Updates the brightness of a screen using xrandr.

brightness.sh

#!/bin/bash

# Changes brightness as a float value in [0,1], using +N -N induces a relative
# change in brightness
# Usage:
# brightness.sh .8 # set brightness to .8
# brightness.sh -.1 # decrement brightness by .1
# brightness.sh +.1 # increment brightness by .1

# helper to do floating point evaluation
evaluate() {
    echo $@ | bc
}

# sometimes the laptop screen is eDP-1 or eDP-1-1, just gotta detect the case
if [ -n "$( xrandr | grep eDP-1-1 )" ]
then
    laptop="eDP-1-1"
else
    laptop="eDP-1"
fi

# brightness is the second token of the 5th entry after the device identifier
CUR_BRIGHT=$( xrandr --verbose --current | grep ^"$laptop" -A5 | tail -n 1 | awk '{print $2}' )

# if no arguments are provided then just print the current brightness.
# otherwise just use the first one
if [ "$#" -lt 1 ] 
then
    echo $CUR_BRIGHT
    exit 1
fi

# check for a sign in the first argument (+/-) use a relative offset, o.w just
# set the first arg to be brightness
ARGSGN=${1::1}
ARGREST=${1:1}
if [ $ARGSGN = '+' ]
    then
    NEW_BRIGHT=$( evaluate $CUR_BRIGHT + $ARGREST )
elif [ $ARGSGN = '-' ]
    then
    NEW_BRIGHT=$( evaluate $CUR_BRIGHT - $ARGREST )
else
    NEW_BRIGHT=$1
fi


# clamp the brightness between 0 and 1
if [ $( evaluate "$NEW_BRIGHT < 0" ) -eq 1 ] 
    then
    NEW_BRIGHT=0
elif [ $( evaluate "$NEW_BRIGHT > 1" ) -eq 1 ] 
    then
    NEW_BRIGHT=1
fi

# finally use xrandr to change brightness
xrandr --output $laptop --brightness $NEW_BRIGHT

This was implemented in allow for brightness controls on my Dell XPS 15 7590, which has an OLED panel (which doesn't work with xbrightness for me).