Userspace devices

Whenever a device is added or removed from the computer, a device node appears or vanishes at /dev. A userspace device manager dynamically executes instructions upon device events. This can be used to change permissions, automatically mount storage media or even just run an arbitrary executable.

Linux

For the Linux Kernel, there is a plethora of implementations available:

systemd-udevd, which has been assimilated by systemd, but is kept available outside of it by projects like Gentoo systemd-utils.
eudev, which is an effort to completely isolate udev from systemd.
mdev, is much simpler and more minimal than (e)udev.
mdevd, in the spirit of mdev and the same configuration format, but more performant. It was written by the author of s6 and s6-rc.
smdev, which is another alternative also compatible with mdev.

Source directories for all options are part of the goetia-linux package.

For compatibility reasons, eudev and systemd-udevd are the main choice for desktop systems. Do not use any of the alternatives unless you know what you are doing. If this is the case, the author recommends mdevd-conf.

FreeBSD

On FreeBSD the default devd is the only choice you have. Its source directory is part of goetia-freebsd.

Setup

After making your choice of the userspace device manager, add the corresponding source directories to the bundle udev, e.g. in case of:

systemd-udevd, add:
- udevd-srv
- udevd-log
- udevadm-settle
- udevadm-trigger
mdevd, add:
- mdevd-srv
- mdevd-log
- mdevd-coldplug
devd, add:
- devd-srv
- devd-log
...