set KDE Plasma backlight brightness using dbus with a bash script

plasma-backlight.sh

#!/bin/bash

# Set this to 1 to reset the backlight to 100% after execution
TEST_MODE=0

DBUS_SERVICE="local.org_kde_powerdevil"
DBUS_PATH="/org/kde/Solid/PowerManagement/Actions/BrightnessControl"
MAX_BRIGHTNESS=$(qdbus $DBUS_SERVICE $DBUS_PATH brightnessMax)


current_brightness() {
    qdbus $DBUS_SERVICE $DBUS_PATH brightness
}


if [ -z "$1" ]; then
    echo "set backlight using qdbus on KDE Plasma"
    echo "usage: $0 50"
    echo "usage: $0 up"
    echo "usage: $0 down"
    echo "usage: $0 -v"
    exit 0
    
elif [ "$1" = "-v" ]; then
    # Use bc for floating point division
    printf %.0f $(echo "$(echo \"$(current_brightness) / $MAX_BRIGHTNESS\" | bc -l) * 100" | bc -l)
    exit 0
    
else
    # Attempt to convert first arg to a number
    # bc returns 0 when given non-numbers
    desired_percent=$(echo "$1" | bc -l)
    if [ $desired_percent -gt 0 ] && [ $desired_percent -le 100 ]; then
        val=$(echo "$(echo \"$desired_percent / 100\" | bc -l) * $MAX_BRIGHTNESS" | bc -l)
        # use printf to strip decimals from bc's output, making it an integer again for dbus
        qdbus $DBUS_SERVICE $DBUS_PATH setBrightness $(printf %.0f $val)
        
    else
        # Check if arg controls relative "steps" up and down
        if [ "$1" = "up" ] || [ "$1" = "down" ]; then
            CURR_BRIGHTNESS=$(current_brightness)
            STEPS_BRIGHTNESS=$(qdbus $DBUS_SERVICE $DBUS_PATH brightnessSteps)
            if [ "$1" = "up" ]; then
                val=$(($CURR_BRIGHTNESS + $(($MAX_BRIGHTNESS / $STEPS_BRIGHTNESS))))
                
            elif [ "$1" = "down" ]; then
                val=$(($CURR_BRIGHTNESS - $(($MAX_BRIGHTNESS / $STEPS_BRIGHTNESS))))
            fi
            qdbus $DBUS_SERVICE $DBUS_PATH setBrightness $val
            
        else
            # Invalid argument
            exit 1
        fi
    fi
fi

if [ $TEST_MODE = 1 ]; then
    sleep 2s
    qdbus $DBUS_SERVICE $DBUS_PATH setBrightness $MAX_BRIGHTNESS
    echo "TEST_MODE is enabled"
fi