Build a service
Choose a service type
Choose Type=simple, exec, notify, forking, or oneshot from the process behavior you actually control.
8 minute lesson
A service type tells systemd two things: when startup has succeeded, and how to identify the main process. Pick it from how your program behaves, not from what you copied last time.
The types that matter
Type=simple is the default. systemd considers the service started the instant the process is forked, before your binary even runs. That means systemctl start can report success for a service whose executable does not exist.
Use Type=exec for a normal foreground process when an execution failure must fail startup. It behaves like simple, but systemctl start waits until the binary has actually been executed, so a wrong path or a permission problem fails the start command itself:
[Service]
Type=exec
ExecStart=/usr/bin/node /opt/app/server.jsType=notify needs application support: the program calls sd_notify() to send READY=1 when it can really serve traffic. That makes ordering meaningful for anything started After= this service. Only use it when the application documents that support.
Type=forking exists for traditional daemons that detach: the parent exits, a child keeps running. Pair it with PIDFile= so systemd can track the right process. New software should stay in the foreground instead.
Type=oneshot runs finite setup work. systemd waits for the process to exit before considering the unit started. Add RemainAfterExit=yes when the unit should stay active (exited) after finishing:
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/bin/prepare-cache-dirsThe failure mode to recognize
The classic mismatch is Type=forking on a program that never forks. systemd waits for the parent to exit, the parent never does, and startup hangs until the timeout kills it. The journal shows a start timeout even though the process was healthy the whole time. The fix is not a longer timeout. It is the right type.
Read the startup behavior of an application you run: does it stay in the foreground, detach, or finish and exit? Choose a type and write down the event that should count as successful startup.
Lesson completed