After the fresh installation of Ubuntu 18.04 at Dell Latitude E6400 laptop its keyboard backlight worked. Nevertheless, after some updates (I do not know when) the keyboard backlight has stopped working. What I can do?
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The following commands will help you check the backlight of your Dell Keyboard:
$ cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness 0
Above command will show the configured brightness
$ cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/max_brightness 4
Above command will help you see the maximum brightness value.
$ echo 4 | sudo tee /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness
Above command will set the maximum keyboard backlight brightness.
You can set the Time_out to your keyboard backlight using the Following Tutorial: