On Wed, Sep 24, 2025 at 8:27 PM Demi Marie Obenour <demiobenour@gmail.com>
wrote:

> There are cases where a RUN+= script needs to do something
> exactly once each time a device appears, such as binding a
> different driver to the device.  If the udev rule matches
> based on a property (such as PCI device information) that
> is set only by the kernel, is it okay to use ACTION=="add"
> in the rule?  The only other options I know of are to either
>

Such events can still be caused by the admin doing "udevadm trigger
--action=". Not sure why one might do that, but probably better to not rely
on nobody doing that.


>
> 1. Add additional code to the script to make sure it is
>    idempotent.  This might require adding a lock.
>

Maybe not necessarily a lock as I *think* udev event processing is
serialized (for a given device at least); a flag file in /run or an xattr
on the /dev node might be enough.


>
> 2. Use a persistent daemon.
>

It might be possible to have a persistent Type=oneshot .service via
ENV{SYSTEMD_WANTS}, with RefuseManualStop. Not sure if that's a good idea.