Frequently Asked Question

What does Type= mean in a service unit (simple, exec, forking, oneshot, notify)?

Type= tells systemd how to decide that a service has finished starting up, important because anything that depends on this service should wait until then. simple (the historic default) says the main process is whatever ExecStart= runs, and the service is considered started the moment fork() returns. exec is the modern, stricter default: systemd waits until the binary has been successfully exec()'d before regarding the service as up. forking matches classic Unix daemons that fork into the background and let the parent exit; you usually also need PIDFile= so systemd can track the surviving child.

oneshot is for short-lived units that run to completion, backups, mount preparation, one-off setup steps, and is treated as "active" while the command is running and (with RemainAfterExit=yes) afterwards. notify is the strongest contract: the service itself calls sd_notify(READY=1) to tell systemd it has finished initialising, typically once it is listening on its socket. Choosing the right type makes the difference between dependent units starting too early and starting at exactly the right moment.