Scripting with Go on NixOS

This is a short HOWTO based on great idea from this blog post on how to utilize gorun and binfmt_misc to use go files for scripting.

First lets install gorun as a system package on NixOS.

Lets create gorun.nix file first:

with import <nixpkgs> {};

buildGoPackage rec {
  name = "gorun-${version}";
  version = "master-85cd5f5e084af0863ed0c3f18c00e64526d1b899";
  goPackagePath = "github.com/erning/gorun";

  src = fetchFromGitHub {
    owner = "erning";
    repo = "gorun";
    rev = "85cd5f5e084af0863ed0c3f18c00e64526d1b899";
    sha256 = "sha256:1hdqimfzpynnpqc7p8m8hkcv9dlfbd8kl22979i6626nq57dvln8";
  };

  meta = with stdenv.lib; {
    description = "gorun is a tool enabling one to put a \"bang line\" in the source code of a Go program to run it";
    homepage = https://github.com/erning/gorun;
    license = licenses.mit;
    maintainers = with maintainers; [ Gonzih ];
    platforms = platforms.linux ++ platforms.darwin;
  };
}

Next lets add this package to systemPackages list:

environment.systemPackages = with pkgs; [
  ...
  (import ./gorun.nix)
  ...
];

Next step would be to utilize boot.binfmtMiscRegistrations to register our gorun in binfmt_misc module which is responsible for parsing shebang scripts and binary files. Drop the following lines in to your configuration.nix:

boot.binfmtMiscRegistrations = {
  go = {
    recognitionType = "extension";
    magicOrExtension = "go";
    interpreter = "/run/current-system/sw/bin/gorun";
    preserveArgvZero = false;
    openBinary = true;
    matchCredentials = true;
    fixBinary = false;
  };
};

What does this configuration do? recognitionType tells binfmt to make a decision based on file extension, magicOrExtension tells it to look for .go extension, interpreter specifies which binary to invoke, openBinary corresponds to O flag of binfmt (which I dont fully understand tbh), matchCredentials tells kernel to run interpreter with permissions set on original script. For more information on binfmt please do check out these amazing kernel docs.

Now lets simply run sudo nixos-rebuild --switch and after everything is done lets give it a try.

First lets create a simple hello world script:

package main

import "fmt"

func main() {
	fmt.Println("Hello World!")
}

Lets make it executable with chmod +x hello.go.

And now lets run it:

$ ./hello.go
Hello World!

Yay! It works!

Hope that helps somewhat.