Backlight utility

get-backlight

#!/bin/bash

MAX_PATH="/sys/class/backlight/intel_backlight/max_brightness"
SET_PATH="/sys/class/backlight/intel_backlight/brightness"

MAX=$(cat $MAX_PATH)
CUR=$(cat $SET_PATH)
CURPERCENT=$(printf '%.2f' $(bc -l <<< "$CUR / $MAX * 100"))
printf '%s\n' "Backlight: $CUR/$MAX ($CURPERCENT%)"

set-backlight

#!/bin/bash

# This needs to echo into a system file.
if [[ $EUID -ne 0 ]]; then
  echo "This script must be run as root."
  exit 1
fi

INPUT=$1
if [ -z "$INPUT" ]; then
  echo "No argument supplied."
  exit 2
fi

if ! [[ "$INPUT" =~ ^[0-9]{1,3}(\+|-)?$ ]]; then
  echo "Invalid input."
  exit 3
fi

OPERATOR=${INPUT: -1}
if [[ "$OPERATOR" =~ ^(\+|-)$ ]]; then
  INPUT=${INPUT::-1}
else
  OPERATOR=""
fi

if [[ "$INPUT" -gt 100 ]] || [[ "$INPUT" -lt 0 ]]; then
  echo "Invalid input."
  exit 4
fi

# Paths.
MAX_PATH="/sys/class/backlight/intel_backlight/max_brightness"
SET_PATH="/sys/class/backlight/intel_backlight/brightness"

# Math.
MAX=$(cat $MAX_PATH)
OLD=$(cat $SET_PATH)
OLDPERCENT=$(bc -l <<< "$OLD / $MAX * 100")
printf 'Backlight: %d/%d (%.2f%%) -> ' $OLD $MAX $OLDPERCENT

# Some more math.
if [ "$OPERATOR" ]; then
  if [ "$OPERATOR" = "+" ]; then
    NEW=$(bc <<< "$OLD + ($MAX * $INPUT / 100)")
  else
    NEW=$(bc <<< "$OLD - ($MAX * $INPUT / 100)")
  fi
else
  NEW=$(bc <<< "$MAX * $INPUT / 100")
fi

# Make sure new value never goes out of bound.
if [[ "$NEW" -gt $MAX ]]; then
  NEW=$MAX
elif [[ "$NEW" -lt 0 ]]; then
  NEW="0"
fi

# A bit more math.
NEWPERCENT=$(bc -l <<< "$NEW / $MAX * 100")
printf '%d/%d (%.2f%%)\n' $NEW $MAX $NEWPERCENT

# Finally, set.
echo $NEW > $SET_PATH

set-backlight needs sudo. It's kinda annoying to use unless you put it into a sudoers file, e.g.

%sudo ALL=(root) NOPASSWD:/usr/local/bin/set-backlight

Usage:

set-backlight 0
set-backlight 25
set-backlight 100
set-backlight 15+
set-backlight 15-
get-backlight

Should be self-explanatory.

I'm using this with Openbox, to be added as shortcuts in Openbox's RC file.
I don't know how to otherwise set/get backlight level without root.