Binding the result of a system command to a variable in Haskell

Question

How does one run a system command in Haskell and bind it's result (i.e., standard output) to a variable? In pseudo-haskell I'm looking for something like the following:

import System.Process

main = do output <- callCommand "echo hi"
          putStrLn output -- result: "hi"

This does not work. Is there something similar that does?

Answer 1

Here is some working code:

import System.Process

main :: IO ()
main = do
  let stdin' = ""
  (errCode, stdout', stderr') <- readProcessWithExitCode "echo" ["hi"] stdin'
  putStrLn $ "stdout: " ++ stdout'
  putStrLn $ "stderr: " ++ stderr'
  putStrLn $ "errCode: " ++ show errCode

As Carsten notes, you can use readProcess, but I prefer this version because with any serious usage, you will soon be very confused as to why your command is silently failing (if you run in the command line, both stdout and stderr are, of course, shown).

Answer 2

Expanding on jamshidh's answer, if instead of getting stdout and stderr as String you want to get them as ByteString or Text, you can use the process-extras package.

You can also use my process-streaming package:

$ import System.Process.Streaming
$ execute (piped (shell "echo foo")) (liftA3 (,,) (foldOut intoLazyBytes)    
                                                  (foldErr intoLazyBytes)
                                                  exitCode)
("foo\n","",ExitSuccess)

Also for Text:

$ import System.Process.Streaming.Text
$ execute (piped (shell "echo foo")) (liftA3 (,,) (foldOut (transduce1 utf8x intoLazyText)) 
                                                  (foldErr (transduce1 utf8x intoLazyText)) 
                                                  exitCode)
("foo\n","",ExitSuccess)