A standalone script to get the current charging percentage of your device's battery, written in Lua. Requires LGI. Linux only (maybe BSD too; probably not, though).

get-battery.lua

#!/usr/bin/env luajit

local script_version = "1.1.1"

local lgi = require("lgi")
local GLib = lgi.GLib
local UPowerGlib = lgi.UPowerGlib

local function get_battery()
	for _, device in pairs(UPowerGlib.Client():get_devices()) do
		if device:get_object_path():match("/battery_BAT[0-9]+$") then
			return device
		end
	end

	error("No battery found! Are you on a desktop PC or server?")
end

local function get_charge(battery)
	battery = battery or get_battery()

	return 100 * (battery["energy"] / battery["energy-full"])
end

local function print_charge(charge)
	print(("%3.02f%%"):format(charge or get_charge()))
end

local function print_help()
	print(([===[
%s (version %s): Get the current charge of the device's battery

Supported arguments:
	-h, --help          Show this help text and exit
	-v, --version       Print the current version and exit
	-w, --watch         Keep printing updates instead of only printing once (blocking)
]===]):format(arg[0]:gsub("^.*/", ""), script_version))
end

local options = {
	watch = false,
}

local function pasrse_arg()
	for k, v in pairs(arg) do
		if k < 1 then
			goto _continue_0
		end

		if (v == "-h") or (v == "--help") then
			print_help()
			os.exit(0)
		elseif (v == "-v") or (v == "--version") then
			print(script_version)
			os.exit(0)
		elseif (v == "-w") or (v == "--watch") then
			options.watch = true
		else
			print("ERROR: Unsupported arguments '" .. v .. "'!")
			print()
			print_help()
			os.exit(1)
		end

		::_continue_0::
	end
end

local function main(watch)
	pasrse_arg()

	print_charge()

	if not watch then
		return
	end

	local battery = get_battery()
	local previous_charge

	function battery:on_notify()
		local current_charge = get_charge(battery)

		if current_charge == previous_charge then
			return
		end

		print_charge()

		previous_charge = current_charge
	end

	GLib.MainLoop():run()
end

if (arg ~= nil and arg[1] ~= nil) or (... == nil) then
	main(options.watch)

	return
end

return {
	main = main,
	print_charge = print_charge,
	get_charge = get_charge,
	get_battery = get_battery,
	version = script_version,
}