# Some Useful Linux Snippets

## LED triggers

The Linux kernel offers both direct access to LEDs and support for many different *triggers* that control when/how an LED should light up.
You can see all available triggers for an LED by reading from its ''trigger'' file. The active trigger is indicated by brackets.

<code>
$ cat /sys/class/leds/input0::capslock/trigger

none kbd-scrolllock kbd-numlock [kbd-capslock] kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock disk-activity disk-read disk-write ide-disk mtd nand-disk cpu cpu0 cpu1 cpu2 cpu3 panic usb-gadget usb-host mmc0 wacom_battery_0-online AC-online BAT0-charging-or-full BAT0-charging BAT0-full BAT0-charging-blink-full-solid rfkill-any rfkill-none audio-mute audio-micmute rfkill0 phy0rx phy0tx phy0assoc phy0radio rfkill73 
</code>

For some nice triggers (e.g., *heartbeat*, *timer*, *oneshot*, *usbport*) you might need to load their module first.

<code>
$ modprobe ledtrig-timer 
$ echo "timer" > /sys/class/leds/input0::capslock/trigger
</code>

Two new files appear: ''delay_on'' and ''delay_off''. Both accept values in milliseconds.

<code>
$ echo 100 > /sys/class/leds/input0::capslock/delay_off
$ echo 100 > /sys/class/leds/input0::capslock/delay_on
</code>

See also:

* [ledtrig-timer.c](https://github.com/torvalds/linux/blob/master/drivers/leds/trigger/ledtrig-timer.c)
* [LED kernel documentation](https://www.kernel.org/doc/Documentation/leds/)