A small script with minimal requirements for interacting with ACPI backlights via sysfs, intended as a stand-in for xbacklight on systems which it does not support. Interface uses percentages. Supports linear fading between values, relative brightness changes, and only-decrease and only-increase options.

acpi-brightness
acpi-brightness [-h|--help]
acpi-brightness [--time|-t TIME [--block|-b]] [--COND] [+-]TARGET

With no argument, report current brightness as % of maximum.

With TARGET %, alter brightness. If --time is set, fade over TIME seconds. If --block is also specified, subsequent attempts to change brightness will fail until this invocation is finished. --force can be used to ignore previously --blocking invocations.

COND is either inc[rement] or dec[rement] -- if decrement is specified, but TARGET is brighter than the current brightness, there will be no change.

• -h, --help : Display usage instructions
• -t, --time TIME : Fade brightness over TIME seconds (positive decimal)
• -b, --block : With --time, prevent other brightness changes until complete
• -f, --force : Stop previous blocking invocations
• --COND : Only change if increasing or decreasing brightness. COND is inc or dec

• Get current brightness in %

  $ acpi-brightness 
  100
  

• Set brightness to 40%

  $ acpi-brightness 40
  

• Set brightness to 40% unless current brightness is lower than 40%

  $ acpi-brightness --dec 40
  

• Decrease brightness by 10pp

  $ acpi-brightness -20
  

• Increase brightness by 20pp over 2.5 seconds

  $ acpi-brightness -t 2.5 +20
  

• Set brightness to 100% over 2 seocnds, and prevent any brightness changes until complete

  $ acpi-brightness --block -t 2 100
  

• Set brightness to 40%, ignoring previous --blocking invocations

  $ acpi-brightness -f 40
  

• awk

• Add $USER to video group

  $ sudo usermod -a -G video $USER
  

• Give video group write-access to ACPI brightness file

  The following udev rule will do this at startup:

  SUBSYSTEM=="backlight", RUN+="/usr/bin/chgrp video /sys/class/backlight/%k/brightness", RUN+="/usr/bin/chmod 660 /sys/class/backlight/%k/brightness"
  

  e.g. with the following command:

  $ sudo tee /etc/udev/rules.d/60-acpi-backlight.rules <<< 'SUBSYSTEM=="backlight", RUN+="/usr/bin/chgrp video /sys/class/backlight/%k/brightness", RUN+="/usr/bin/chmod 660 /sys/class/backlight/%k/brightness"'
  

• Place acpi-brightness on your $PATH

• Test everything is working

  $ acpi-brightness 100
  $ acpi-brightness
  100
  $ acpi-brightness 30
  $ acpi-brightness
  30
  

Hook for redshift to decrease brightness throughout the day: redshift-hooks

acpi-brightness is licensed under the GNU General Public Licence v3.0.