Comment 26 for bug 280518

Thank you for the clarification. The brightness did not change. I don't
have to reboot or restart X for that change, do I?

Steve Langasek wrote:
> On Fri, Apr 10, 2009 at 08:55:02PM -0000, Jeremy LaCroix wrote:
>> The contents of each of those files is "7". (If it makes any difference,
>> I had the laptop plugged in to AC power which makes the brightness go up
>> as high as it can during POST).
>
>> The command that started with "echo 2" did nothing because I prefixed it
>> with "sudo" and it still wouldn't let me edit it. I edited it with Kate
>> and nothing changed, not sure if I have to log out after I change that
>> file or not.
>
> You'll need to edit the file by running:
>
> sudo -s
> echo 2 > /sys/devices/virtual/backlight/acpi_video0/brightness
>
> (redirection happens in the parent shell, not as part of the sudo command,
> so is not happening as root; editors are unlikely to work correctly on the
> special files under /sys.)
>