Understanding systemd units
On the Unit

Systemd units use files to control resources that Systemd manages.
Whether you like it or not: Systemd has become ubiquitous. Linux distributions that rely on other init systems are becoming increasingly rare. If you run one of the mainstream distributions, you need to familiarize yourself with the concepts and working methods of systemd.
Systemd units and their corresponding configuration files require close attention. The term "unit" means any type of resource that cooperates with Systemd – which includes timers, mountpoints, network resources, sockets, partitions, and devices on top of services.
Configuration units, known as unit files, let you define how and when a service starts, which resources it is allowed to access, and which dependencies need to be met. Unit files are similar in function to the init scripts in SysVinit or Upstart (Figure 1), but they are usually easier to create and easier to maintain. They follow the conventions of simple INI files (Figure 2).
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