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nix-super/src/nix/registry.md
Alex Wied cd361b31fa doc: Fix typo
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R""(

Description

nix registry provides subcommands for managing flake registries. Flake registries are a convenience feature that allows you to refer to flakes using symbolic identifiers such as nixpkgs, rather than full URLs such as git://github.com/NixOS/nixpkgs. You can use these identifiers on the command line (e.g. when you do nix run nixpkgs#hello) or in flake input specifications in flake.nix files. The latter are automatically resolved to full URLs and recorded in the flake's flake.lock file.

In addition, the flake registry allows you to redirect arbitrary flake references (e.g. github:NixOS/patchelf) to another location, such as a local fork.

There are multiple registries. These are, in order from lowest to highest precedence:

  • The global registry, which is a file downloaded from the URL specified by the setting flake-registry. It is cached locally and updated automatically when it's older than tarball-ttl seconds. The default global registry is kept in a GitHub repository.

  • The system registry, which is shared by all users. The default location is /etc/nix/registry.json. On NixOS, the system registry can be specified using the NixOS option nix.registry.

  • The user registry ~/.config/nix/registry.json. This registry can be modified by commands such as nix registry pin.

  • Overrides specified on the command line using the option --override-flake.

Registry format

A registry is a JSON file with the following format:

{
  "version": 2,
  "flakes": [
    {
      "from": {
        "type": "indirect",
        "id": "nixpkgs"
      },
      "to": {
        "type": "github",
        "owner": "NixOS",
        "repo": "nixpkgs"
      }
    },
    ...
  ]
}

That is, it contains a list of objects with attributes from and to, both of which contain a flake reference in attribute representation. (For example, {"type": "indirect", "id": "nixpkgs"} is the attribute representation of nixpkgs, while {"type": "github", "owner": "NixOS", "repo": "nixpkgs"} is the attribute representation of github:NixOS/nixpkgs.)

Given some flake reference R, a registry entry is used if its from flake reference matches R. R is then replaced by the unification of the to flake reference with R.

Matching

The from flake reference in a registry entry matches some flake reference R if the attributes in from are the same as the attributes in R. For example:

  • nixpkgs matches with nixpkgs.

  • nixpkgs matches with nixpkgs/nixos-20.09.

  • nixpkgs/nixos-20.09 does not match with nixpkgs.

  • nixpkgs does not match with git://github.com/NixOS/patchelf.

Unification

The to flake reference in a registry entry is unified with some flake reference R by taking to and applying the rev and ref attributes from R, if specified. For example:

  • github:NixOS/nixpkgs unified with nixpkgs produces github:NixOS/nixpkgs.

  • github:NixOS/nixpkgs unified with nixpkgs/nixos-20.09 produces github:NixOS/nixpkgs/nixos-20.09.

  • github:NixOS/nixpkgs/master unified with nixpkgs/nixos-20.09 produces github:NixOS/nixpkgs/nixos-20.09.

)""