The main capability of the system daemon that most people know is its support for system services. Unlike the traditional SysV-compatible init systems, systemd does not use scripts to manage services. Instead, it uses a declarative approach for the various services, documenting the wanted state and configuration parameters while using its own logic to ensure that the right set of services is started in due time.
Systemd uses unit files to declare how a service should behave. These unit files use the INI-style syntax, supporting sections and key/value pairs within each file. A service can have multiple unit files that influence the service at large. It is important to remember that different unit files for the same service are all related:
The
*.serviceunit files define how a system service should be launched, what its dependencies are, how systemd should treat sudden failures, and so on.The
*.socketunit files define which socket(s) should...