This chapter describes several testing builders which are available in the testers namespace.


Checks whether a package exposes a given list of pkg-config modules. If the moduleNames argument is omitted, hasPkgConfigModules will use meta.pkgConfigModules.


passthru.tests.pkg-config = testers.hasPkgConfigModules {
  package = finalAttrs.finalPackage;
  moduleNames = [ "libfoo" ];

If the package in question has meta.pkgConfigModules set, it is even simpler:

passthru.tests.pkg-config = testers.hasPkgConfigModules {
  package = finalAttrs.finalPackage;

meta.pkgConfigModules = [ "libfoo" ];


Checks the command output contains the specified version

Although simplistic, this test assures that the main program can run. While there's no substitute for a real test case, it does catch dynamic linking errors and such. It also provides some protection against accidentally building the wrong version, for example when using an 'old' hash in a fixed-output derivation.


passthru.tests.version = testers.testVersion { package = hello; };

passthru.tests.version = testers.testVersion {
  package = seaweedfs;
  command = "weed version";

passthru.tests.version = testers.testVersion {
  package = key;
  command = "KeY --help";
  # Wrong '2.5' version in the code. Drop on next version.
  version = "2.5";

passthru.tests.version = testers.testVersion {
  package = ghr;
  # The output needs to contain the 'version' string without any prefix or suffix.
  version = "v${version}";


Make sure that a build does not succeed. This is useful for testing testers.

This returns a derivation with an override on the builder, with the following effects:

  • Fail the build when the original builder succeeds
  • Move $out to $out/result, if it exists (assuming out is the default output)
  • Save the build log to $out/testBuildFailure.log (same)


runCommand "example" {
  failed = testers.testBuildFailure (runCommand "fail" {} ''
    echo ok-ish >$out
    echo failing though
    exit 3
} ''
  grep -F 'ok-ish' $failed/result
  grep -F 'failing though' $failed/testBuildFailure.log
  [[ 3 = $(cat $failed/testBuildFailure.exit) ]]
  touch $out

While testBuildFailure is designed to keep changes to the original builder's environment to a minimum, some small changes are inevitable.

  • The file $TMPDIR/testBuildFailure.log is present. It should not be deleted.
  • stdout and stderr are a pipe instead of a tty. This could be improved.
  • One or two extra processes are present in the sandbox during the original builder's execution.
  • The derivation and output hashes are different, but not unusual.
  • The derivation includes a dependency on buildPackages.bash and expect-failure.sh, which is built to include a transitive dependency on buildPackages.coreutils and possibly more. These are not added to PATH or any other environment variable, so they should be hard to observe.


Check that two paths have the same contents.


testers.testEqualContents {
  assertion = "sed -e performs replacement";
  expected = writeText "expected" ''
    foo baz baz
  actual = runCommand "actual" {
    # not really necessary for a package that's in stdenv
    nativeBuildInputs = [ gnused ];
    base = writeText "base" ''
      foo bar baz
  } ''
    sed -e 's/bar/baz/g' $base >$out


Checks that two packages produce the exact same build instructions.

This can be used to make sure that a certain difference of configuration, such as the presence of an overlay does not cause a cache miss.

When the derivations are equal, the return value is an empty file. Otherwise, the build log explains the difference via nix-diff.


  "The hello package must stay the same when enabling checks."
  (hello.overrideAttrs(o: { doCheck = true; }))


Use the derivation hash to invalidate the output via name, for testing.

Type: (a@{ name, ... } -> Derivation) -> a -> Derivation

Normally, fixed output derivations can and should be cached by their output hash only, but for testing we want to re-fetch everytime the fetcher changes.

Changes to the fetcher become apparent in the drvPath, which is a hash of how to fetch, rather than a fixed store path. By inserting this hash into the name, we can make sure to re-run the fetcher every time the fetcher changes.

This relies on the assumption that Nix isn't clever enough to reuse its database of local store contents to optimize fetching.

You might notice that the "salted" name derives from the normal invocation, not the final derivation. invalidateFetcherByDrvHash has to invoke the fetcher function twice: once to get a derivation hash, and again to produce the final fixed output derivation.


tests.fetchgit = testers.invalidateFetcherByDrvHash fetchgit {
  name = "nix-source";
  url = "https://github.com/NixOS/nix";
  rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
  hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";


A helper function that behaves exactly like the NixOS runTest, except it also assigns this Nixpkgs package set as the pkgs of the test and makes the nixpkgs.* options read-only.

If your test is part of the Nixpkgs repository, or if you need a more general entrypoint, see "Calling a test" in the NixOS manual.


pkgs.testers.runNixOSTest ({ lib, ... }: {
  name = "hello";
  nodes.machine = { pkgs, ... }: {
    environment.systemPackages = [ pkgs.hello ];
  testScript = ''


Run a NixOS VM network test using this evaluation of Nixpkgs.

NOTE: This function is primarily for external use. NixOS itself uses make-test-python.nix directly. Packages defined in Nixpkgs reuse NixOS tests via nixosTests, plural.

It is mostly equivalent to the function import ./make-test-python.nix from the NixOS manual, except that the current application of Nixpkgs (pkgs) will be used, instead of letting NixOS invoke Nixpkgs anew.

If a test machine needs to set NixOS options under nixpkgs, it must set only the nixpkgs.pkgs option.


A NixOS VM test network, or path to it. Example:

  name = "my-test";
  nodes = {
    machine1 = { lib, pkgs, nodes, ... }: {
      environment.systemPackages = [ pkgs.hello ];
      services.foo.enable = true;
    # machine2 = ...;
  testScript = ''
    machine1.succeed("hello | foo-send")


A derivation that runs the VM test.

Notable attributes:

  • nodes: the evaluated NixOS configurations. Useful for debugging and exploring the configuration.

  • driverInteractive: a script that launches an interactive Python session in the context of the testScript.