Working with users and groups

In this section, we will learn how to create and remove users and groups, and how to add groups to users. Also, we will see how Linux internally stores user information and passwords, and how to retrieve user information programmatically. Finally, we will learn how to substitute user accounts while staying logged in. Linux is a multiuser system, which means more than one user can work with the system simultaneously. Therefore, a system is needed, which guarantees common access to Linux objects such as files using measurements of excess protection. For example, all files which have been created by one user should not be allowed to be erased by another user. Every Linux user is defined and identified by a unique user ID, as humans more easily can work with names than numbers. There also exists a literal username connected to each user ID, but Linux...