gistfile1.txt

#!/usr/bin/env bash
#
# syntax:
# 'sudo zbright +N' increments screen brightness by N percent (N=0-100)
# 'sudo zbright -N' decrements screen brightness by N percent
# 'sudo zbright N'  sets screen brightness to N percent
# 'sudo zbright'    w/o any args reports current brightness 
#
# note: must be run as sudo
#
# 2008-12-24   Ryan M. Eustice   eustice@umich.edu
# 2009-02-23   Yuichiro MASUI    masui@masuidrive.jp

# probe current brightness
ho=0x$(setpci -s 00:02.0 F4.B); # hex     (0-FF)
do=$(printf "%d" $ho);          # decimal (0-255)
po=$(($do*100/255));            # percent (0-100)
#echo "$ho $do $po"

if [ $1 ]; then
    if [ "$(echo $1 | tr -d '\-')" != "$1" ]; then
        delta=$(echo $1 | tr -d '\-');
        d=$((do-delta));
    elif [ "$(echo $1 | tr -d '\+')" != "$1" ]; then
        delta=$(echo $1 | tr -d '\+');
        d=$((do+delta));
    else
        d=$1;
    fi

    if [ $d -ge 256 ]; then
        d=255;
    fi
    if [ $d -le 0 ]; then
        d=0;
    fi

    h=0x$(printf "%x" $d);    # hex     (0-FF)
    H=$(printf "%x" $h);      # strip 0x hex prefix
    setpci -s 00:02.0 F4.B=$H
else
    echo "$ho $do $po";
fi