thoughtpolice / bootstrap.nix

code to bootstrap my nix projects

bootstrap.nix

This project contains a very simple file named bootstrap.nix that I use in my Nix-built projects. It acts as the "bottom layer" of any more advanced packaging abstractions. It is responsible for coherently fetching a copy of nixpkgs for you to use (via fetchTarball) in a number of ways, as well as constructing version information for your version fields based on repo information.

Ultimately, I found myself reproducing this logic enough that I extracted it into its own project. The source code is very simple (< 100 lines of easily understood code), and intended to be used "inline".

All you have to do is copy the file nix/bootstrap.nix into your project and import it from somewhere. There are multiple ways to do this, but an easy one can be seen inside the mini.nix demo, which completely specifies a snapshot of Nixpkgs to use:

with (import ./nix/bootstrap.nix {
  repo = builtins.fetchGit ./.;
  versionFile = ./.version;
  nixpkgs = {
    url    = "https://github.com/nixos/nixpkgs-channels/archive/c4196cca9acd1c51f62baf10fcbe34373e330bb3.tar.gz";
    sha256 = "0jsisiw8yckq96r5rgdmkrl3a7y9vg9ivpw12h11m8w6rxsfn5m5";
  };
});

pkgs.stdenv.mkDerivation {
  pname = "demo";
  inherit version;

  src = ./.;

  installPhase = ''
    mkdir -p $out
    touch $out/test.txt
  '';
}

The import of bootstrap.nix returns an attrset, with the following structure:

{
  pkgs    = { ... };  # nixpkgs package set
  version = "...";    # first line of .version
  relname = "...";    # second line of .version
}

So in the above example, both version and pkgs come from this import.

Other uses

You can bootstrap nixpkgs from any directory (such as a git clone), an HTTP(s) tarball (like above), any upstream NixOS channel, or a pre-configured snapshot that you specify inline (like above), or with a "lockfile" named nixpkgs.json.

The demo.nix file shows a more full-featured example that specifies its nixpkgs snapshot using the lockfile nix/nixpkgs.json. This lock file can be updated with nix/update.sh (but you can use your own, too.) The lockfile can also be overridden by any user, another feature not exposed by mini.nix. For example, try the following builds:

nix build -f demo.nix # use lockfile snapshot
nix build -f demo.nix --arg nixpkgs '<nixpkgs>' # use system nixpkgs
nix build -f demo.nix --arg nixpkgs channel:nixos-19.03 # use channel
nix build -f demo.nix --arg nixpkgs https://... # some .tar.gz file

License

MIT. See LICENSE.txt for terms of copyright and redistribution.