Write shell scripts with Conduit

Version on this page:4.5.2
LTS Haskell 8.2:4.5.2
Stackage Nightly 2017-02-22:4.5.2
Latest on Hackage:4.5.2

shell-conduit Hackage

Write shell scripts with Conduit. Still in the experimental phase.

Haddock API documentation.


Cloning and initializing a repo
import Control.Monad.IO.Class
import Data.Conduit.Shell
import System.Directory

main =
  run (do exists <- liftIO (doesDirectoryExist "fpco")
          if exists
             then rm "fpco/.hsenvs" "-rf"
             else git "clone" ""
          liftIO (setCurrentDirectory "fpco")
          shell "./dev-scripts/"
          shell "./dev-scripts/"

Piping of processes and normal conduits is possible:

λ> run (ls $| grep ".*" $| shell "cat" $| conduit ( ( toUpper)))
Running actions in sequence and piping

Results are outputted to stdout unless piped into other processes:

λ> run (do shell "echo sup"; shell "echo hi")
λ> run (do shell "echo sup" $| sed "s/u/a/"; shell "echo hi")

Live streaming between pipes like in normal shell scripting is possible:

λ> run (do tail' "/tmp/example.txt" "-f" $| grep "--line-buffered" "Hello")
Hello, world!
Oh, hello!

(Remember that grep needs --line-buffered if it is to output things line-by-line).

Handling exit failures

Process errors can be ignored by using the Alternative instance.

import Control.Applicative
import Control.Monad.Fix
import Data.Conduit.Shell

main =
  run (do ls
          echo "Restarting server ... ?"
          killall name "-q" <|> return ()
          fix (\loop ->
                 do echo "Waiting for it to terminate ..."
                    sleep "1"
                    (ps "-C" name >> loop) <|> return ())
          shell "dist/build/ircbrowse/ircbrowse ircbrowse.conf")
  where name = "ircbrowse"
Running custom things

You can run processes directly:

λ> run (proc "ls" [])
dist      LICENSE    Setup.hs         src
examples  shell-conduit.cabal  TAGS

Or shell commands:

λ> run (shell "ls")
dist      LICENSE    Setup.hs         src
examples  shell-conduit.cabal  TAGS

Or conduits:

λ> run (cat $| conduit (awaitForever yield))
Keyboard configuration
import Data.Conduit.Shell
main =
  run (do xmodmap ".xmodmap"
          xset "r" "rate" "150" "50")

How it works

All executable names in the PATH at compile-time are brought into scope as runnable process conduits e.g. ls or grep.

All processes are bound as variadic process calling functions, like this:

rmdir :: ProcessType r => r
ls :: ProcessType r => r

But ultimately the types end up being:

rmdir "foo" :: Segment r
ls :: Segment r
ls "." :: Segment r


Run all shell scripts with

run :: Segment r -> IO r

The Segment type has a handy Alternative instance.

String types

If using OverloadedStrings so that you can use Text for arguments, then also enable ExtendedDefaultRules, otherwise you'll get ambiguous type errors.

{-# LANGUAGE ExtendedDefaultRules #-}

But this isn't necessary if you don't need to use Text yet. Strings literals will be interpreted as String. Though you can pass a value of type Text or any instance of CmdArg without needing conversions.

Other modules

You might want to import the regular Conduit modules qualified, too:

import qualified Data.Conduit.List as CL

Which contains handy functions for working on streams in a list-like way. See the rest of the handy modules for Conduit in conduit-extra.

Also of interest is csv-conduit, html-conduit, and http-conduit.

Finally, see the Conduit category on Hackage for other useful libraries:

All of these general purpose Conduits can be used in shell scripting.

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