Automatic screen backlight dimming in Linux when AC adapter is unplugged

This collection of scripts is used to provide some functionality in Linux to automatically dim the screen brightness when the AC adapter is unplugged (and to increase the brightness when it's plugged back in). This is done by listening to acpi events. Functionality to check the AC adapter status at boot or resume from suspend is done with systemd services.

acpi (acpi package in Ubuntu) is required for the AC adapter state to be queried on boot/resume, and acpid is required for listening for the AC adapter events (acpid package in Ubuntu, but it should be installed by default). acpi_listen (included in the acpid package in Ubuntu) is used for determining the AC adapter plug and unplug events for configuration.

1. Set the various variables in config.sh as desired:

• BATTERYDIM is the location where the scripts will be installed. This is only used during installation

• ACPI_UNPLUG and ACPI_PLUG are the ACPI events for unplugging the AC adapter and plugging it in, respectively. To determine the unplug event, plug in the adapter if it isn't already plugged in, run acpi_listen, and then unplug the adapter. Look for the event starting with ac_adapter and set ACPI_UNPLUG to that whole line. Plug the adapter back in to generate the plug event, and similarly set ACPI_PLUG to the line for the plug event.

• UNPLUG_BRIGHTNESS and PLUG_BRIGHTNESS indicate the brightness for the unplugged and plugged states, respectively. These are specified as percentages of the maximum value, so 25 means 25% of the max and 100 is 100% of the max (i.e. the brightest possible).

• BACKLIGHT specifies the backlight device to use in the filesystem, since there may be more than one such device available. (My laptop, for example, used to have two backlight devices—one for the keyboard backlight, and one for the screen.) Mine is /sys/class/backlight/intel_backlight. Take a look in /sys/class/backlight to see the different backlight options you have. To test one out, manually decrease the brightness to, say 50% of the max, and then use the brightness and max_brightness files to set the brightness to the max value; if you've got the right device, you should see your brightness go up to max.

  For example, if I hypothesized that /sys/class/backlight/intel_backlight was the right device, I'd decrease my screen brightness so that it's signifiantly dimmer, and then run (as root):

  # cat /sys/class/backlight/intel_backlight/max_brightness >
  /sys/class/backlight/intel_backlight/brightness

  Or if you want to do it via sudo:

  $ cat /sys/class/backlight/intel_backlight/max_brightness | sudo tee
  /sys/class/backlight/intel_backlight/brightness

  And I should see my screen brightness go up to the max value to confirm that I've got the right device.

• MIN_BACKLIGHT specifies the minimum allowable backlight percentage. This is to align with the increments used by the desktop environment for the brightness buttons. The default value of 1 is correct for GNOME.

2. Run install.sh as root:

   $ sudo ./install.sh

3. Enjoy the functionality that Windows and OSX have out of the box! (A reboot may be necessary, but I think you should be okay manually starting the relevant systemd services too.)

Once you have it installed, if you decide you want to change the brightness in the unplug or plug states, just set the appropriate values in config.sh in the installation directory; the scripts will read from that file automatically.

1. Run uninstall.sh as root. Note that this wipes out the installation directory as well as stopping and removing the systemd services and acpid event listeners, so if you've put anything in that folder or made any changes you want to keep, get them to a safe place first.

I've tested this on Ubuntu 15.04–20.04 as well as Manjaro (as of mid-2022); it should work on any other system that uses systemd and acpid. At some point, my 18.04 system stopped producing the relevant ACPI events, but the problem can be fixed by upgrading to 18.04.2.

Also, the config and installation are not particularly user-friendly. The ideal would be to get this functionality into GNOME, KDE, etc. so that it is there by default and is configurable via GUI. Alas delving into those DEs is a bit more than I want to do. ;)