How do you create your own flake check?

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1

Hello!

As in the title; how do you create flake checks? I can’t find any examples anywhere.

Thank you kindly for the help!

2

A check is a derivation. If the derivation builds successfully, then the check is a success.

$ cat flake.nix
{
  outputs = { nixpkgs, ... }:
  {
    checks.x86_64-linux.math = with nixpkgs.legacyPackages.x86_64-linux;
      runCommand "math-test" {} ''
        test $(( 1 + 1 )) -eq  3
        touch $out
      '';
  };
}

$ nix flake check
error: builder for '/nix/store/zxjran6ax2dmfc65603h1qpj3984sgr4-math-test.drv' failed with exit code 1

Changing the command to test $(( 1 + 1 )) -eq 2 makes the check succeed.

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3

Can we run individual nix commands within these checks, such as nix build?

4

Not exactly, they still need to be derivations. Your derivation can just be an indicidual command, however, as long as you produce some kind if $out file, e.g.:

checks.x86_64-linux.test = with nixpkgs.legacyPackages.x86_64-linux;
      runCommand "test" {} ''
        echo test > $out
      '';

nix build specifically won’t work, since it’s a nix command. You can do the exact same thing as nix build by just setting the check to equal one of your packages, though:

checks.x86_64-linux.flake-build = self.packages.x86_64-linux.default;

Or you could write a test for the package, which will also build it, since it’s needed to run the test:

checks.x86_64-linux.test =
    let
      package = self.packages.x86_64-linux.default;
    in
      with nixpkgs.legacyPackages.x86_64-linux;
        runCommand "test" {} ''
          ${package}/bin/package > $out
        '';
```:
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5

Ah; got it. Thanks so much for the detailed explanation!

6

Hi thanks for this explanation, a furthur question maybe:

Is there some way to write a check to do proper evaluation of a nixosConfigurations output?

TIL the default nix flake check only verifies the type of nixosConfigurations is a ‘derivation’, but doesn’t actually eval nixosConfigurations thanks to @raphi

checks.x86_64-linux.myhost = self.nixosConfigurations.myhost.config.system.build.toplevel;
checks.x86_64-linux.myotherchecks = runCommand "test" {} ''echo ..";

If I do something like above, it’d build the host, which is time consuming and unwanted. What I want is nix flake check to run nix eval --raw .#nixosConfigurations.myhost.config.system.build.toplevel in essences.

There is nix flake check --no-build, but I do actually need to build in order for the some other hand written checks to run, so… :smiling_face_with_tear:

1 Like
7

Sounds like that’s worth reconsidering, I personally assumed it would actually evaluate the nixosConfigurations at least. Anyone know why it doesn’t?

1 Like