$ sysctl -a | grep brightness
hw.acpi.video.lcd0.brightness: 50
compat.linuxkpi.i915_invert_brightness: 0
So if you want to write a script that works both, you need to put fujitsu-specific knowledge in your script. Vs. alternatively, an interface that works everywhere.(Incidentally I think this sysctl doesn't do much and I instead use a utility called intel_backlight(1) from ports.)