Argthtjtr

I'd like to flash SD cards with a custom script (writes partition table, then copies over a rootfs, etc.) whenever an SD card is inserted.

At first glance this seemed straightforward -- I'd just use a udev rule to detect the SD card then call the script. The first problem is that since the reader already exists, it doesn't trigger an "add" just a "change", and not only that, it would cause multiple events and trigger the script multiple times. After some more filters I was able to get a simple script to trigger only once (/etc/udev/rules.d/99-sd-detect.rules):

ACTION=="change", KERNEL=="sd[b-z]", SUBSYSTEM=="block", ENV{DISK_MEDIA_CHANGE}=="1", ENV{DEVTYPE}=="disk", ATTRS{idVendor}=="8564", ATTRS{idProduct}=="4000", ENV{ID_DRIVE_FLASH_CF}=="1", RUN+="/bin/sh -c '/root/udev_flash_sd.sh %E{DEVNAME}'"

However, when I actually have the script write data to the SD card it is killed after a few seconds. Apparently in 2012 udev made changes that kill processes that don't return after a few seconds. I tried various forking/detaching/nohup etc. trying to keep it running, but to no avail.

It seems the proper solution is to have it call a systemd service to run anything that takes longer, e.g., something like:

ACTION=="change", KERNEL=="sd[b-z]", SUBSYSTEM=="block", ENV{DISK_MEDIA_CHANGE}=="1", ENV{DEVTYPE}=="disk", ATTRS{idVendor}=="8564", ATTRS{idProduct}=="4000", ENV{ID_DRIVE_FLASH_CF}=="1", TAG+="systemd", PROGRAM="/bin/systemd-escape -p --template=flash_sd@.service $env{DEVNAME}", ENV{SYSTEMD_WANTS}+="%c"

Where /etc/systemd/system/flash_sd@.service has:

[Unit]

BindTo=%i.device

After=%i.device



[Service]

Type=oneshot

TimeoutStartSec=360

ExecStart=/root/udev_flash_sd.sh /%I

This works if you call "systemctl start flash_sd@dev-sde.service", which is also what the PROGRAM part of the udev rule outputs (at least for /dev/sde). After much searching it looks like udev will not call the service on a "change" event, only an "add". ("udevadm monitor -p" shows the SYSTEMD_WANTS and tag correctly, not to mention testing a simple add rule that calls a systemd service works fine.)

So, how do I call a custom flashing script automatically upon insertion of an SD card?

Edit: It seems the best method is to pipe your command to "at now" to keep udev from killing it. E.g., "echo mycommand.sh | at now".

Originally it seemed that "& disown" would work, but it actually encounters the same problem as udev killing it, just after 300s (5m) rather than ~2-3s.