As mentioned by the first person to follow up, you need to set line buffering in the "copier" program. It's filling up its buffer while you write small lines to it - unlike the test run at the terminal prompt, where it's connected to a TTY device and therefore behaved differently. In a situation where you can confidently say your software will only ever be run in a POSIX environment, I like to use the Posix functions directly, because a buffered Handle on a pipe is nothing but extra trouble, and similarly the convenient wrapping of the POSIX API seems to cause more trouble than it's worth. Did your first attempt fail because the file name was not a complete path? Do that on purpose and see if runInteractiveProcess gives you a useful error message. Engineer some other kind of problem into your copier program, and see what happens to the error message ... oh, rats, it looks like runInteractiveProcess puts unit 2 on another pipe, when you might have worked better for you to leave it the way it was. (This kind of thing is what we call a "user friendly" API, which is why we say "with friends like that, who needs enemies!") Here's how the start process function could be written - import System.Posix.IO import System.Posix.Process startproc = do ip <- createPipe op <- createPipe pid <- forkProcess $ do forRead ip >>= onStdUnit 0 forWrite op >>= onStdUnit 1 executeFile "./copier" False [] Nothing to <- forWrite ip from <- forWrite op return (to, from, pid) where forRead (i, o) = closeFd o >> return i forWrite (i, o) = closeFd i >> return o onStdUnit i fd = do dupTo fd i closeFd fd -- and then for I/O to the file descriptors (remember to supply -- explicit newlines, if the other process is line buffered, e.g. "boo\n", -- But if the other process also uses Posix.readFd, it isn't buffered -- at all, so you don't need newlines. "cat" is in the latter category, -- but most command line applications are in the former, like, say, "awk".) sendrecv to from s = do forkIO $ fdWrite to s >> return () (v, _) <- fdRead from 1024 putStrLn ("recvd: " ++ show v) -- try it Donn