This small tool listens to file "actual_brightness" modification and reports current brightness percentage. Written in C. It can be used with i3blocks persistent block.

make release
The result executable will be placed into bin directory.
[brightness]
command=./brightness/bin/brightness -a /sys/class/backlight/intel_backlight/actual_brightness -m /sys/class/backlight/intel_backlight/max_brightness
interval=persist
brightness 0.1 - read actual brightness value in non-blocking style.
Usage: brightness [options]
Options:
-a, --actual_brightness_path path to file with actual brightness string
-m, --max_brightness_path path to file with max brightness string
-h, --help print this help.
-V, --version print version and exit.