sh.lua

local type, setmetatable, tostring, select, unpack = type, setmetatable, tostring, select, unpack
local ioo, iop, osrem = io.open, io.popen, os.remove

local tmpfile = os.tmpname()
local _EXIT = "exit"
local _SIGNAL = "signal"
local _TABLE = "table"
local _FUNC, _STR, _NUM, _BOOL = "function", "string", "number", "boolean"
local _TRIM = "^%s*(.-)%s*$"
local ignoreKeys = {
	["__cmd"] = true,
	["__input"] = true,
	["__exitcode"] = true,
	["__signal"] = true,
}

local function posixify(key, value)
	if ignoreKeys[key] then return "" end
	if type(key) == "function" then key = key(value) end
	if type(key) ~= "string" or key == "" then return "" end
	local t = type(value)
	if t == _FUNC then
		value = value(key)
		t = type(value)
		-- Silent return if the funcref returns nil.
		if t == "nil" then return "" end
	end

	if key:sub(1, 1) ~= "_" then
		if #key == 1 then key = " -" .. key
		else
			key = key:gsub("_", "-")
			key = " --" .. key
		end
	end

	if t == _TABLE then
		local build = {}
		local fmt = "%s='%s'"
		for _, v in next, value do
			if type(v) == _NUM or type(v) == _STR then
				build[#build+1] = fmt:format(key, tostring(v))
			end
		end
		return table.concat(build, " ")
	elseif t == _STR then
		-- Return --key='value'
		if #value > 0 then return key .. "='" .. value .. "'" end
		return "" -- 'value' is zero-length, so return nothing
	elseif t == _NUM then
		-- Return --key=value
		return key .. "=" .. tostring(value)
	elseif t == _BOOL then
		if value == true then return key end
		return ""
	end
	error("invalid argument type", t, value)
end

local process
process = function(n, s, input, ...)
	for i = 1, n do
		local a = (select(i, ...))
		if type(a) == _TABLE then
			if a.__input then
				input = input .. " " .. a.__input
			end
			if #a ~= 0 then
				process(#a, s, input, unpack(a))
			else
				for k, v in pairs(a) do s = s .. posixify(k, v) end
			end
		else
			s = s .. " " .. tostring(a)
		end
	end
	return s, input
end

local function run(cmd)
	local p = iop(cmd, "r")
	local output = p:read("*a")
	local _, exit, status = p:close()
	osrem(tmpfile)
	return output, exit, status
end

local command

local invokedMt = {
	__index = function(_, k) return command(k) end,
	__tostring = function(self) return self.__input:match(_TRIM) end
}
local function invoke(cmd)
	local output, exit, status = run(cmd)
	-- If you add new keys here, add them to ignoreKeys
	return setmetatable({
		__cmd = cmd,
		__input = output,
		__exitcode = exit == _EXIT and status or 127,
		__signal = exit == _SIGNAL and status or 0,
	}, invokedMt)
end

local cmdMt = {
	__call = function(self, ...)
		local s = self.__cmd
		local input
		local n = select("#", ...)
		if n ~= 0 then
			local args, data = process(n, "", "", ...)
			s = s .. args
			input = data
		end
		if input and input ~= "" then
			local f = ioo(tmpfile, "w")
			f:write(input)
			f:close()
			s = s .. " <" .. tmpfile
		end
		return invoke(s)
	end,
	__tostring = function(self)
		return (run(self.__cmd)):match(_TRIM)
	end,
}

local cache = {}
command = function(cmd)
	if not cache[cmd] then cache[cmd] = setmetatable({ __cmd = cmd }, cmdMt) end
	return cache[cmd]
end

return setmetatable({
	command = command,
	_ = function(cmd, ...) return command(cmd)(...) end,
	fork = "folknor",
	version = 4,
}, {
	__div = function(_, v) return command(v) end,
	__mod = function(_, v) return (run(v)):match(_TRIM) end,
	__call = function(_, ...)
		local n = select("#", ...)
		if n == 1 then return command(...) end
		local ret = {}
		for i = 1, n do
			ret[#ret+1] = command((select(i, ...)))
		end
		return unpack(ret)
	end
})