I think this was the reason that setting WantedBy=sysinit.target was preventing SystemD from starting my script because it generated a circular dependency conflict. Units will implicitly have dependencies of type Requires= and After= Unless DefaultDependencies= in the '' is set to false, service The important thing was to set DefaultDependencies=false because Starts /opt/gpio-test 6.2 seconds after power-on which is almost directly after the kernel is ready. I ended up with the following SystemD Unit file /lib/systemd/system/rvice: Īctivating this with sudo systemctl enable gpio-test I think default.target is more appropriate but that is just a casual opinion and in any case, again, this is about whether or not to run it, not when to run it. It is used to determine in what context the service should be run at boot basic.target probably covers any of them. What would be a good WantedBy= directive to allow an early start of the application? Double check to make sure that's actually in use first ( systemclt list-units | grep rsyslog), otherwise this won't make any difference. Then it should happen before many or most other things. Since a lot of other things are dependent on the system logger, if you use: Before=rvice However, you can use the Before= directive (see man systemd.unit). You can't make something else depend upon this unless you modify its service file (a bad idea if that something else wasn't also added by you). The GPIO functionality is built into the kernel, so by the time init (systemd) runs there's not really anything required, although since you're using the root filesystem you probably do want local-fs.target (that could be only true if you need read-write access, I'm not sure - the fact that it doesn't work without that implies you do need it). What is the correct SystemD Requires= directive(s) for using GPIO pins? If this isn't required by anything else, and isn't required to happen before anything else, then it won't be a priority. I believe there's a degree of indeterminacy with regard to execution order. What is the correct SystemD Requires= directive(s) for using GPIO pins? And what would be a good WantedBy= directive to allow an early start of the application? Loaded: loaded (/lib/systemd/system/gpio_rvice enabled) After logging in via ssh I got this: $ sudo service gpio_test status So I tried to optimize the startup time by changing the WantedBy= directive to WantedBy=sysinit.target but then the application never gets started. When using the following SystemD unit file to start the application gpio-test, it starts correctly but very late in the boot process (later than with a /etc/rc.local script): The application makes use of GPIO pins with the help of the wiringPi library. I want to start an application on system boot as early as possible.

Fader start gpio