Perhaps influenced by industrial mandatory practices, I wanted to add an emergency button to my home server in order to shut it down fast. For example in case of power shortage, I’d like to trigger a clean shutdown before the UPS runs out of power and without requiring a screen or keyboard attached. Therefore I bought an USB “emergency” button.

That was the easiest part. The button acts as a single key keyboard emitting space (" “) character. Getting input from console is part of basics of UNIX training and documentation abounds. How do you do when there is no console attached? Quickly, I found that attached USB keyboards could be listed by browsing the /sys/bus/usb/devices/ tree where keyboard are of Class 03 and Protocol 01. The following script gets the list:

for dev in /sys/bus/usb/devices/*-*:*
do
if [ -f $dev/bInterfaceClass ]
then
	if [[ "$(cat $dev/bInterfaceClass)" == "03" && "$(cat $dev/bInterfaceProtocol)" == "01" ]]
	then
	echo "Keyboard detected: $dev"
	fi
fi
done

Where to go now? After further searches I figured out that I had to handle low level events from devices, USB or not, to get input in headless mode. Event sources are listed in /proc/bus/input/devices. A keyboard is an event source of type 120013. This value corresponds to a bitmap of flags. Then, finding all devices emitting keyboard events boils down to the following single-liner:

grep -E 'Handlers|EV=|Bus=' /proc/bus/input/devices | grep -B2 'EV=120013' | grep -v -E 'EV=|--'

The resulting list contains vendor ID and product ID of both USB devices, like the emergency button and non USB like the keyboard of a laptop. Once the Vendor and Product ID are known, it is possible to find the corresponding event source by searching in the /dev/input/event list.

Finally, when the right event file descriptor is found, it can be opened and read. It is however not so simple than getting characters from console: not only ona has to cope with key-codes instead of character values but also key bounce management is left to the user.

With all these findings, I managed to write a service (daemon) able to get button press and trigger a user defined script, for example a shutdown command. The result is available, along with tools to detect devices, in a dedicated repository.

If you know a simpler way to do, please contact me!