https://wiki.postmarketos.org/wiki/Amazon_Echo_Dot_2nd_gen_(amazon-biscuit)

rainbow_daemon.sh

#!/bin/ash

# usage: rainbowd [--help|-h|--version|-V|options...]
#
# --fork           do not read or write pid file                 (FORK=true)
# --dump           print initialized color buffer and exit       (DUMP_FRAME=true)
# --verbose    -v  print color buffer table                      (DEBUG=true)
# --loop       -L  play animations, rotate by default            (LOOP=true)
# --no-loop    -l  only render first frame                       (LOOP=false)
# --reverse    -r  reverse animations                            (REVERSE=true)
# --fade       -F  play fade animation                           (FADE=true)
# --blink      -B  play blink animation
#
# --dim(-[0123])?  set led intesity (DIM_LED=[0123]) based on ambient light (DIM_LED=true)
# --no-dim     -d  do not set led intesity                       (DIM_LED=false)
# --invert         invert the color buffer                       (INVERT=true)
# --brightness -b  dim the color buffer (0-1.0)
# --timeout    -t  time in secs to wait before killing self
# --interval   -i  time in secs to wait between updates
#
# --nop        -n  do not reset color buffer
# --clear      -C  fill color buffer with "blank" color
# --color      -c  fill color buffer with color
# --bg             fill "blank" color buffer with color
# --spinner    -s  append color to hind of color buffer
# --frame      -f  set color buffer (12 hex colors with no spaces)
# --pastel         use pastel rainbow color palette
#
# misc env vars: IO_RING IO_AMBIENT IO_LED_DIM
#
# rainbow_daemon.sh v0.9
# [email protected]

# default pattern (12 hex colors)
frame='0000ff0000ff4f00cf4f00cfff0000ff0000af2f00af2f00ffff00ffff0000ff0000ff00'

# misc defaults
[ -z "$DEBUG" ]      && DEBUG=false
[ -z "$DUMP_FRAME" ] && DUMP_FRAME=false
[ -z "$LOOP" ]       && LOOP=true
[ -z "$DIM_LED" ]    && DIM_LED=true
brightness='1'
interval='0.1'
blank_frame='000000000000000000000000000000000000000000000000000000000000000000000000'

# devices: led array; ambient light sensor; led power
[ -z "$IO_RING"    ] && IO_RING='/sys/bus/i2c/devices/0-003f/frame'
[ -z "$IO_AMBIENT" ] && IO_AMBIENT='/sys/bus/iio/devices/iio:device0/calibrated_lux'
[ -z "$IO_LED_DIM" ] && IO_LED_DIM='/sys/bus/i2c/devices/0-003f/led_current'

while [ $# -gt 0 ]; do case $1 in
	"--help"    | "-h") sed -n "/^# usage/,/^# ~/p" "$(which $0)" | sed -E 's/(# |#)//'; exit ;;
	"--version" | "-V") grep "rainbow_daemon.sh\sv" "$(which $0)" | sed -E 's/(# |#)//'; exit ;;
	"--fork"          ) FORK=true; shift ;;
	"--dump"          ) DUMP_FRAME=true; shift ;;
	"--verbose" | "-v") DEBUG=true; shift ;;
	"--loop"    | "-L") LOOP=true; shift ;;
	"--no-loop" | "-l") LOOP=false; shift ;;
	"--reverse" | "-r") REVERSE=true; shift ;;
	"--blink"   | "-B") blink=true; shift ;;
	"--fade"    | "-F") FADE=true; shift ;;

	"--dim"              ) DIM_LED=true; shift ;; "--dim-0") DIM_LED=0; shift ;; "--dim-1") DIM_LED=1; shift ;; "--dim-2") DIM_LED=2; shift ;; "--dim-3") DIM_LED=3; shift ;;
	"--no-dim"     | "-d") DIM_LED=false; shift ;;
	"--invert"           ) INVERT=true; shift ;;
	"--brightness" | "-b") brightness=$2; shift 2 ;;
	"--interval"   | "-i") interval=$2; shift 2 ;;
	"--timeout"    | "-t") timeout=$2; shift 2 ;;

	"--nop"     | "-n") LOOP=false; frame=$(cat /sys/bus/i2c/devices/0-003f/frame); blank_frame=$frame; shift ;;
	"--clear"   | "-C") LOOP=false; blink=false; frame=$blank_frame; shift ;;
	"--color"   | "-c") LOOP=false; frame=$(seq 12 | awk "{printf \"$2\"}"); shift 2 ;;
	"--bg"            ) blank_frame=$(seq 12 | awk "{printf \"$2\"}"); shift 2;;
	"--spinner" | "-s") LOOP=true; frame=$2$blank_frame; shift 2 ;;
	"--frame"   | "-f") frame=$2; shift 2 ;;
	"--pastel") frame='881177aa3355cc6666ee9944eedd0099dd5544dd8822ccbb00bbcc0099cc3366bb663399'; blank_frame=$frame; shift ;;

	*) echo "unknown argument: $1"; exit ;;
esac; done

# helper functions
transcolor() {
	r=$(printf "%02x" $(printf "($2)/1" 0x$(echo $1 | cut -c 1-2) | bc))
	g=$(printf "%02x" $(printf "($2)/1" 0x$(echo $1 | cut -c 3-4) | bc))
	b=$(printf "%02x" $(printf "($2)/1" 0x$(echo $1 | cut -c 5-6) | bc))
	echo -n "$r$g$b"
}

darken() { transcolor $1 "%d*$2"; }
invert() { transcolor $1 "255-%d"; }

invertframe() { for i in $(seq 1 12); do echo -n $(invert $(echo $1 | cut -c $(( i*6 - 5 ))-$(( i*6 ))) $2); done; }
darkenframe() { for i in $(seq 1 12); do echo -n $(darken $(echo $1 | cut -c $(( i*6 - 5 ))-$(( i*6 ))) $2); done; }

# debug functions
printc() {
	r=$(printf "%d" 0x$(echo $1 | cut -c 1-2))
	g=$(printf "%d" 0x$(echo $1 | cut -c 3-4))
	b=$(printf "%d" 0x$(echo $1 | cut -c 5-6))
	echo -n "\033[48;2;${r};${g};${b}m    \033[0m"
}

printframe() {
	for i in $(seq 1 12); do echo -n $(printc $(echo $1 | cut -c $(( i*6 - 5 ))-$(( i*6 )))); done
	echo -n "\033[0m"
}

# init rainbowd
# * get initial color buffer
frame=$(echo $frame | cut -c -72)
[ "$DEBUG" = true ] && echo -e "<        $(printframe $frame)" &
#   →  apply invert filer
[ "$INVERT" = true ] && { frame=$(invertframe $frame)
	[ "$DEBUG" = true ] && echo -e "*        $(printframe $frame)" &
} # →  apply darken filer
[ ! "$brightness" = 1 ] && { frame=$(darkenframe $frame $brightness)
	[ "$DEBUG" = true ] && echo -e "*        $(printframe $frame)" &
}

[ "$DUMP_FRAME" = true ] && { echo "$frame"; exit; }

# * replace previous instance of rainbowd
if [ -z "$FORK" ]; then
	if [ -e /run/rainbow_daemon.pid ] && [ -d /proc/$(cat /run/rainbow_daemon.pid) ]; then
		kill -9 $(cat /run/rainbow_daemon.pid)
	fi
	echo $$ > /run/rainbow_daemon.pid
fi

# * set timeout
if [ -n "$timeout" ]; then
	{ sleep $timeout; kill -9 $$; rainbowd --fork --clear; } &
fi

# * set led brightness
case "$DIM_LED" in
	0|1|2|3) echo $DIM_LED > $IO_LED_DIM ;;
esac

# blink animation
if [ "$blink" = true ]; then while true; do
	FORK=true DEBUG=$DEBUG LOOP=false DIM_LED=$DIM_LED INVERT=$INVERT rainbowd --frame $blank_frame
	sleep $interval
	FORK=true DEBUG=$DEBUG LOOP=false DIM_LED=$DIM_LED INVERT=$INVERT rainbowd --frame $frame
	sleep $interval
done; exit; fi

# render
i=12
while true; do
	if [ $i = 12 ] && [ "$DIM_LED" = true ]; then
		ambient=$(cat $IO_AMBIENT)
		if [ $ambient -gt 300 ]; then echo 0 > $IO_LED_DIM
		elif [ $ambient -gt 200 ]; then echo 1 > $IO_LED_DIM
		elif [ $ambient -gt 100 ]; then echo 2 > $IO_LED_DIM
		else echo 3 > $IO_LED_DIM
		fi
	fi
	i=$((i-1))
	[ $i -le 1 ] && i=12

	shift=000000
	if [ "$LOOP" = true ]; then
		if [ "$REVERSE" = true ]; then
			shift=$(echo $frame | cut -c 67-72)
			frame=$(echo "${shift}${frame}" | cut -c -72)
		else
			shift=$(echo $frame | cut -c 1-6)
			frame=$(echo "${frame}${shift}" | cut -c 7-78)
		fi
	fi


	if [ "$FADE" = true ]; then
		[ ! "$LOOP" = true ] && { [ "$REVERSE" = true ] && shift=$(echo $frame | cut -c 67-72) || shift=$(echo $frame | cut -c 1-6); }
		currentcolor=$(seq 12 | awk "{printf \"$shift\"}")
		echo "$currentcolor" > $IO_RING
		[ "$DEBUG" = true ] && echo -e "+ $(printc $shift) = $(printframe $currentcolor)" &
	else
		echo "$frame" > $IO_RING
		[ "$DEBUG" = true ] && [ "$LOOP" = true ] && echo -e "+ $(printc $shift) = $(printframe $frame)" &
	fi

	[ ! "$LOOP" = true ] && exit
	sleep $interval
done