WARNING

This is a PUBLIC repository. Contributors are advised to NOT commit any sensitive information!

Swift Nix Channel

This repository contains the Nix expressions providing packages used at Swift Navigation that are otherwise not available in nixpkgs.

Usage

Add the GitHub host key

To quickly add the GitHub host key to the known hosts file, run the following command as root and answer yes when prompted:

$> ssh -T [email protected]

The authenticity of host 'github.com (140.82.116.4)' can't be established.
ED25519 key fingerprint is SHA256:+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'github.com' (ED25519) to the list of known hosts.

It is OK if the command fails with the following message:

[email protected]: Permission denied (publickey).

Adding the channel

To add the channel to a NixOS machine, run the following commands as root:

$> nix-channel --add https://github.com/swift-nav/nix-channel/archive/main.tar.gz swiftpkgs
$> nix-channel --list
$> nix-channel --update

The nix-channel --update command will download the Nix expressions from the main branch of the repository and build it as a derivation. The packages will then be available via the <swiftpkgs> expression.

Using the packages

In the configuration.nix of the NixOS machine:

1. Import the channel and assign it to a variable.
2. Add the desired package(s) to the environment.systemPackages list.

let
  swiftpkgs = import <swiftpkgs> {};
in
{
  environment.systemPackages = with pkgs; [
    swiftpkgs.sd-mux-ctrl
  ];
}

Rebuild NixOS (nixos-rebuild switch) to apply the changes.

Verify that the packages are available

To verify that the packages installed correctly, use the which command to check if the executable is in the PATH:

$> which sd-mux-ctrl

/run/current-system/sw/bin/sd-mux-ctrl

Theory of Operation

According to one Luc Perkins (paraphrased):

Nix channels are essentially tarballs that are made available via an unchanging URL. Conveniently, GitHub has a built-in archive feature that automatically turns every branch in every GitHub repository into a tarball available at a URL with this structure:

https://github.com/<username>/<repo>/archive/<branch>.tar.gz

Therefore, it is possible to create and maintain a Nix channel without any additional work beyond committing and updating code in a GitHub repository.

Although the manual entry for nix-channel --add URL [name] claims that:

A channel URL must point to a directory containing a file nixexprs.tar.xz.

In practice, the URL can point to a tarball file. The extracted contents of the tarball need only contain a top-level default.nix file acting as channel's entry point to work as intended.

Finally, the GitHub repository MUST be public because the nix-channel --update command's download mechanism only supports HTTP Basic Authentication at this time; therefore, it is not capable of authenticating to GitHub in order to access a private repository.

References

1. Nix Pills
2. Luc Perkins' Blog "The easiest way (I've found) to create your own Nix channel"
3. INRIA Nix Tutorial
4. NixOS Manual
5. Nix Manual