Rainbow Light Cycle for the UniFi LED Bar

UniFi devices that expose /proc/ubnt_ledbar let you set the LED bar color by writing an R,G,B value to /proc/ubnt_ledbar/custom_color. The short /bin/sh script below uses this to fade the LED through red, orange, yellow, green, blue and purple, then back to red, in an endless loop. A compiled Go version also exists, but this post covers only the shell script.

The Script

#!/bin/sh

# This function takes RGB values and writes them to the LED device
set_led_color() {
  echo -n "$1,$2,$3" > /proc/ubnt_ledbar/custom_color
}

# This function smoothly transitions from one color to another
transition_color() {
  local startR=$1
  local startG=$2
  local startB=$3
  local endR=$4
  local endG=$5
  local endB=$6
  local steps=20  # Use fewer steps since we're using a longer sleep duration
  local i=0

  while [ $i -le $steps ]; do
    local r=$(awk "BEGIN {print int($startR + ($endR - $startR) * $i / $steps)}")
    local g=$(awk "BEGIN {print int($startG + ($endG - $startG) * $i / $steps)}")
    local b=$(awk "BEGIN {print int($startB + ($endB - $startB) * $i / $steps)}")
    set_led_color $r $g $b
    sleep 1  # Use full second sleep
    i=$((i + 1))
  done
}

# The main loop to cycle through colors
while true; do
  # Transition from red to orange
  transition_color 254 0 0 254 165 0
  # Transition from orange to yellow
  transition_color 254 165 0 254 254 0
  # Transition from yellow to green
  transition_color 254 254 0 0 254 0
  # Transition from green to blue
  transition_color 0 254 0 0 0 254
  # Transition from blue to purple
  transition_color 0 0 254 160 32 240
  # Transition from purple back to red
  transition_color 160 32 240 254 0 0
done

How It Works

set_led_color R G B joins its three arguments into an R,G,B string and writes it to /proc/ubnt_ledbar/custom_color. The -n flag keeps echo from adding a trailing newline.

transition_color startR startG startB endR endG endB fades linearly between two colors:

• With steps=20 and the condition $i -le $steps, i runs from 0 to 20 inclusive. Each transition therefore makes 21 writes, beginning on the start color and ending exactly on the end color.
• Each channel is computed in awk as start + (end - start) * i / steps, and int() truncates the result to an integer.
• sleep 1 pauses one second between writes, so one transition takes about 21 seconds.

The main loop repeats six transitions forever. Each one ends on the color the next one starts from:

A transition’s last write and the next transition’s first write set the same color, so each key color holds for about two seconds before the next fade starts. A full cycle takes a little over two minutes.

local and echo -n aren’t strictly POSIX, but both work in common /bin/sh implementations such as BusyBox ash and dash.

Usage

Copy the script to the device, make it executable, and start it in the background with nohup so it keeps running after you log out:

scp light-cycle.sh <user>@<device-ip>:/tmp/
ssh <user>@<device-ip>
chmod +x /tmp/light-cycle.sh
nohup /tmp/light-cycle.sh > /dev/null 2>&1 &

Run it as a user that can write to /proc/ubnt_ledbar/custom_color. To stop it, find its PID with ps and kill it.