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Managing shell scripts on NixOS

Posted on 24. February 2019 by Severin Fürbringer

So you’ve got a bunch of shell scripts laying around to get various small userspace tasks done. On a regular GNU/Linux or unix-like operating system you’d simply chuck the scripts into /usr/bin/ or /usr/sbin. The extra fancy variant would be to use GNU stow to have them symlinked from a versioned directory.

On NixOS this would be quite ugly. This fact becomes apparent once one takes a look at the /usr/bin directory on NixOS:

$ ls -l /usr/bin
total 4
lrwxrwxrwx 1 root root 66 Feb 23 10:26 env ->
/nix/store/68z2cvbzws1pn0z8dhgfkmws75r2z7gm-coreutils-8.29/bin/env

Nope! This is the territory of the Nix system. When you use NixOS, you shall play ball with Nix. Packages on NixOS are defined as so-called derivations on NixOS. They become available once imported into the environments packages. So why not just define our scripts as Nix derivations? This may sound complicated, but it’s not.

I recently wrote a bash script1 that spawns a virtualenv inside of a Nix shell to download Bandcamp2 albums. Let’s see how that works as a derivation:

with import <nixpkgs> {};

stdenv.mkDerivation rec {
  name = "bc-dl";
  version = "1.0.0";

  buildInputs = [ makeWrapper ];

  unpackPhase = "true";

  installPhase = ''
    mkdir -p $out/bin
    cp ${./bc-dl} $out/bin/${name}
    for f in $out/bin/*; do
      wrapProgram $f
        --prefix PATH : ${stdenv.lib.makeBinPath [ coreutils ]}
    done
  '';

}

This default.nix file simply takes bc-dl, the shell script, and tells NixOS to put it in its long hash-like directories you’re used to. This involves patching the shebang and bringing packages into scope, like coreutils.

To actually get this installed the derivation has to have the script present and be in a reasonable place where your /etc/nixos/configuration.nix has access to it. In your environment.systemPackages define the folder where your default.nix resides:

environment.systemPackages = [
  (import ./pkgs/bc-dl)
  # Other packages
  # ...
];

After a nixos-rebuild switch your script should be available like it would be from /usr/bin or similar. A full example can be found in my nixfiles.

Footnotes

  1. bc-dl
  2. Bandcamp is great and I do buy albums there. Support drm-free music!

Post was last generated on 11-08-2019 19:50:03.