Copied from Ji m's Ubuntu handbook:
Open your file browser and navigate to “Computer-> sys -> class ->
backlight” directory. You’ll see two or three folders there:
In each folder there’s a file called actual_brightness, you can see
its content (brightness value) through the thumbnail icon.
Remember them and do:
Change your screen brightness through Fn + Arrow key.
Check out the actual_brightness files in each folder (Re-open each
folder to refresh the value).
Does the value change?
Remember the folder name in which the value of actual_brightness
changes. Replace acpi_video0 in below commands with that folder name
Press Ctrl+Alt+T on keyboard to open the terminal. When it opens, run the command below will give you the maximum level of your laptop
Mine is 9, so I can set backlight level from 0 to 9.
Run the command below one by one. You’ll get the super user privilege and open the /etc/rc.local file, a script executed at the
end of each multiuser runlevel, with gedit editor.
Add the line below before the last. Change the number 0 to the
brightness level you want.
echo 0 > /sys/class/backlight/acpi_video0/brightness