Shebang nix-shell

Posted on 05 December 2019

Using nix-shell this way allows us to create scripts that describe their own dependencies.

Usage

Basic usage:

#!/usr/bin/env nix-shell
#!nix-shell [OPTIONS] -i INTERPRETER [--packages PACKAGES|-p PACKAGES|PATH]
[#!nix-shell OPTIONS]

As shown above line #!nix-shell can be repeated. These lines are parsed by nix-shell itself, which differs slightly from how shell would parse them. For example single quotes do not work, and double quotes must be used when e.g. specifying Nix expression.

Somewhat useful documentation can be found in nix-shell(1) manual page.

Bash Script

#!/usr/bin/env nix-shell
#!nix-shell -i bash -p curl

# shellcheck shell=bash

set -euo pipefail

function main() {
    : TODO
}

main "$@"

Haskell (Shakefile)

#!/usr/bin/env nix-shell
#!nix-shell -i runghc
#!nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [ directory executable-path shake ])"

{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE RecordWildCards #-}

{-# OPTIONS_GHC -Wall -Wcompat #-}

-- |
-- Description: An example of Shake-based build system using nix-shell
-- Copyright:   (c) 2019 Peter Trško
-- License:     BSD3
--
-- An example of Shake-based build system using @nix-shell@ to provide all
-- dependencies needed by the buildsystem itself. This generalises to any
-- Haskell binary.
module Main (main)
  where

import Development.Shake


main :: IO ()
main = do
    opts@Options{..} <- getOptions
    shakeMain opts shakeOptions
        { shakeColor = True
--      , shakeFiles = ...
--      , shakeVersion = ...
--      , shakeChange = ChangeDigest
--      , shakeExtra = ...
        }

data Options = Options
    { shakeOptions :: ShakeOptions
--  , ...
    }

-- | Figure out some things that don't need to be tracked by Shake.
getOptions :: IO Options
getOptions = undefined  -- TODO

shakeMain :: ShakeOptions -> IO ()
shakeMain = shareArgs opts do
    ...

    want
        [ ...
        ]

    ...