If you want to adjust your screen brightness in Linux (Debian / Ubuntu) from the command line you can write to a file:

/sys/devices/virtual/backlight/acpi_video0/brightness

The maximum value for this variable is listed in the file:

/sys/devices/virtual/backlight/acpi_video0/max_brightness

A script can easily be written to have a variable argument:

if [ $# -eq 0 ]; then
        VAR="0"
else
        VAR=`echo $1`
fi
sudo sh -c "echo $VAR > /sys/devices/virtual/backlight/acpi_video0/brightness"