nix-shebang

Run scripts as executables with automatic dependency management via Nix.

Currently supports Haskell. More languages are planned.

How it works

Nix flakes cache evaluation results but do not cache nix shell --expr. We exploit this by using a flake to instantly bring in cached-nix-shell, which can cache arbitrary Nix expressions. The script is then compiled and cached on first run via cached-nix-shell.

Haskell

Add a shebang to your Haskell file:

#!/usr/bin/env nix
#!nix shell --no-write-lock-file git+https://github.com/ymeister/nix-shebang?submodules=1#haskell.nix-haskell --command sh -c ``nix-haskell-shebang --opts -O2 --deps shh -- "$@"`` sh

{-# LANGUAGE ExtendedDefaultRules #-}
{-# LANGUAGE TemplateHaskell #-}

import System.Environment
import Shh

$(loadEnv SearchPath)

main :: IO ()
main = do
  args <- getArgs
  if null args then do
    echo "Hello World!"
  else do
    echo $ "Hello" : args

Then make it executable and run it:

chmod +x script.hs
./script.hs

Shebang options

Option	Description
--deps <packages...>	Haskell package dependencies
--opts <flags...>	GHC compiler flags (e.g. -O2, -threaded)
--with <packages...>	Non-Haskell system dependencies from nixpkgs
--module <expr>	Nix-haskell module override as a raw Nix expression (repeatable)
--	Separator between shebang args and script args

Bare positional arguments are treated as --deps.

Packages

Two variants are available via the flake:

• haskell.nix-haskell -- Uses haskell.nix for dependency resolution. Supports --module for overrides like custom compilers and cabal project settings.
• haskell.nixpkgs -- Uses haskellPackages.ghcWithPackages from nixpkgs.

Commands

Each package provides two commands:

• nix-haskell-shebang -- Compiles and runs the script.
• nix-haskell-repl -- Opens GHCi with the script and its dependencies loaded.