Integration testing using virtual machines#

This tutorial aims to be compatible with NixOS release 22.11.

What will you learn?#

This tutorial introduces the functionality of Nixpkgs to write automated tests to debug NixOS configurations independent of a working NixOS installation.

What do you need?#


Nixpkgs provides a test environment to automate integration testing for distributed systems. It allows defining tests based on a set of declarative NixOS configurations and using a Python shell to interact with them through QEMU as the backend. Those tests are widely used to ensure that NixOS works as intended, so in general they are called NixOS Tests. They can be written and launched outside of NixOS, on any Linux machine[1].

The nixosTest function#

NixOS VM tests are defined using the nixosTest function. The pattern for NixOS VM tests looks like this:

 2  nixpkgs = fetchTarball "";
 3  pkgs = import nixpkgs { config = {}; overlays = []; };
 5  pkgs.nixosTest {
 6    name = "test-name";
 7    nodes = {
 8      machine1 = { config, pkgs, ... }: {
 9        # ...
10      };
11      machine2 = { config, pkgs, ... }: {
12        # ...
13      };
14    }
15    testScript = { nodes, ... }: ''
16      # ...
17    '';
18  }

The function nixosTest takes an attribute set that follows the module convention to specify the test. Because the attribute set only defines options, one can use the abbreviated form of the module convention. The attribute set needs to define the following options:

  • name defines the name of the test.

  • nodes contains a set of named configurations, because a test script can involve more than one virtual machine. Each virtual machine is setup using a NixOS configuration.

  • testScript defines the Python test script, either as literal string or as a function that takes a nodes attribute. This Python test script can access the virtual machines via the names used for the nodes. It has super user rights in the virtual machines. In the Python script is each virtual machine is accessible via the machine object. NixOS provides various methods to run tests on these configurations.

The test framework automatically starts the virtual machines and runs the Python script.

Minimal example#

As a minimal test on the default configuration, we will check if the user root and alice can run Firefox. We will build the example up from scratch.

As recommended we use an explicitly pinned version of Nixpkgs, and explicitly set configuration options and overlays to avoid them being inadvertently overridden by global configuration:

2  nixpkgs = fetchTarball "";
3  pkgs = import nixpkgs { config = {}; overlays = []; };
5  pkgs.nixosTest {
6    # ...
7  }



Label the test with a descriptive name such as “minimal-test”:

1name = "minimal-test";


Because this example only uses one virtual machine, the node we specify is simply called machine. This name is arbitrary and can be chosen freely. As configuration you use the relevant parts of the default configuration, that we used in a previous tutorial:

 1nodes.machine = { config, pkgs, ... }: {
 2  users.users.alice = {
 3    isNormalUser = true;
 4    extraGroups = [ "wheel" ];
 5    packages = with pkgs; [
 6      firefox
 7      tree
 8    ];
 9  };
11  system.stateVersion = "22.11";

Test script#

This is the test script:

2machine.succeed("su -- alice -c 'which firefox'")"su -- root -c 'which firefox'")

This Python script is referring to machine which is the name chosen for the virtual machine configuration used in the nodes attribute set.

The script waits until systemd reaches It uses the su command to switch between users and the which command to check if the user has access to firefox. It expects that the command which firefox to succeed for user alice and to fail for root.

This script will be the value of the testScript attribute.

Test file#

The complete minimal-test.nix file content looks like the following:

    nixpkgs = fetchTarball "";
  pkgs = import nixpkgs { config = {}; overlays = []; };
  pkgs.nixosTest {
    name = "minimal-test";
    nodes.machine = { config, pkgs, ... }: {

      users.users.alice = {
        isNormalUser = true;
        extraGroups = [ "wheel" ];
        packages = with pkgs; [

      system.stateVersion = "22.11";
    testScript = ''
      machine.succeed("su -- alice -c 'which firefox'")"su -- root -c 'which firefox'")

Running tests#

To set up all machines and run the test script:

$ nix-build minimal-test.nix
test script finished in 10.96s
cleaning up
killing machine (pid 10)
(0.00 seconds)

Interactive Python shell in the virtual machine#

When developing tests or when something breaks, it’s useful to interactively tinker with the test or access a terminal for a machine.

To start an interactive Python session with the testing framework:

$ $(nix-build -A driverInteractive minimal-test.nix)/bin/nixos-test-driver

Here you can run any of the testing operations. Execute the testScript attribute from minimal-test.nix with the test_script() function.

If a virtual machine is not yet started, the test environment takes care of it on the first call of a method on a machine object.

But you can also manually trigger the start of the virtual machine with:

>>> machine.start()

for a specific node,


>>> start_all()

for all nodes.

You can enter a interactive shell on the virtual machine using:

>>> machine.shell_interact()

and run shell commands like:

uname -a
Linux server 5.10.37 #1-NixOS SMP Fri May 14 07:50:46 UTC 2021 x86_64 GNU/Linux
Re-running successful tests

Because test results are kept in the Nix store, a successful test is cached. This means that Nix will not run the test a second time as long as the test setup (node configuration and test script) stays semantically the same. Therefore, to run a test again, one needs to remove the result.

If you would try to delete the result using the symbolic link, you will get the following error:

nix-store --delete ./result
finding garbage collector roots...
0 store paths deleted, 0.00 MiB freed
error: Cannot delete path '/nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed' since it is still alive. To find out why, use: nix-store --query --roots

Instead, remove the symbolic link and only then remove the cached result:

rm ./result
nix-store --delete /nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed

This can be also done with one command:

result=$(readlink -f ./result) rm ./result && nix-store --delete $result

Tests that need multiple virtual machines#

Tests can involve multiple virtual machines.

This example uses the use-case of a REST interface to a PostgreSQL database. The following example Nix expression is adapted from How to use NixOS for lightweight integration tests.

This tutorial follows PostgREST tutorial, a generic RESTful API for PostgreSQL.

If you skim over the official tutorial, you’ll notice there’s quite a bit of setup in order to test if all the steps work.

The setup includes:

  • A virtual machine named server running PostgreSQL and PostgREST.

  • A virtual machine named client running HTTP client queries using curl.

  • A testScript orchestrating testing logic between client and server.

The complete postgrest.nix file looks like the following:

  # Pin Nixpkgs, as some packages are broken in the 22.11 release
  nixpkgs = fetchTarball "";
  pkgs = import nixpkgs { config = {}; overlays = []; };

  # Single source of truth for all tutorial constants
  database = "postgres";
  schema        = "api";
  table         = "todos";
  username      = "authenticator";
  password      = "mysecretpassword";
  webRole       = "web_anon";
  postgrestPort = 3000;

  # NixOS module shared between server and client
  sharedModule = {
    # Since it's common for CI not to have $DISPLAY available, explicitly disable graphics support = false;

  pkgs.nixosTest {
    # NixOS tests are run inside a virtual machine, and here you specify its system type
    system = "x86_64-linux";
    name = "postgres-test";
    nodes = {
      server = { config, pkgs, ... }: {
        imports = [ sharedModule ];

        networking.firewall.allowedTCPPorts = [ postgrestPort ];

        services.postgresql = {
          enable = true;

          initialScript = pkgs.writeText "initialScript.sql" ''
            create schema ${schema};

            create table ${schema}.${table} (
                id serial primary key,
                done boolean not null default false,
                task text not null,
                due timestamptz

            insert into ${schema}.${table} (task) values ('finish tutorial 0'), ('pat self on back');

            create role ${webRole} nologin;
            grant usage on schema ${schema} to ${webRole};
            grant select on ${schema}.${table} to ${webRole};

            create role ${username} inherit login password '${password}';
            grant ${webRole} to ${username};

        users = {
          mutableUsers = false;
          users = {
            # For ease of debugging the VM as the `root` user
            root.password = "";

            # Create a system user that matches the database user so that you
            # can use peer authentication. The tutorial defines a password,
            # but it's not necessary.
            "${username}".isSystemUser = true;
        }; = {
          wantedBy = [ "" ];
          after = [ "postgresql.service" ];
          script =
              configuration = pkgs.writeText "tutorial.conf" ''
                  db-uri = "postgres://${username}:${password}@localhost:${toString}/${database}"
                  db-schema = "${schema}"
                  db-anon-role = "${username}"
            in "${pkgs.haskellPackages.postgrest}/bin/postgrest ${configuration}";
          serviceConfig.User = username;

      client = {
        imports = [ sharedModule ];

    # Disable linting for simpler debugging of the testScript
    skipLint = true;

    testScript = ''
      import json
      import sys


      server.wait_for_open_port(${toString postgrestPort})

      expected = [
          {"id": 1, "done": False, "task": "finish tutorial 0", "due": None},
          {"id": 2, "done": False, "task": "pat self on back", "due": None},

      actual = json.loads(
              "${pkgs.curl}/bin/curl http://server:${toString postgrestPort}/${table}"

      assert expected == actual, "table query returns expected content"

Unlike the previous example, the virtual machines need an expressive name to distinguish them. For this example we choose client and server.

Set up all machines and run the test script:

nix-build postgrest.nix
test script finished in 10.96s
cleaning up
killing client (pid 10)
killing server (pid 22)
(0.00 seconds)

## Additional information regarding NixOS tests:
  - Running integration tests on CI requires hardware acceleration, which many CIs do not support.

    To run integration tests on [GitHub Actions](<github-actions>) see [how to disable hardware acceleration](
  - NixOS comes with a large set of tests that serve also as educational examples.

    A good inspiration is [Matrix bridging with an IRC](

<!-- TODO: move examples from to the NixOS manual and troubleshooting tips to -->