a "shell" in lua

shell.lua

local unistd = require("posix.unistd")
local stdlib = require("posix.stdlib")
local wait = require("posix.sys.wait")

local topcmd = nil

local Cmd = {}

function Cmd:exec()
	local cmds = {}
	table.insert(cmds, self)
	local p = self.pipeto
	while p do
		table.insert(cmds, p)
		p = p.pipeto
	end
	local pipes = {}
	for i = 1, #cmds - 1 do
		local p0, p1 = unistd.pipe()
		if not p0 then
			error(p1)
		end
		pipes[i] = {p0, p1}
	end

	local pids = {}

	for i = 1, #cmds do
		local pid, errmsg = unistd.fork()
		if pid == nil then
			error(errmsg)
		elseif pid == 0 then
			-- child
			if i > 1 then
				-- stdin
				local fd, errmsg = unistd.dup2(pipes[i-1][1], 0)
				if not fd then
					error(errmsg)
				end
			end

			if i < #cmds then
				-- stdout
				local fd, errmsg = unistd.dup2(pipes[i][2], 1)
				if not fd then
					error(errmsg)
				end
			end

			for i = 1, #pipes do
				unistd.close(pipes[i][1])
				unistd.close(pipes[i][2])
			end

			local ok, err = unistd.execp(cmds[i].cmd, cmds[i].argt)
			if not ok then
				io.stderr:write(err .. "\n")
				unistd._exit(1)
			end
		else
			table.insert(pids, pid)
			-- parent
		end
	end

	for i = 1, #pipes do
		unistd.close(pipes[i][1])
		unistd.close(pipes[i][2])
	end

	for i = 1, #pids do
		local status, errmsg = wait.wait(pids[i])
		if status == nil then
			error(errmsg)
		end
	end
	return nil
end

function Cmd:__call(...)
	self:arg(select(1, ...))
	return self
end

function Cmd:__bor(right)
	local c = self
	if self.parent then
		c = self.parent
	end
	c.pipeto = right
	return self
end

function Cmd:__band(what)
	-- it's a fake Command, convert it to arg
	local v = stdlib.getenv(what.cmd)
	if not v then
		error("no such environment variable " .. what.cmd)
	end
	self:arg(v)

	-- set parent so we can track pipes correctly
	what.parent = self

	return self
end

function Cmd:arg(a)
	self.argt[#self.argt + 1] = a
	return self
end

local function newcmd(c)
	return setmetatable({
		cmd = c, argt = {}
	}, {
		__index = Cmd,
		__call = Cmd.__call,
		__bor = Cmd.__bor,
		__band = Cmd.__band,
	})
end

local function getenv(arg)
	if arg then
		print(stdlib.getenv(arg))
	else
		for k,v in pairs(stdlib.getenv()) do
			print(string.format("%s=%s", k, v))
		end
	end
end

local function setenv(k, v)
	stdlib.setenv(k,v)
end

local gbls = {
	print = print,
	quit = function() os.exit(0) end,
	help = function()
		print([[Syntax:
    help: this help
    print("hello")
    setenv("X", "Y")
    setenv("X")
    getenv("X")
    cmd "arg1" "arg2"
    cmd1 | cmd2
    cmd1 "arg" | cmd2 "arg" | cmd3 "arg"
    cmd1 &ENVVAR
]])
	end,
	getenv = getenv,
	setenv = setenv,
}

local function gindex(t, k)
	if gbls[k] then return gbls[k] end

	local cmd = newcmd(k)
	if not topcmd then
		topcmd = cmd
	end
	return cmd
end

local env = setmetatable({
	prompt = "lsh# ",
}, {__index = gindex})

repeat
	io.write(env.prompt)
	local line = io.read()
	if not line then break end
	local chk, err = load("return " .. line, "stdin", "t", env)
	if not chk then
		print(err)
	else
		local ok, result = pcall(chk)
		if not ok then
			print(result)
			goto continue
		end

		if ok and type(result) == "function" and not topcmd then
			result()
			goto continue
		end

		if topcmd then
			local ok, err = pcall(function() topcmd:exec() end)
		end
		topcmd = nil
		if not ok then
			print(err)
		end
	end
	::continue::
until false