diff --git a/build.lua b/build.lua
index 5dea716..af4623f 100644
--- a/build.lua
+++ b/build.lua
@@ -161,13 +161,20 @@ local function exportFile(srcpath, dstpath)
 end
 
 local function buildKernel(dst)
-	local sources = fs.list('src/kernel/')
+	local srcdir = 'src/kernel/'
+	local sources = fs.list(srcdir)
 	table.sort(sources)
 
+	for i, path in ipairs(sources) do
+		path = srcdir .. path
+		sources[i] = path
+		assert(loadfile(path)) --validate syntax
+	end
+
 	function sources:lines()
 		return coroutine.wrap(function()
 			for i, path in ipairs(sources) do
-				local f = assert(io.open('src/kernel/' .. path))
+				local f = assert(io.open(path))
 				for l in f:lines() do
 					coroutine.yield(l)
 				end
diff --git a/src/kernel/00_preamble.lua b/src/kernel/00_preamble.lua
index b5cc316..379f036 100644
--- a/src/kernel/00_preamble.lua
+++ b/src/kernel/00_preamble.lua
@@ -3,7 +3,6 @@ local bootfs = ...
 -- ...but [lua bios, openloader] don't, so fall back to the usual way
 bootfs = bootfs or computer.getBootAddress()
 
-
 --store all functions we need in locals so userspace mischief can't replace them
 --prefix 'k' (for kernel) on any functions userspace will also have a version of to ensure kernelspace
 --function usage is intentional (ie prevent accidentally using the kernel's `load` in a userspace `loadfile`)
@@ -12,9 +11,9 @@ local assert, checkArg, error, kload, ipairs, next, pairs, rawequal, rawset, set
 local invoke = component.invoke
 local kpullSignal, shutdown, uptime = computer.pullSignal, computer.shutdown, computer.uptime
 local co_create, co_status, kresume, kyield = coroutine.create, coroutine.status, coroutine.resume, coroutine.yield
-local huge = math.huge
-local format = string.format
-local pack, unpack = table.pack, table.unpack
+local huge, math_type = math.huge, math.type
+local find, format, match, sub = string.find, string.format, string.match, string.sub
+local concat, pack, unpack = table.concat, table.pack, table.unpack
 
 local big = math.maxinteger or huge --some large amount to read at once in kreadfile
 
@@ -26,4 +25,44 @@ _ENV = setmetatable({}, {
 	__newindex = function(self, key) return error(format('assigned global %q', key), 2) end,
 })
 
+
+local isFile --function(obj:any):boolean, boolean -- returns (is obj a file?), (is obj an open file?)
+
+--[[function(index:number, val:any, types:string...):string -- asserts `types` contains the type of `val`. similar to `checkArg`.
+	recognizes all possible returns from `type()` as well as ['integer', 'float', 'file']. unlike io.type(), 'file' ignores whether the file is open or closed.
+	on success, returns the element of `types` that matched.
+	on failure, throws an error originating from the caller's caller that mimicks built-in type errors. `index` should be the 1-based index of the argument that was the wrong type.
+]]
+local checkArgEx
+do
+	-- tail-recursive varargs part of checkArgEx
+	local function check(val, t, want, ...)
+		if t == want then
+			return want
+		elseif want == 'integer' or want == 'float' then
+			if math_type(val) == want then
+				return want
+			end
+		elseif want == 'file' then
+			if isFile(val) then
+				return want
+			end
+		elseif not want then
+			return nil
+		else
+			return check(val, t, ...)
+		end
+	end
+
+	function checkArgEx(i, val, ...)
+		local t = type(val)
+		local ok = check(val, t, ...)
+		if ok then
+			return ok
+		else
+			error(format('bad argument #%d (%s expected, got %s)', i, concat({...}, ' or '), t), 3)
+		end
+	end
+end
+
 -- vi: set ts=2:
diff --git a/src/kernel/02_io.lua b/src/kernel/02_io.lua
new file mode 100644
index 0000000..771dc03
--- /dev/null
+++ b/src/kernel/02_io.lua
@@ -0,0 +1,307 @@
+local openNull, openPath
+do
+	local filedata = setmetatable({}, { __mode = 'k' }) --weak-keyed mapping of file object to data/inner (fh of wrapFile)
+	local filemt = { __metatable = '<file>' }
+	filemt.__index = filemt
+
+	function isFile(obj)
+		if filedata[obj] then
+			-- if this is true, it's an object we control, so it's safe to trust attribute access
+			return true, not obj._closed
+		else
+			return false
+		end
+	end
+
+	local function wrapFile(mode, fh)
+		local rwa, bin, plus, b2 = match(mode or 'r', '^([rwa])(b?)(%+?)(b?)$') --match all of x, xb, x+, xb+, x+b
+		if not rwa or (bin ~= '' and b2 ~= '') then return nil, 'bad mode' end
+		bin = bin .. b2 --when (bin == '') xor (b2 == ''), this is logical or
+
+		local read = rwa == 'r' or plus
+		local write = rwa ~= 'r' or plus
+		local append = rwa == 'a'
+		local update = plus ~= ''
+
+		local file = setmetatable({}, filemt)
+		filedata[file] = fh
+		if read then
+			fh._file_rbuf = ''
+		end
+		if write then
+			fh._file_wbuf, fh._file_wmode, fh._file_wcap = '', 'full', nil --nil capacity means default, which may vary with e.g. system memory usage
+		end
+
+		return file
+	end
+
+	local function null_close() return true end
+	local function null_read() return nil end --always eof
+	local function null_write() return true end --eats data
+	--null devices can't be seeked
+
+	function openNull(mode)
+		return assert(wrapFile(mode, { close = null_close, read = null_read, write = null_write }))
+	end
+
+	local function path_close(fh) return invoke(fh.address, 'close', fh.handle) end
+	local function path_read(fh, n) return invoke(fh.address, 'read', fh.handle, n) end
+	local function path_write(fh, data) return invoke(fh.address, 'write', fh.handle, data) end
+	local function path_seek(fh, ...) return invoke(fh.address, 'seek', fh.handle, ...) end
+
+	function openPath(mode, addr, path)
+		local fd, err = invoke(addr, 'open', path, mode) --should we validate `mode` first?
+		if not fd then
+			return nil, err
+		end
+		return assert(wrapFile(mode, { address = addr, handle = fd, close = path_close, read = path_read, write = path_write, seek = path_seek }))
+	end
+
+
+	--file method helpers
+	local function assertOpen(file, how)
+		local fh = filedata[file]
+		if fh._file_closed then error('attempt to use a closed file', 3) end
+		if how == 'read' then
+			if not fh._file_rbuf then return nil, 'file not readable' end
+		elseif how == 'write' then
+			if not fh._file_wbuf then return nil, 'file not writable' end
+		elseif how == 'seek' then
+			if not fh.seek then return nil, 'file not seekable' end
+		elseif how then
+			error('bad how: should be read, write, seek, or nil', 2)
+		end
+		return fh
+	end
+
+	local function flush(fh)
+		local ok, err = fh:write(fh._file_wbuf)
+		if ok then
+			fh._file_wbuf = ''
+			return true
+		else
+			return nil, err
+		end
+	end
+
+	local function read1(fh, n)
+		if type(n) == 'number' then
+			while #fh._file_rbuf < n do
+				local blocksz = n - #fh._file_rbuf
+				if blocksz < 512 then blocksz = 512 end --TODO tune default capacity to e.g. free memory
+				local data, err = fh:read(blocksz)
+				if err then
+					return nil, err
+				elseif not data then --eof
+					break
+				end
+				fh._file_rbuf = fh._file_rbuf .. data
+			end
+
+			local data = sub(fh._file_rbuf, 1, n)
+			fh._file_rbuf = sub(fh._file_rbuf, n + 1)
+			return data
+		elseif n == 'a' then
+			local parts = { fh._file_rbuf }
+			local chunk, err
+			repeat
+				chunk, err = fh:read(512)
+				parts[#parts+1] = chunk
+			until not chunk
+
+			local cat = concat(parts)
+
+			if err then
+				fh._file_rbuf = cat
+				return nil, err
+			else
+				fh._file_rbuf = ''
+				return cat
+			end
+		else
+			error('TODO read1 patterns')
+		end
+	end
+
+	local function write1(fh, data)
+		local wbuf = fh._file_wbuf .. data
+		fh._file_wbuf = wbuf
+
+		local mode = fh._file_wmode
+		if mode == 'no' then
+			if wbuf ~= '' then return flush(fh) end
+		elseif mode == 'full' then
+			local cap = fh._file_wcap or 512 --TODO tune default capacity to e.g. free memory
+			if #fh._file_wbuf >= cap then
+				return flush(fh)
+			end
+		else --for line buffering, only flush complete lines
+			local i1, i2 = find(fh._file_wbuf, '.+[\r\n]') --`i2` is the last cr or lf in the buffer
+			if i1 then --if there are any newlines
+				local rest = sub(fh._file_wbuf, i2 + 1)
+				fh._file_wbuf = sub(fh._file_wbuf, 1, i2)
+				local ok, err = flush(fh)
+				if ok then fh._file_wbuf = rest end
+				return ok, err
+			else --no newlines, nothing to flush
+				return true
+			end
+		end
+	end
+
+
+	--file methods
+	function filemt:close()
+		local fh = assertOpen(self)
+		if fh._file_wbuf then
+			local ok, err = flush(fh)
+			if not ok then return nil, err end --XXX should close ever fail? what do we expect the caller to do?
+		end
+		fh:close()
+		fh._file_closed = true
+	end
+
+	filemt.__close = filemt.close --for future lua 5.4 compat
+
+	function filemt:flush()
+		local fh, err = assertOpen(self, 'write')
+		if not fh then return nil, err end
+
+		local ok, err = flush(fh)
+		if not ok then return nil, err end
+
+		return self
+	end
+
+	function filemt:lines(...)
+		local pats = pack(...)
+		for i = 1, pats.n do
+			local pat = pats[i]
+			local t = checkArgEx(i, pat, 'string', 'integer')
+			if t == 'string' and pat ~= 'n' and pat ~= 'a' and pat ~= 'l' and pat ~= 'L' then
+				error(format('bad argument #%d to \'lines\' (invalid format)', i), 2)
+			end
+		end
+		if pats.n == 0 then
+			pats[1], pats.n = 'l', 1
+		end
+
+		local ok, err = assertOpen(self, 'read')
+		if not ok then error(err, 2) end --more informative than the error from `for line in nil, err do end`
+
+		return function()
+			local ret = {}
+			for i = 1, pats.n do
+				local ok, err = read1(fh, pats[i])
+				if not ok then
+					if err then error(err, 2) end
+					break
+				end
+				ret[i] = ok
+			end
+			return unpack(ret, 1, pats.n)
+		end
+	end
+
+	function filemt:read(...)
+		local args = pack(...)
+		for i = 1, args.n do
+			local pat = args[i]
+			local t = checkArgEx(i, pat, 'string', 'integer')
+			if t == 'string' and pat ~= 'n' and pat ~= 'a' and pat ~= 'l' and pat ~= 'L' then
+				error(format('bad argument #%d to \'read\' (invalid format)', i), 2)
+			end
+		end
+		if args.n == 0 then
+			args[1], args.n = 'l', 1
+		end
+
+		local fh, err = assertOpen(self, 'read')
+		if not fh then return nil, err end
+
+		local args = pack(...)
+		local iend = args.n
+		for i = 1, args.n do
+			local ok, err = read1(fh, args[i])
+			if ok then
+				args[i] = ok
+			else
+				iend = i + 1
+				args[i] = nil
+				args[i + 1] = err
+				break
+			end
+		end
+		return unpack(args, 1, iend)
+	end
+
+	function filemt:seek(whence, offset)
+		if type(whence) == 'number' and rawequal(offset, nil) then
+			checkArgEx(1, whence, 'integer') --produce a better error message citing arg 1
+			whence, offset = 'cur', whence
+		end
+		checkArgEx(1, whence, 'string', 'nil')
+		checkArgEx(2, offset, 'integer', 'nil')
+
+		local fh, err = assertOpen(self, 'seek')
+		if not fh then return nil, err end
+
+		--before seeking, flush any buffered writes
+		if fh._file_wbuf then
+			local ok, err = flush(fh)
+			if not ok then return nil, err end
+		end
+
+		local ok, err = fh:seek(whence or 'cur', offset or 0)
+		if not ok then return nil, err end
+
+
+		local tell = (whence == 'cur' or not whence) and (offset == 0 or not offset) --true if we didn't move ie ftell()
+		--after seeking, if we moved, invalidate the read buffer
+		if (whence and whence ~= cur) or (offset and offset ~= 0) then
+			fh._file_rbuf = fh._file_rbuf and ''
+		end
+
+		return ok
+	end
+
+	function filemt:setvbuf(mode, size)
+		checkArg(1, mode, 'string')
+		if mode ~= 'no' and mode ~= 'line' and mode ~= 'full' then
+			error(format('bad argument #1 to \'setvbuf\' (invalid option %q)', mode), 2)
+		end
+		checkArgEx(2, size, 'integer', 'nil')
+
+		local fh, err = assertOpen(self)
+		if not fh then return nil, err end
+
+		if not fh._file_wbuf then
+			return true --on readonly files, setvbuf is a noop
+		end
+
+		fh._file_wmode, fh._file_wcap = mode, size
+		--don't flush now; we don't have a way to propagate errors. the next write (or close) will take care of it.
+		return true
+	end
+
+	function filemt:write(...)
+		local args = pack(...)
+		for i = 1, args.n do
+			local arg = args[i]
+			local t = checkArgEx(i, arg, 'string', 'number')
+			if t ~= 'string' then args[i] = tostring(arg) end
+		end
+
+		local fh, err = assertOpen(self, 'write')
+		if not fh then return nil, err end
+
+		for i = 1, args.n do
+			local ok, err = write1(fh, args[i])
+			if not ok then return nil, err end
+		end
+
+		return self
+	end
+end
+
+-- vi: set ts=2:
diff --git a/src/kernel/02_processes.lua b/src/kernel/03_processes.lua
similarity index 100%
rename from src/kernel/02_processes.lua
rename to src/kernel/03_processes.lua
diff --git a/src/kernel/03_environment.lua b/src/kernel/04_environment.lua
similarity index 70%
rename from src/kernel/03_environment.lua
rename to src/kernel/04_environment.lua
index 22dc5af..0b9956e 100644
--- a/src/kernel/03_environment.lua
+++ b/src/kernel/04_environment.lua
@@ -9,7 +9,7 @@ given <https://ocdoc.cil.li/tutorial:custom_oses#what_s_available>:
 ]]
 
 
-envBase._OSVERSION = 'kitn 0.1.2' --prefer [openloader, openos] "$name $ver" over plan9k "$name/$ver"
+envBase._OSVERSION = 'kitn 0.2.0-beta.1' --prefer [openloader, openos] "$name $ver" over plan9k "$name/$ver"
 envBase.kitn = {}
 envBase._G, envBase.load = nil
 
@@ -84,6 +84,50 @@ function fillEnv(proc, uenv)
 
 	uenv.load, uenv.loadfile, uenv.dofile = uload, uloadfile, udofile
 
+	local stdin, stdout = openNull('r'), openNull('w')
+	local uio; uio = {
+		stdin = stdin, stdout = stdout, stderr = stdout,
+		close = function(file) return (file or uio.stdout):close() end,
+		flush = function() return uio.stdout:flush() end,
+		input = function(file)
+			local t = checkArgEx(1, file, 'string', 'table', 'nil')
+			if t ~= 'nil' then
+				uio.stdin = (t == 'string' and assert(uio.open(file, 'r')) or file)
+			end
+			return uio.stdin
+		end,
+		lines = function(filename, ...)
+			checkArg(1, filename, 'string', 'nil')
+			error('TODO io.lines') --iff we open a file, we need to close it
+		end,
+		open = function(filename, mode) return openPath(mode, bootfs, filename) end,
+		output = function(file)
+			local t = checkArgEx(1, file, 'string', 'table', 'nil')
+			if t ~= nil then
+				uio.stdout = (t == 'string' and assert(uio.open(file, 'w')) or file)
+			end
+			return uio.stdout
+		end,
+		--popen is unimplemented because we don't have a notion of a shell
+		read = function(...) return uio.stdin:read(...) end,
+		tmpfile = function()
+			--use math.random() to pick a filename on computer.tmpAddress() that doesn't exist yet and create it
+			error('TODO io.tmpfile') --file is deleted on process exit, requires atExit mechanism
+		end,
+		type = function(obj)
+			local file, opened = isFile(obj)
+			if opened then
+				return 'file'
+			elseif file then
+				return 'closed file'
+			else
+				return nil
+			end
+		end,
+		write = function(...) return uio.stdout:write(...) end,
+	}
+	uenv.io = uio
+
 
 	--env will automatically mirror envBase.kitn here
 	local ukitn = uenv.kitn
diff --git a/src/kernel/04_scheduler.lua b/src/kernel/05_scheduler.lua
similarity index 100%
rename from src/kernel/04_scheduler.lua
rename to src/kernel/05_scheduler.lua