Script to control the screen brightness (Backlight) for Linux

README

Controlling the screen brightness on a Macbook Pro running linux is harder than it should be. 

For ...reasons the bundled `xbacklight` doesn't work, so we're left with no option but to roll 
up our sleaves and get our hands dirty. Fortunately various bash commands and `bc` makes this
task pretty trivial so here is a script to control the brightness.

This script supports the following api:

$ brightness -set 10
$ brightness -set +10
$ brightness -set 10%
$ brightness -set +10%
$ brightness -inc 10
$ brightness -dec 10

As with all things linux, its not as easy as save this to `/usr/local/bin` and you're ready
to go.

By default, only `root` can change the brightness by this method. To allow users in the `video` 
group to change the brightness, a `udev` rule such as the following can be used:

  ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="gmux_backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
  ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="gmux_backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"
  ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="gmux_backlight", RUN+="setpci -v -H1 -s 00:01.00 BRIDGE_CONTROL=0"

To add a user to the `video` group:

sudo gpasswd -a <USER> video

brightness

#!/bin/sh
#
# Brightness control for linux.
#
# Author: ktec

set -o errexit # abort on nonzero exitstatus
set -o nounset # abort on unbound variable

usage() {
cat <<EOF
Usage: ${0} [options]
    -set value
    -set value%
    -set +value%
    -set -value%
    -get
    -inc value
    -inc value%
    -dec value
    -dec value%
EOF
}

parse_args() {
  case ${1} in
    \+[0-9]*%) set_increment_by_percentage ${1//[!0-9]/} ;;
    \-[0-9]*%) set_decrement_by_percentage ${1//[!0-9]/} ;;
    \+[0-9]*)  set_increment ${1//[!0-9]/} ;;
    \-[0-9]*)  set_decrement ${1//[!0-9]/} ;;
    [0-9]*%)   set_percentage ${1//[!0-9]/} ;;
    *)         set_brightness ${1} ;;
  esac
}

set_increment() {
  current=$(get_brightness)
  value=$(echo $current + ${1} | bc)
  set_brightness $value
}

set_decrement() {
  current=$(get_brightness)
  value=$(echo $current - ${1} | bc)
  set_brightness $value
}

set_percentage() {
  max=$(get_max_brightness)
  value=$(echo "$max/100 * ${1}" | bc -l)
  set_brightness ${value%.*}
}

set_increment_by_percentage() {
  max=$(get_max_brightness)
  current=$(get_brightness)
  value=$(echo "${current} + ($max/100 * ${1})" | bc -l)
  set_brightness ${value%.*}
}

set_decrement_by_percentage() {
  max=$(get_max_brightness)
  current=$(get_brightness)
  value=$(echo "${current} - ($max/100 * ${1})" | bc -l)
  set_brightness ${value%.*}
}

set_brightness() {
  tee /sys/class/backlight/*/brightness <<< ${1} &>/dev/null
}

get_brightness() {
  cat /sys/class/backlight/*/brightness
}

get_max_brightness() {
  cat /sys/class/backlight/*/max_brightness
}

if [ "$#" -eq "0" ]; then
  usage
  exit 2
fi

case "$1" in
  "-set") parse_args $2;;
  "-get") get_brightness;;
  "-inc") parse_args "+$2";;
  "-dec") parse_args "-$2";;
  *) usage;;
esac