That's not going to be sufficient; you could end up in a deadlock by not flushing the output buffer of the child process. Have a look at the implementation of readProcess to see how to deal with some of these tricky issues in the lazy I/O world: http://hackage.haskell.org/package/process-1.2.0.0/docs/src/System-Process.h... On Thu Dec 11 2014 at 6:40:54 PM Derek McLoughlin < derek.mcloughlin@gmail.com> wrote:

Something like this would work:

import System.Process import System.IO import System.Exit

readExecOutput :: String -> [String] -> IO () readExecOutput cmdName cmdOptions = do (_, Just hOut, _, jHandle) <- createProcess (proc cmdName cmdOptions) { cwd = Just "." , std_out = CreatePipe } exitCode <- waitForProcess jHandle exec_output <- hGetContents hOut case exitCode of ExitSuccess -> putStr $ "Success!\n" ++ exec_output _ -> putStrLn "Command failed"

Derek.

On Thursday, 11 December 2014 13:29:34 UTC, Ernesto Rodriguez wrote:

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Hi Michael,

Thank you for your answer. Conduits certainly are the best approach, but the output I am going to read is very short and the external program will only be called once or twice per execution so I think Lazy IO won't be problematic here.

Best regards,

Ernesto

On Thu, Dec 11, 2014 at 10:18 AM, Michael Snoyman

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wrote:

You can use readProcess[1], but it has some downsides:

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1. Can't specify all options to CreateProcess, e.g., working directory and environment variables. 2. Relies on lazy I/O

I use Data.Conduit.Process[2] for this kind of thing, which has a full tutorial[3]. However, it *does* involve non-base dependencies.

[1] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ process-1.2.0.0/System-Process.html#v:readProcess [2] http://www.stackage.org/haddock/2014-12-10-ghc78-exc/ conduit-extra-1.1.4.2/Data-Conduit-Process.html [3] https://www.fpcomplete.com/user/snoyberg/library- documentation/data-conduit-process?show=tutorials

On Thu Dec 11 2014 at 11:11:12 AM Ernesto Rodriguez wrote:

Dear All,

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...

I have a program which invokes an external program via exec. I wish to capture the standard output of the program without using external (temporary) files. Is it possible to create an in memory handle which I can tell exec to use as standard output of the invoked program. It would be great if it can be done without any extra dependencies.

Best,

-- Ernesto Rodriguez

Masters Student Computer Science Utrecht University

www.netowork.me github.com/netogallo

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-- Ernesto Rodriguez

Masters Student Computer Science Utrecht University

www.netowork.me github.com/netogallo