change_keyboard_backlight

#!/bin/bash
#
# Increase/decrease keyboard backlight.
#
# Dependencies: coreutils, dbus, upower, util-linux

set -o nounset -o pipefail

readonly DBUS_CONNECTION="org.freedesktop.UPower"
readonly DBUS_INTERFACE="${DBUS_CONNECTION}.KbdBacklight"
readonly DBUS_OBJECT_PATH="/org/freedesktop/UPower/KbdBacklight"

readonly L_OPTS="help,version"
readonly OPTS="hVv"

readonly CMD="${0##*/}"
readonly USAGE="${CMD} [OPTION]... +|-"
readonly USAGE_ERR_MSG=$(
  cat <<END
Usage: ${USAGE}
Try '${CMD} --help' for more information.
END
)
readonly VERSION_INFO=$(
  cat <<END
${CMD} (scripts) 1.0.0

This is free and unencumbered software released into the public domain.
For more information, please refer to <https://unlicense.org/>.
END
)
readonly HELP=$(
  cat <<END
SYNOPSIS
  ${USAGE}

DESCRIPTION
  Increase (+) or decrease (-) keyboard backlight.

OPTIONS
  -h, --help
    display this help and exit
  -V, -v, --version
    display version and exit
END
)

#######################################
# Get the integer value of kbd backlight via dbus
# Globals:
#  DBUS_CONNECTION - name of the dbus connection to receive the message
#  DBUS_INTERFACE - name of the D-Bus interface
#  DBUS_OBJECT_PATH - path to the D-Bus object
# Arguments:
#  method - dbus interface method (e.g., GetBrightness)
#######################################
function get_dbus_kbd_backlight() {
  local -r method="$1"

  dbus-send --print-reply="literal" --system \
    --dest="${DBUS_CONNECTION}" \
    "${DBUS_OBJECT_PATH}" "${DBUS_INTERFACE}.${method}" \
    | cut --delimiter " " --fields 5
}

#######################################
# Set the integer value of kbd backlight via dbus
# Globals:
#  DBUS_CONNECTION - name of the dbus connection to receive the message
#  DBUS_INTERFACE - name of the D-Bus interface
#  DBUS_OBJECT_PATH - path to the D-Bus object
# Arguments:
#  brightness - int32 value to set kbd brightness to
#######################################
function set_dbus_kbd_backlight() {
  local -ir brightness="$1"

  dbus-send --system --type=method_call \
    --dest="${DBUS_CONNECTION}" \
    ${DBUS_OBJECT_PATH} "${DBUS_INTERFACE}.SetBrightness" \
    "int32:${brightness}"
}

#######################################
# Main function
# Globals:
#   CMD - script basename
#   HELP - help message
#   L_OPTS - long non-positional options (e.g. --help)
#   OPTS - short non-positional options (e.g. --h)
#   USAGE_ERR_MSG - usage error message
#######################################
function main() {
  local opts
  opts=$(
    getopt \
      --options=${OPTS} --longoptions=${L_OPTS} \
      --name "${CMD}" \
      -- "$@"
  )
  if (($? != 0)); then
    echo "${USAGE_ERR_MSG}"
    exit 2
  fi

  eval set -- "${opts}"

  local need_help=false
  local need_version=false

  while true; do
    case "$1" in
      -h | --help)
        need_help=true
        shift
        break
        ;;
      -V | -v | --version)
        need_version=true
        shift
        break
        ;;
      --)
        shift
        break
        ;;
    esac
  done

  if [[ $need_help == true ]]; then
    echo "${HELP}"
    exit 0
  fi

  if [[ $need_version == true ]]; then
    echo "${VERSION_INFO}"
    exit 0
  fi

  if (($# != 1)) || [[ $1 != "+" && $1 != "-" ]]; then
    if (($# != 1)); then
      echo "${CMD}: expected 1 argument; got $#"
    else
      echo "${CMD}: expected '+' or '-'; got $1"
    fi
    echo "${USAGE_ERR_MSG}"
    exit 2
  fi

  local -ir min_brightness=0
  local -ir max_brightness=$(get_dbus_kbd_backlight "GetMaxBrightness")

  local -i brightness=$(get_dbus_kbd_backlight "GetBrightness")

  if [[ $1 == "+" ]]; then
    ((brightness += 1))
  else
    ((brightness -= 1))
  fi

  if ((brightness > max_brightness)); then
    brightness=max_brightness
  elif ((brightness < min_brightness)); then
    brightness=min_brightness
  fi

  set_dbus_kbd_backlight "${brightness}"
}

main "$@"