Linux: systemd services, timers and udev rules

Customs write for the third time: not a single declaration has reached the single window in three days, and containers are stuck at the port. The same morning, the dock supervisor reports that the label printer has printed nothing since the dock workstation was plugged back in. In lab 1, you had put the collection script in the risks column: started at boot by cron, detached by nohup, its output thrown into /dev/null. The risk has materialised. You start with the investigation: the log that stops without a word, the UPS log revealing that only power cuts ever restarted the collection, then the script run by hand that dies before your eyes at the first refusal from the customs window. Before touching anything, you put the configuration under Git. You then hand the collection over to systemd: a unit that restarts it without wearing itself out, under its own account, with bounded memory and a journal where nothing gets lost any more. The nightly summary, which cron forgot on power-cut nights, becomes a timer that catches up on missed runs. That leaves the barcode scanner. The kernel numbers devices in the order they arrive, and the labelling software now listens to the keyboard. You read what the kernel and udev know about a device, meet the modules that drive it, and write the udev rule that names it by what it is, not by its place in the queue. To finish, a colleague "simplifies" the configuration on a Friday evening, and Git tells you in ten seconds what changed. The customs window and the dock hardware are simulated on the machine, and behave like the real ones. At the end, the server's state and your Git history are checked, and the result counts for your ranking.