Toggle LEDs using triggers. Made for discrete notifications (e.g. while sitting in a lecture hall at cfgmgmtcamp :).

To blink the caps lock trigger on a ThinkPad when host.example.org responds to ping requests:

until ping -w2 -c1 -D host.example.org &>/dev/null; do
  echo -n .
  sleep 1
done && while sleep 0.5; do
  ./build/toggle-leds input4::capslock
done

In the above we send a single ICMP packet (-c1), exit after a deadline of 2 seconds (-w2) and use quiet mode (-q) and throw stdout and stderr away (&>/dev/null). Until ping exits with an exit code of 0 we’ll print a single dot with no newline (echo -n .) and sleep 1 second (which also means that we have an easier time killing the process if we want to cancel it).

When ping exits with 0 we’ll toggle the input4::capslock LED every 0.5 seconds until terminated.

The available LEDs differ between platforms. On a ThinkPad A485 the following are available:

$ find /sys/class/leds/ -mindepth 1 -maxdepth 1 -type d -follow -exec basename {} \; | sort
input4::capslock
input4::numlock
input4::scrolllock
input8::capslock
input8::compose
input8::kana
input8::numlock
input8::scrolllock
phy0-led
platform::micmute
platform::mute
tpacpi::kbd_backlight
tpacpi::power
tpacpi::standby
tpacpi::thinklight
tpacpi::thinkvantage

Note that not all of these actually work; Lenovo sadly decided to do away with the awesome thinklight…

To show a list of LEDs on your platform, run the following command:

find /sys/class/leds/ -mindepth 1 -maxdepth 1 -type d -follow -exec basename {} \; |
  sort

The binary needs read/write access to the brightness file and read access to the max_brightness file for LED devices under /sys/class/leds/ to work. The Makefile does this automatically, but for posterity this is the command you need to run — …but only if you understand what it actually does; setuid is dangerous!

setuid u+s ./build/toggle-leds