Pipe string to shell command in Haskell

Question

How do I feed a string to shell command and get the output it produces in Haskell?

For example, given this:

> myHaskellProg 
  "blah"

> myHaskellProg | wc
  4

I want

> myHaskellProg

to print 4 by calling wc and printing out the result

I was able to call shell commands using process's callCommand but how do I pipe a string to it and get back the result or stderr?

Thomas M. DuBuisson · Accepted Answer · 2018-08-07 02:45:27Z

5

You are looking for System.Process and you can use the shell function to create a description of a process then use readCreateProcess to run the shell command, provide stdin and read stdout.

import System.Process
main :: IO ()
main =
  do result <- readCreateProcess (shell "ls") myHaskellString
     putStrLn result

myHaskellString :: String
myHaskellString = "string"

answered Aug 7, 2018 at 2:45

Thomas M. DuBuisson

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2 Comments

sam46 Over a year ago

Can you please show me how to print stderr produced from the process?

Thomas M. DuBuisson Over a year ago

Use readCreateProcessWithExitCode to capture both the return value and stderr.

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