shh: Simple shell scripting from Haskell

[ bsd3, library, program, system ] [ Propose Tags ]

Provides a shell scripting environment for Haskell. It helps you use external binaries, and allows you to perform many shell-like functions, such as piping and redirection.

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Dependencies async (>=2.2.1 && <2.3), base (>=4.9 && <5), bytestring, containers (>=0.5.11 && <0.7), deepseq (>=1.4.3 && <1.5), directory (>=1.3.1 && <1.4), filepath (>=1.4.2 && <1.5), mtl (>=2.2.2 && <2.3), process (>=1.6.3 && <1.7), shh, split (>=0.2.3 && <0.3), stringsearch (>= && <0.4), template-haskell (>=2.13.0 && <2.18), temporary, unix (>=2.7.2 && <2.8), utf8-string [details]
License BSD-3-Clause
Copyright (c) 2018, 2019 Luke Clifton
Author Luke Clifton
Revised Revision 1 made by lukec at 2020-11-10T01:54:42Z
Category System
Source repo head: git clone
Uploaded by lukec at 2020-09-05T04:03:18Z
Distributions NixOS:
Executables shh-example, shh
Downloads 10380 total (17 in the last 30 days)
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Status Hackage Matrix CI
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Note: This package has metadata revisions in the cabal description newer than included in the tarball. To unpack the package including the revisions, use 'cabal get'.

Maintainer's Corner

For package maintainers and hackage trustees

Readme for shh-

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<details><summary> Shh is a library to enable convenient shell-like programming in Haskell. It works well in scripts, and from GHCi, allowing you to use GHCi as a shell. </summary>

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ExtendedDefaultRules #-}
module Readme (test) where

import Shh

import Control.Concurrent.Async
import Prelude hiding (head)
import Test.Tasty
import Test.Tasty.HUnit
import Test.Tasty.QuickCheck
import qualified System.Directory
import qualified Data.ByteString.Lazy.Char8 as Char8
import Data.List (nub)
import Data.Char

load SearchPath ["echo", "base64", "cat", "head", "sleep", "mktemp", "ls", "wc", "find", "tr", "users", "sha256sum", "false", "true"]

curl :: Cmd
curl = true

test :: IO ()
test = do


It's primary purpose is in replacing shell scripts. As such, many functions are provided to mimic the shell environment, and porting shell scripts to shh should be fairly straightforward. A simple "cargo culting" port should work in most situations, and perhaps be even more robust than the original.

It is also a wrapper tool around launching GHCi as a shell.

It supports

  • Automatically defining a function for each executable on your $PATH using template Haskell, as well as a runtime check to ensure they all exist on startup.

  • Redirection of stdout and stderr

      -- Redirect stdout
      echo "Hello" &> StdErr
      echo "Hello" &> Truncate ".tmp_file"
      -- Redirect stderr
      echo "Hello" &!> Append "/dev/null"
      echo "Hello" &!> StdOut
  • Piping stdout or stderr to the input of a chained process

      cat "/dev/urandom" |> base64 |> head "-n" 5
  • Multiple processes sequentially feeding a single process

      (echo 1 >> echo 2) |> cat
  • Use of Haskell's concurrency primitives.

      race (sleep 1 >> echo "Slept for 1") (sleep 2 >> echo "Slept for 2")
      mapConcurrently_ (\url -> curl "-Ls" url |> wc)
        [ ""
        , ""
  • Capturing of process output

      s <- echo "Hello" |> tr "-d" "l" |> capture
      print s
      loggedIn <- nub . Char8.words <$> (users |> capture)
      putStrLn $ "Logged in users: " ++ show loggedIn
      mapM_ Char8.putStrLn =<< (find "-maxdepth" 1 "-print0" |> captureEndBy0)
  • Capturing infinite output of a process lazily

      cat "/dev/urandom"
        |> base64
        |> readInput (mapM_ Char8.putStrLn . take 3 . Char8.lines)
  • Write strings to stdin of a process.

      writeOutput "Hello\n" |> cat
      -- Hello
      "Hello" >>> sha256sum
      sha256sum <<< "Hello"
  • Proper exceptions, when a process exits with a failure code, an exception is thrown. You can catch these normally. The exception includes the error code, the command, and all it's arguments.

      false "Ha, it died"
      --  *** Exception: Command `false "Ha, it died"` failed [exit 1]
      exitCode false
      --  1
  • "Native" processes, i.e. Haskell functions that behave like a process.

      echo "Hello" |> pureProc ( toUpper) |> tr "-d" "L"
      -- HEO
  • And much, much more! Look at the documentation on Hackage for a comprehensive overview of all the possibilities.


Shh has many symbols that might seem intimidating at first, but there is a simple mnemonic for them.

|     Piping. Looks like a pipe, same as in POSIX shells.
&     Redirection, think of the shell `2>&1`
>,<   The direction of flow of a command
!     Operate on stderr instead of stdout

So, for example,

ls |> cat      Pipe the stdout of `ls` into stdin of `cat`
cat <| ls      Same as above
ls &> StdErr   Redirect stdout of `ls` to wherever stderr is going.
StdErr <& ls   Same as above
ls &!> StdOut  Redirect stderr of `ls` to wherever stdout is going.
StdOut <!& ls  Same as above


Currently Shh does not have any built in globbing support. Rather, it is currently suggested to use another library to do globbing. For example, using the Glob package, it is possible to do something like

wc "--" =<< glob "*.md"

Certainly more verbose than the Bash equivalent, however, also more explicit, which is probably a good thing. If this turns out to be too cumbersome, we might introduce a more succinct globbing feature, though it will always be explicit, and thus always more verbose than most other shells.


Some of the features in Shh require that you use the threaded runtime. Please compile with the -threaded flag to avoid deadlocks.

Enable Template Haskell and load the environment. It is also strongly recommended to use ExtendedDefaultRules. This is especially important if you want to use OverloadedStrings.

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ExtendedDefaultRules #-}
$(loadEnv SearchPath)

You now have all your executables available as simple to Haskell functions. If you don't want to load your entire environment you can load specific commands directly:

load SearchPath ["echo", "grep", "cat", "ls"]

If you want to check that all the dependencies still exist, you can use missingExecutables :: IO [String], which will tell you if anything is missing.

For use in a project, it makes sense to have a dedicated module for your project which does the template Haskell above. This will prevent recompilation of all the executables, and also allow you to easily namespace them to avoid collisions with normal Haskell functions.

Usage in GHCi

If you want ^D to be recognised as a EOF marker (when running commands that read from stdin) when running in GHCi, you will need to run the initInteractive function. This sets the line buffering appropriately and ensures the terminal is in canonical mode.

Shh as a Shell

There is a tool called shh which is a fairly small wrapper around launching GHCi which automatically loads your environment and allows you to have custom config when using GHCi as a shell.

To install it, one option is to use cabal new-install

cabal new-install --lib shh
cabal new-install --lib ssh-extras

The shh binary will look in your $SHH_DIR (defaults to $HOME/.shh) for a Shell.hs, init.ghci and wrapper files. If these don't exist default ones will be created.

The Shell.hs file should contain any top level definitions that you would like to be available in your Shell. By default it loads your environment.

The init.ghci file is loaded by GHCi after your .ghci files. This lets you specify settings that you want to take effect when using GHCi as a shell. By default it sets a shell-like prompt.

The wrapper file is an executable that is called with the command that is to be executed. By default it just calls exec with the arguments passed to it. The use-case for this is to be able to set up the environment for shh. You might, for example, wrap the execution in a nix-shell. Either way, it is up to you to make sure that the compiler, and packages you require are available, either globally, or provided by the wrapper script.

Faster Startup

shh precompiles your Shell.hs file so that starting up shh is very quick on subsequent launches. Unfortunately, shh isn't quite able to detect this perfectly. If you see GHCi telling you that it is Compiling Shell, and you notice the delay when starting shh, try manually forcing a rebuild by passing in the --rebuild argument to shh.

This is particularly likely to happen if you upgrade your GHC, or installed packages, or even shh itself.

Nix Wrapper Example

The following snippet could act as a wrapper file to set up a suitable environment using nix-shell

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p "(haskellPackages.ghcWithPackages (p: with p; [shh shh-extras]))"
exec "$@"

Script Usage


Nixpkgs provides a writeHaskellBin function which is very convenient for writing quick scripts for your Nix setup.

writers.writeHaskellBin "example" {libraries = [haskellPackages.shh];} ''
  {-# LANGUAGE TemplateHaskell #-}
  import Shh

  -- Load binaries from Nix packages. The dependencies will be captured
  -- in the closure.
  loadFromBins ["${git}", "${coreutils}", "${curl}"]

  main :: IO ()
  main = do
    cat "/a/file"
    cp "/a/file" "/b/file"


There are quite a few players in the "shell programming for Haskell" field.

This table attempts to summarise some of the differences.

  • Pipe Style refers to how processes are joined together, "native" means that the mechanisms provided by the OS are used, while "via Haskell" means that the data is read into the Haskell process, and then written into the subprocess.
  • Via Shell refers to whether subprocesses are launched directly or via a shell (which can provide a "native" piping solution at the cost of composability)
  • Run in IO refers to whether commands need to be prefixed with run or similar functions to actually execute them.
  • TH Helper refers to whether the use of TH to generate Haskell functions based on commands found at compile time is encouraged in the main library.
  • Monadic Subshell refers to the ability to join multiple processes together and feed them all from the same input and to the same output. echo a | (cat; echo b) | wc -l should report that 2 lines appeared.

| Library | Pipe Style | Via Shell | Run in IO | Threadsafe cd | TH Helper | Monadic Subshell | Redirect stderr | |---------|-------------|-----------|-----------|-----------------|-----------|------------------|-------------------| | Shh | Native | No | Yes | No | Yes | Yes | Yes | | Shelly | Via Haskell | Yes | No | Yes | No | No | Yes | | Turtle | Via Haskell | Optional | No | ? | No | No (Alternative) | Yes | | shell-conduit | Via Haskell | Optional | No | ? | Yes | Yes | No? |


| Library | Exception on non-zero | Contains arguments | Contains stderr | Terminates pipeline | |---------|-----------------------|--------------------|-------------------|---------------------| | Shh | Yes | Yes | Optional | Yes | | Shelly | Yes | Yes | Yes | Yes | | Turtle | Sometimes | No | No | ? | | shell-conduit | Yes | Yes | No | No |