set_brightness_level.sh

#!/bin/bash
# We want to find the directory where all the backlight data
# is stored , based on the fact that there's always "actual_brightness"
# file in that directory
DIR=$(find /sys/devices/ -type f -name "actual_brightness" -printf "%h" )

# Graphic cards all have different integer value for
# maximum level of brightness, so we need to get it
MAX=$(cat "$DIR"/max_brightness  )

# Percentage which we want to set
PERCENT="0.4"

# Calculate new value
NEW=$( bc <<< "$MAX*$PERCENT" )

# Print out the resulting float number
# as int to the controlling file
printf "%.0f" "$NEW" > "$DIR"/brightness