Users in Linux could be managed using command-line tools or via graphical applications. The most common command-line tools in Linux for user creation are adduser and useradd. At the same time, most desktop environments and Linux distributions will have a graphical tool that is just a frontend to these command-line tools.
These tools will eventually write related user information in /etc/passwd, /etc/group, and /etc/shadow files. They'll also create a user home directory and populate the home directory with necessary files stored in /etc/skel.
You can manually add user information to these files to add new users in Linux or use dedicated tools such as adduser. adduser is an interactive command-line tool available by default in most Linux distributions.
Graphical methods to create user for different Linux distributions:
$ sudo adduser username
[sudo] password for user:
Current user needs to have administrator privilege on the system.
Adding user `username' ... Adding new group `username' (1001) ... Adding new user `username' (1001) with group `username' ... Creating home directory `/home/username' ... Copying files from `/etc/skel' ...
New password: Retype new password: passwd: password updated successfully
Changing the user information for username Enter the new value, or press ENTER for the default Full Name : Room Number : Work Phone : Home Phone : Other :
Is the information correct? [Y/n] y
Manual: adduser(8) - Linux man page
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