brightness.lua

local wibox = require("wibox")
local awful = require("awful")
 
brightness_widget = wibox.widget.textbox()
brightness_widget:set_align("right")

function round(num, idp)
    return tonumber(string.format("%." .. (idp or 0) .. "f", num))
end

function update_brightness(widget)
    local brightness = round(tonumber(awful.util.pread("xbacklight -get")) or 0, 0)
    brightness = " Brightness : " .. brightness .. "%" .. " |"

    widget:set_markup(brightness)
end

update_brightness(brightness_widget)

rc.lua

-- Brightness control display
local brightness = require("brightness")

right_layout:add(brightness_widget)


    awful.key({  }, "XF86MonBrightnessUp", function() update_brightness(brightness_widget) end),
    awful.key({  }, "XF86MonBrightnessDown", function() update_brightness(brightness_widget) end)