How to change brightness before logging in?

Question

Whenever I boot, the brightness is set to full. I read this and make changes so that it do not reset every time I boot into my laptop.

But before this, I tried to change brightness before login, but brightness buttons only work when I log in. So after my problem of Brightness is reset to Maximum on every Restart is solved, I just want to know any way to change the brightness just after OS start, that is before login.

Answer 1

Running a script before or once login screen appears

There are two ways to approach this:

1. Place command or call to script in /etc/rc.local. For instance

   #!/bin/sh -e
   #
   # rc.local
   #
   # This script is executed at the end of each multiuser runlevel.
   # Make sure that the script will "exit 0" on success or any other
   # value on error.
   #
   # In order to enable or disable this script just change the execution
   # bits.
   #
   # By default this script does nothing.
   
   # path to my script. Note the & at the end, it's important
   /home/serg/bin/brightness_set.sh &
   exit 0
   

2. The login screen is actually known as desktop manager, and Ubuntu in particular uses lightdm desktop manager. Its configuration file /etc/lightdm/lightdm.conf can take greeter-setup-script= parameter to execute something before login screen actually shows up. For instance, you could do:

   [Seat:*]
   greeter-setup-script=/opt/set_brightness.py
   

   NOTE: Older versions of the header [SeatDefaults] is deprecated now, use [Seat:*]. Also, if you have never edited /etc/lightdm/lightdm.conf it will be blank - that's the normal behavior since 14.04 Ubuntu version.

Controlling the brightness:

Setting brightness will have to be done by writing to brightness file in /sys/class/backlight/<NAME> directory. There are couple different <NAME> versions than can appear, for example mine is /sys/class/backlight/intel_backlight , so you will need to figure out the name your own computer uses or alternatively use /sys/class/backlight/*/brightness. There is also max_brightness file in the same location, which you can use as 100% value to calculate brightness to set. Both files accept integer value.

Common way to write into that file is via echo 123 | sudo tee /sys/class/backlight/*/brightness from command line. If you are running a script via /etc/rc.local or via greeter-setup-script= parameter mentioned above, sudo is not necessary, as both method run scripts with root privillege ( which is also important to remember for security reasons, so ensure your script is accessible only to your user or root only ) .

There are also alternatives to writing into /sys , but not all of them are good. In my experience xbacklight doesn't work for Ubuntu users and xrandr --output SCREEN_NAME --brightness INT is only a software solution (i.e., it doesn't actually decrease power of the screen, only makes screen turn darker color).

Additional resources

• If you want more flexible control over brightness and set specific level based on time of day, see my script on the related question here

• Arch Wiki article contains plenty of information about backlight and ways to control it.