When is a path in a Nix expression copied into the Nix store?

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1

Is there a list?

For example:

  • “Nix path to string” scenarios1:

    • builtins.toString ./a_file
      returns absolute path to a_file as a string (or an error if it doesn’t exist
      not copied to the Nix store

    • string interpolation (e.g., "${./a_file}")
      coptied to the Nix store and returns the absolute path to it as a string

  • “bare” Nix paths in mkShell when assigning to environment variables:

    { pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = [
    # ...
  ];

  # environment variable
  GIT_CONFIG_GLOBAL = ./assets/git.conf;

  # equivalent to:
  shellHook =
    let
      gitConf =
        pkgs.writeText
          "git.conf"
          (builtins.readFile ./assets/git.conf)
      ;
    in
      ''
      #                         VV.......V
      export GIT_CONFIG_GLOBAL="${gitConf}"
      #                         ^^.......^
      #
      # That's Nix's string interpolation!
      # Use ''\{..} for Bash's string interpolation.
      ''
  ;
}

  • Haven’t packaged anything yet with Nix, but what I remember from the manuals is that any path that appears in builder shell scripts will get put into the Nix store as well. (Right?)

[1]: Relevant threads:

1 Like
2

You’re in luck, thanks to funding by Antithesis we expanded on that subject the manual in the past months:

https://nixos.org/manual/nix/stable/language/operators.html#string-concatenation

https://nixos.org/manual/nix/stable/language/string-interpolation.html

https://nixos.org/manual/nix/stable/language/values.html#type-path

https://nixos.org/manual/nix/stable/language/builtins.html#builtins-toString

Right. Paths are baked into derivations to make them immutable.

1 Like
3

Thank you for collecting these Nix manual pages! It’s been a while since I read through the manual, but they have changed quite a lot, and now have significantly more practical info in them. Thank you for all of you who contributed!

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