Frequently Asked Question

What is a unit file and what types of unit are there?

Everything systemd manages is called a unit, and every unit is described by a small text file in INI-style format. The file extension tells systemd what kind of unit it is. The most common are .service (a program to run and supervise), .socket (a listening socket that activates a service on demand), .timer (a schedule that triggers another unit), .mount and .automount (filesystem mount points), .swap (a swap area), .path (watch a filesystem path for changes), and .target (a grouping unit that bundles other units together).

A handful of other types, .device, .slice, .scope, are mostly generated automatically rather than written by hand. Unit files live in three directories that systemd merges, with later directories overriding earlier ones: /usr/lib/systemd/system/ ships with packages, /run/systemd/system/ is for runtime-generated units, and /etc/systemd/system/ is where you put your own. The same scheme repeats under the user's home for per-user units (~/.config/systemd/user/).