[Haskell-cafe] Call external program and get stdout

Allan Clark a.d.clark at ed.ac.uk
Thu Nov 22 14:17:08 EST 2007

Jules Bean wrote:
> Maurí­cio wrote:
>> Hi,
>> How can I call a program (like, for instance,
>> 'grep text *') and get the standard output?
>> All actions I found (executeFile, system) do
>> not give me the output of the program.
> http://haskell.org/ghc/docs/latest/html/libraries/process- 
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I was using the following:
  A small function for blindly running a process until it completes
  its output and then waiting for its exit code.
  We return both the output (excluding stderr) plus the exit code.
getProcessOutput :: String -> IO (String, ExitCode)
getProcessOutput command =
     -- Create the process
  do (_pIn, pOut, pErr, handle) <- runInteractiveCommand command
     -- Wait for the process to finish and store its exit code
     exitCode <- waitForProcess handle
     -- Get the standard output.
     output   <- hGetContents pOut
     -- return both the output and the exit code.
     return (output, exitCode)

You'll need the following imports:
import System.IO
  ( hGetContents )
import System.Process
  ( runInteractiveCommand
  , waitForProcess
import System.Exit
  ( ExitCode ( .. ) )


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