#!/bin/bash

step=10
file=/sys/class/leds/chromeos::kbd_backlight/brightness

case "$1" in
    -i|--increase) ((val = +10));;
    -d|--decrease) ((val = -10));;
esac

if !((val)); then
  echo "Increase of decrease keyboard brightness"
  echo "Usage: ${0##*/} --increase | --decrease"
  exit
fi

read -r cur < "$file"
((val = cur + val))

if ((val <   0)); then ((val =   0));fi
if ((val > 100)); then ((val = 100));fi

printf '%d' "$val" > "$file"

printf 'Before: %3d\n' "$cur"
printf 'After : %3d\n' "$val"