So I have a slightly bizarre problem....

I have an openembedded linux setup.

Sometimes it will be on hardware that has a cellular USB modem installed on the PCB, and sometimes not.

I won't know before I boot.

So udev correctly sees the usb ID and I can create a udev rule to do...

Something.

That is the question. What something?

Ultimately I want to run up pppd and chatscript at the right point in the boot process.

So should I make the udev action something like
"systemctl enable pppd@ttyACM0.service"

Or should I just symbolic link in the .service file?

Suggestions?

i don't see why it shouldn't work like that.
have you tried?

but i'm not sure if udev is the best tool; i think it's at its most useful when you un/plug devices during runtime, but in your case a systemd service running a script that checks for a condition(is that hardware present?) and then executes systemctl enable pppd@ttyACM0.service?