See 0README. Power management.

0README

These change the background color in X (and I think they don't work if you are
using a desktop system, so the `xset` command ought to be changed for whatever
Gnome/KDE/LXDE/XFCE/ASDFJKLSEMICOLON thing you are using if you are using such a
thing) based on battery power remaining.  The Lua one requires luaposix to be
installed.

The reason there are two versions is that Ruby was, when it ought to have been
sleeping, doing some select() loop.  I figured Lua would not do that to me, and
it does not.  (Porting is usually more fun than fighting the language.)

These are dirty scripts.  The Ruby one in particular.  It was written way back
in 2008 and occasionally patched.  They are not particularly well-designed,
either internally or from a usability perspective.  You've been warned.

power.lua

#!/usr/bin/env lua
colors = {
	red = {min = 99, max = 30},
	green = {min = 10, max = 99},
	blue = {min = 10, max = 99}
}
fcache = {}
battery_path = io.popen('ls -d /proc/acpi/battery/BAT*/ | sed 1q'):lines()()

function cstr(t, frac)
	return string.format("%d", t.min + ((t.max - t.min) * frac))
end

function cmd(frac)
	return "xsetroot -solid rgb:" ..
		cstr(colors.red, frac) .. "/" ..
		cstr(colors.green, frac) .. "/" ..
		cstr(colors.blue, frac)
end

function pull_state()
	local state = {}

	for i, fname in pairs({"alarm", "info", "state"}) do
		 for line in io.lines(battery_path .. fname) do
			local colon = string.find(line, ":")
			local n = string.find(line, "%d")
			if colon and n then
				local key = string.sub(line, 1, colon -1)
				local val = tonumber(string.match(line, "%d+"))

				if key == "present rate" then state.rate = val
				elseif key == "last full capacity" then state.capacity = val
				elseif key == "remaining capacity" then state.present = val end
			end
		end
	end

	state.plugged_in = state.rate == 0
	state.pct = (state.present * 100) / state.capacity

	return state
end

function set_color()
	local state = pull_state()
	local fraction = math.floor(state.pct)
	if not fcache[fraction] then fcache[fraction] = cmd(state.pct / 100) end
	os.execute(fcache[fraction])
end

if arg[1] == "-bg" then
	set_color()
elseif arg[1] == "-d" then
	local p = require("posix")
	local pid = p.fork()
	if pid == 0 then
		p.chdir("/")
		devnull = p.open("/dev/null", p.O_RDWR)
		p.dup2(devnull, 0)
		p.dup2(devnull, 1)
		p.dup2(devnull, 2)
		pid = p.fork()
		if pid == 0 then
			while true do
				set_color()
				p.sleep(61)
			end
		else
			os.exit(0)
		end
	else
		os.exit(0)
	end
else
	local state = pull_state()
	local rem = state.present / state.rate

	local hls
	if state.plugged_in then hls = "(plugged in)"
	else hls = string.format("(%0.02f hours remaining)", rem) end
			
	print(string.format("%6.02f%% ", state.pct) .. hls)
end

power.rb

#!/usr/bin/env ruby

Colors = {
	:red => [99, 30],
	:blue => [10, 99],
	:green => [10, 99],
}
FCache = []

BatteryPath = Dir['/proc/acpi/battery/BAT*/'].first

def ccalc(min, max, fraction)
	min + ((max - min) * fraction)
end

def pull_state
	bat_info = []
	%w(alarm info state).each { |file|
		bat_info.concat File.readlines(BatteryPath + file)
	}

	bat_info.map! { |line| line.split(':').each { |i| i.strip! } }

	pbi = Hash.new { |h,k| h[k] = bat_info.assoc(k).last }

	state = {
		:rate => pbi['present rate'].to_i,
		:capacity => pbi['last full capacity'].to_i,
		:present => pbi['remaining capacity'].to_i,
	}
	state[:plugged_in] = state[:rate].zero?

	state
end

def set_color
	state = pull_state
	fraction = (state[:present] * 100) / state[:capacity]
	FCache[fraction] ||= begin
		f = fraction.to_f / 100
		'xsetroot -solid rgb:%d/%d/%d' % [:red, :green, :blue].map! { |c|
			ccalc(Colors[c].first, Colors[c].last, f)
		}
	end
	system(FCache[fraction])
end

case ARGV.first
when nil
	state = pull_state
	fullness = (state[:present] * 100.0) / state[:capacity]
	hours_left = state[:present].to_f / state[:rate]
	hlstring = if state[:plugged_in]
			"plugged in"
		   else
			sprintf "%0.2f hours remaining", hours_left
		   end
	printf "%6.02f%% (#{hlstring})\n", fullness
when '-bg'
	set_color
when '-d'
	fork {
		Dir.chdir '/'
		fork { loop { set_color; sleep 61 } }
	}
end