There was a brief introduction to systemd in the chapter "Booting and init." The basic concept for systemd is unit – it can be a description of a service, an action, or a rule. As a rule, a unit is a file with a standard structure in the style of ini-file, i.e., the file is divided into sections, and the name of the section is given in square brackets. Within a section, each line has the form "key = value."