. To switch to user_a, run the command below.
su - user_a
You can see that # changed to $ as you are back to the normal user. Also, you can see that the home directory has changed to the user_a directory by running the pwd command.
pwd
/home/user_a
To go back to the ubuntu user, use the exit command twice as shown below.
exit
exit
exit
ubuntu:~$
You can also learn this topic offline. Click 
.