builtins.findFile

Takes 2 arguments

search-path, lookup-path

Find lookup-path in search-path.

Lookup path expressions are desugared using this and builtins.nixPath:

<nixpkgs>

is equivalent to:

builtins.findFile builtins.nixPath "nixpkgs"

A search path is represented as a list of attribute sets with two attributes:

• prefix is a relative path.
• path denotes a file system location

Examples of search path attribute sets:

• {
    prefix = "";
    path = "/nix/var/nix/profiles/per-user/root/channels";
  }
  

• {
    prefix = "nixos-config";
    path = "/etc/nixos/configuration.nix";
  }
  

• {
    prefix = "nixpkgs";
    path = "https://github.com/NixOS/nixpkgs/tarballs/master";
  }
  

• {
    prefix = "nixpkgs";
    path = "channel:nixpkgs-unstable";
  }
  

• {
    prefix = "flake-compat";
    path = "flake:github:edolstra/flake-compat";
  }
  

The lookup algorithm checks each entry until a match is found, returning a path value of the match:

• If a prefix of lookup-path matches prefix, then the remainder of lookup-path (the "suffix") is searched for within the directory denoted by path. The contents of path may need to be downloaded at this point to look inside.

• If the suffix is found inside that directory, then the entry is a match. The combined absolute path of the directory (now downloaded if need be) and the suffix is returned.

Example

A search-path value

[
  {
    prefix = "";
    path = "/home/eelco/Dev";
  }
  {
    prefix = "nixos-config";
    path = "/etc/nixos";
  }
]

and a lookup-path value "nixos-config" causes Nix to try /home/eelco/Dev/nixos-config and /etc/nixos in that order and return the first path that exists.

If path starts with http:// or https://, it is interpreted as the URL of a tarball to be downloaded and unpacked to a temporary location. The tarball must consist of a single top-level directory.

The URLs of the tarballs from the official nixos.org channels can be abbreviated as channel:<channel-name>. See documentation on nix-channel for details about channels.

Example

These two search path entries are equivalent:

• {
    prefix = "nixpkgs";
    path = "channel:nixpkgs-unstable";
  }
  

• {
    prefix = "nixpkgs";
    path = "https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz";
  }
  

Search paths can also point to source trees using flake URLs.

Example

The search path entry

{
  prefix = "nixpkgs";
  path = "flake:nixpkgs";
}

specifies that the prefix nixpkgs shall refer to the source tree downloaded from the nixpkgs entry in the flake registry.

Similarly

{
  prefix = "nixpkgs";
  path = "flake:github:nixos/nixpkgs/nixos-22.05";
}

makes <nixpkgs> refer to a particular branch of the NixOS/nixpkgs repository on GitHub.