I have a USB device that contains a modem (a long with a few other cans of worms).

The modem (an u-blox LARA) enumerates as /dev/ttyACM[1-6] on this embedded Linux (Yocto) box. This box has Busybox as init system (so no systemd) and udev to manage devices.

It has little else on board and I cannot change it (I am not in control of the Yocto build), although I can load whatever I want under /opt. / is mounted as rootfs, so changes are lost on power cycle: currently I symlink required files from /opt on boot.

My goal: I need to start pppd when /dev/ttyACM1 appears.

The problem is: udev, by design, will terminate forked, detached & co. processes after handling the event. The board does not have at, so even the at now trick does not work.

By the way, the other cans of worms above also require daemons, so best (but not required) to have not pppd-specific answers.

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