exponential backlight for Linux on Dell Inspiron 15R

lb-i15r.sh

#!/bin/sh
set -euf

# `dell_backlight` is exp (percieved lin),
# `intel_backlight` is lin (percieved log),
# but Intel's `0` is dimmer than Dell's,
# so I prefer `acpi_backlight` set to "intel", not "vendor".
BL=/sys/class/backlight/intel_backlight/brightness
[ -w "$BL" ] || sudo chmod a+w "$BL"
# BL API has built-in input-validation,
# so there's no need to worry about security
# (unless you run `chmod a+x`).

# 15 for Dell.
MAX=4882 #./max_brightness
BITS=7 #bit_len MAX - 6
B2=152 #$((MAX >> (BITS - 2)))
#B1=0

# ChromeOS behavior
B0() {
	# "b" can't change while off,
	# and we want a "smooth" transition to pure-black
	echo 0 > "$BL"
	# X11
	xset dpms force off 2>/dev/null \
	|| \ # GNOME Wayland fallback
	dbus-send \
		--session \
		--dest=org.gnome.ScreenSaver \
		--type=method_call \
		/org/gnome/ScreenSaver \
		org.gnome.ScreenSaver.SetActive \
		boolean:true
}

b="$1"

if [ "_$b" = _+ ]
then
	b="$(cat "$BL")"

	if [ "$b" -lt "$B2" ]
	then
		b="$B2"
	else
		b="$((b << 1))"
		if [ "$b" -gt "$MAX" ]
		then
			b="$MAX"
		fi
	fi

	echo "$b" > "$BL"

elif [ "_$b" = _- ]
then
	b="$(cat "$BL")"

	if [ "$b" -le 0 ]
	then
		B0
	elif [ "$b" -le "$B2" ]
	then
		echo 0 > "$BL"
	else
		echo "$((b >> 1))" > "$BL"
	fi

else
	b="$((b))" # safety parse

	if [ "$b" -le 1 ]
	then
		if [ "$b" -le 0 ]
		then
			B0
		else
			echo 0 > "$BL"
		fi
	else
		if [ "$b" -ge "$BITS" ]
		then
			b=0
		else
			b="$((BITS - b))"
		fi
		echo "$((MAX >> b))" > "$BL"
	fi
fi