haskell System.Process.createProcess redirect stdout and stderr

Question

How can I redirect std_in and std_err to a file using createProcess in System.Process?

Answer 1

Use the std_in and std_err fields of the CreateProcess type:

foo cmd fin ferr =
    withFile fin  ReadMode  $ \hin  ->
    withFile ferr WriteMode $ \herr -> do
        (_, _, _, ph) <- createProcess (proc cmd [])
            { std_in  = UseHandle hin
            , std_err = UseHandle herr
            }
        waitForProcess ph