Difference between revisions of "How to fetch Nixpkgs with an empty NIX PATH"

Latest revision as of 10:55, 6 April 2024

You can fetch Nixpkgs with an empty NIX_PATH. This comes in handy if you want to remove impure references to the NIX_PATH from your code base.

To do so, you can use builtins.fetchTarball, like this:

let
  nixpkgs = builtins.fetchTarball {
    url    = "https://github.com/NixOS/nixpkgs/archive/3389f23412877913b9d22a58dfb241684653d7e9.tar.gz";
    sha256 = "0wgm7sk9fca38a50hrsqwz6q79z35gqgb9nw80xz7pfdr4jy9pf8";
  };

  pkgs = import nixpkgs { config = {}; };

in
  ...

… replacing 3389f2… with the desired revision of Nixpkgs and replacing 0wgm7s… with the corresponding SHA256 hash.

You can use the following command to obtain the correct SHA256 hash to use if you prefer not to use trial and error:

$ nix-prefetch-url --unpack "https://github.com/NixOS/nixpkgs/archive/${REVISION}.tar.gz"

@@ Line 1: / Line 1: @@
-This recipe provides a way to fetch Nixpkgs with an empty <code>NIX_PATH</code>.  This
+You can fetch Nixpkgs with an empty <code>NIX_PATH</code>.  This
 comes in handy if you want to remove impure references to the <code>NIX_PATH</code> from
-your code base
+your code base.
-Save the following code to a <code>fetchNixpkgs.nix</code> file within your project:
+To do so, you can use <code>builtins.fetchTarball</code>, like this:
 <syntaxhighlight lang="nix">
-{ rev                             # The Git revision of nixpkgs to fetch
+let
-, sha256                          # The SHA256 of the downloaded data
+  nixpkgs = builtins.fetchTarball {
-, system ? builtins.currentSystem # This is overridable if necessary
+    url    = "https://github.com/NixOS/nixpkgs/archive/3389f23412877913b9d22a58dfb241684653d7e9.tar.gz";
-}:
+    sha256 = "0wgm7sk9fca38a50hrsqwz6q79z35gqgb9nw80xz7pfdr4jy9pf8";
+  };
-with {
+  pkgs = import nixpkgs { config = {}; };
-  ifThenElse = { bool, thenValue, elseValue }: (
-    if bool then thenValue else elseValue);
-};
-ifThenElse {
+in
-   bool = (0 <= builtins.compareVersions builtins.nixVersion "1.12");
+   ...
-  # In Nix 1.12, we can just give a `sha256` to `builtins.fetchTarball`.
-  thenValue = (
-    builtins.fetchTarball {
-      url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-      inherit sha256;
-    });
-  # This hack should at least work for Nix 1.11
-  elseValue = (
-    (rec {
-      tarball = import <nix/fetchurl.nix> {
-        url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-        inherit sha256;
-      };
-      builtin-paths = import <nix/config.nix>;
-      script = builtins.toFile "nixpkgs-unpacker" ''
-        "$coreutils/mkdir" "$out"
-        cd "$out"
-        "$gzip" --decompress < "$tarball" | "$tar" -x --strip-components=1
-      '';
-      nixpkgs = builtins.derivation {
-        name = "nixpkgs-${builtins.substring 0 6 rev}";
-        builder = builtins.storePath builtin-paths.shell;
-        args = [ script ];
-        inherit tarball system;
-        tar       = builtins.storePath builtin-paths.tar;
-        gzip      = builtins.storePath builtin-paths.gzip;
-        coreutils = builtins.storePath builtin-paths.coreutils;
-      };
-    }).nixpkgs);
-}
 </syntaxhighlight>
-... and then you can use the saved file to fetch Nixpkgs like this:
+… replacing <code>3389f2…</code> with the desired revision of Nixpkgs and replacing <code>0wgm7s…</code> with the corresponding SHA256 hash.
-<syntaxhighlight lang="nix">
-let
-  fetchNixpkgs = import ./fetchNixpkgs.nix;
-  nixpkgs = fetchNixpkgs {
+You can use the following command to obtain the correct SHA256 hash to use if you prefer not to use trial and error:
-     rev = "76d649b59484607901f0c1b8f737d8376a904019";
-     sha256 = "01c2f4mj4ahir0sxk9kxbymg2pki1pc9a3y6r9x6ridry75fzb8h";
-  };
-  pkgs = import nixpkgs { config = {}; };
+<syntaxhighlight lang="bash">
-in
+$ nix-prefetch-url --unpack "https://github.com/NixOS/nixpkgs/archive/${REVISION}.tar.gz"
-  ...
 </syntaxhighlight>
 [[Category:Nix]] [[Category:Nixpkgs]] [[Category:Cookbook]]