Reduce screen brightness in Ubuntu LTS 12.04.3

One of the things that kept bugging me was how the Ubuntu login screen would be so much brighter than I needed it to be. Coming directly from a black / purple background for the terminal and boot sequence, it would explode into colour and momentarily daze me.

It's almost part of my muscle memory now to tap the Fn-Brightness Down key combination until it reaches a more comfortable level at every startup. So I decided to take a different approach - find a command that allows me to issue the brightness level myself.

And it turns out that in Linux, there is a simple text file that controls the brightness of your backlight. For each laptop, it will differ, but I found mine using:

# find / -name brightness

That means, when logged in as the root user (the prompt typically shows a #), find files recursively starting from the root of the filesystem / with the -name brightness. This yielded the following files:

/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/brightness /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness /sys/devices/pci0000:00/0000:00:02.0/backlight/acpi_video1/brightness /sys/devices/pci0000:00/0000:00:1c.1/0000:03:00.0/leds/ath9k-phy0/brightness /sys/devices/pci0000:00/0000:00:1c.5/0000:07:00.0/leds/mmc0::/brightness

There we can immediately rule out the last two entries for leds since we are looking for entries to change screen brightness and not LEDs. Since the first three also involve backlight we can safely say we are on the right track. Then I changed to the pci0000:00/ directory to save a little typing*:

# cd /sys/devices/pci0000:00/ # cat 0000:00:01.0/0000:01:00.0/backlight/acpi_video0/brightness # cat 0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness # cat 0000:00:02.0/backlight/acpi_video0/brightness

Initially, I got 10 for both the acpi_video files, and 976 for the Intel one. This suggested that the Intel file offered finer control over the brightness settings than the other two, which seemed to be supported by the max_brightness files I found poking about in the same directories, which had values 10, 4882, 10 respectively.

So, while logged in as root, I tried:

# echo 0 > 0000:00:01.0/0000:01:00.0/backlight/acpi_video0/brightness

and that dimmed the screen somewhat. And trying the same for the Intel file turned off the screen completely, so I had to type** the last in the dark:

# echo 0 > 0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness # echo 200 > 0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness

And then I played around with the values for a bit to see what brightness values I would be comfortable with. Once I'd had enough, I decided to set up a shortcut so that I could adjust it without logging in as root. First, I made it writeable by all, then I set up a link to it in my home directory:

# chmod 666 0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness # ln -s /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness ~/brightness

Now I can adjust it to whatever value I want by issuing the following command within my home directory:

$ echo MY_PREFERRED_VALUE > brightness

Naturally, you might prefer to place the command in your startup scripts to use a preferred value automatically. That also means you don't have to change the permissions of that file, since root is active at startup. But that would need a bit more explaining, so I will leave it out of this post.

1. * if tab completion works, it saves a lot of typing.
2. ** here, press Up to retrieve the previous command, Home or Ctrl-A to go to the beginning, then replace 0 with 200 manually.