Combine inputs from multiple line-oriented sources. Useful for displaying things in status bars like dzen or bar.

multiplex.hs

import Control.Monad (liftM, mapM, forever)
import Control.Monad.Loops (untilM_)
import Control.Concurrent (forkIO, threadDelay)
import Control.Concurrent.STM (atomically)
import Control.Concurrent.STM.TMVar (TMVar, newTMVar, swapTMVar, readTMVar)
import System.IO (hGetLine, hIsEOF, hFlush, Handle, stdin, stdout)
import System.Process (createProcess, shell, CreateProcess(..), StdStream(..))
import System.Environment (getArgs)
import Data.Maybe (catMaybes)
import Data.List (nub)

hProducer :: Handle -> TMVar String -> IO ()
hProducer h var = untilM_ (hGetLine h >>= atomically . swapTMVar var) (hIsEOF h)

spawnHandle :: String -> IO (Maybe Handle)
spawnHandle cmd = do
  (_, stdin, _, _) <- createProcess ((shell cmd) { std_in = CreatePipe, std_out = CreatePipe, std_err = CreatePipe })
  return stdin

createProducer :: Handle -> IO (TMVar String)
createProducer h = do
  v <- atomically $ newTMVar ""
  forkIO $ hProducer h v
  return v

printer :: [TMVar String] -> IO ()
printer vs = mapM (atomically . readTMVar) vs >>= putStrLn . concat >> hFlush stdout

argToHandle "-" = return . Just $ stdin
argToHandle cmd = spawnHandle cmd

main = do
  handles <- liftM catMaybes $ getArgs >>= mapM argToHandle . nub
  vs <- mapM createProducer handles
  forever $ printer vs >> threadDelay 500000