# Writing Tests

A NixOS test is a Nix expression that has the following structure:

    import ./make-test.nix {

      # Either the configuration of a single machine:
      machine =
        { config, pkgs, ... }:
        { configuration…
        };

      # Or a set of machines:
      nodes =
        { machine1 =
            { config, pkgs, ... }: { … };
          machine2 =
            { config, pkgs, ... }: { … };
          …
        };

      testScript =
        ''
          Perl code…
        '';
    }

The attribute `testScript` is a bit of Perl code that executes the test (described below). During the test, it will start one or more virtual machines, the configuration of which is described by the attribute `machine` (if you need only one machine in your test) or by the attribute `nodes` (if you need multiple machines). For instance, `login.nix` only needs a single machine to test whether users can log in on the virtual console, whether device ownership is correctly maintained when switching between consoles, and so on. On the other hand, `nfs.nix`, which tests NFS client and server functionality in the Linux kernel (including whether locks are maintained across server crashes), requires three machines: a server and two clients.

There are a few special NixOS configuration options for test VMs:

`virtualisation.memorySize`

  The memory of the VM in megabytes.

`virtualisation.vlans`

  The virtual networks to which the VM is connected. See `nat.nix` for an example.

`virtualisation.writableStore`

  By default, the Nix store in the VM is not writable. If you enable this option, a writable union file system is mounted on top of the Nix store to make it appear writable. This is necessary for tests that run Nix operations that modify the store.

For more options, see the module `qemu-vm.nix`.

The test script is a sequence of Perl statements that perform various actions, such as starting VMs, executing commands in the VMs, and so on. Each virtual machine is represented as an object stored in the variable `$name`, where name is the identifier of the machine (which is just `machine` if you didn’t specify multiple machines using the `nodes` attribute). For instance, the following starts the machine, waits until it has finished booting, then executes a command and checks that the output is more-or-less correct:

    $machine->start;
    $machine->waitForUnit("default.target");
    $machine->succeed("uname") =~ /Linux/ or die;

The first line is actually unnecessary; machines are implicitly started when you first execute an action on them (such as `waitForUnit` or `succeed`). If you have multiple machines, you can speed up the test by starting them in parallel:

    startAll;

The following methods are available on machine objects:

`start`

  Start the virtual machine. This method is asynchronous — it does not wait for the machine to finish booting.

`shutdown`

  Shut down the machine, waiting for the VM to exit.

`crash`

  Simulate a sudden power failure, by telling the VM to exit immediately.

`block`

  Simulate unplugging the Ethernet cable that connects the machine to the other machines.

`unblock`

  Undo the effect of `block`.

`screenshot`

  Take a picture of the display of the virtual machine, in PNG format. The screenshot is linked from the HTML log.

`getScreenText`

  Return a textual representation of what is currently visible on the machine's screen using optical character recognition.

  This requires passing `enableOCR` to the test attribute set.

`sendMonitorCommand`

  Send a command to the QEMU monitor. This is rarely used, but allows doing stuff such as attaching virtual USB disks to a running machine.

`sendKeys`

  Simulate pressing keys on the virtual keyboard, e.g., `sendKeys("ctrl-alt-delete")`.

`sendChars`

  Simulate typing a sequence of characters on the virtual keyboard, e.g., `sendKeys("foobar\n")` will type the string `foobar` followed by the Enter key.

`execute`

  Execute a shell command, returning a list `(status, stdout)`.

`succeed`

  Execute a shell command, raising an exception if the exit status is not zero, otherwise returning the standard output.

`fail`

  Like `succeed`, but raising an exception if the command returns a zero status.

`waitUntilSucceeds`

  Repeat a shell command with 1-second intervals until it succeeds.

`waitUntilFails`

  Repeat a shell command with 1-second intervals until it fails.

`waitForUnit`

  Wait until the specified systemd unit has reached the “active” state.

`waitForFile`

  Wait until the specified file exists.

`waitForOpenPort`

  Wait until a process is listening on the given TCP port (on `localhost`, at least).

`waitForClosedPort`

  Wait until nobody is listening on the given TCP port.

`waitForX`

  Wait until the X11 server is accepting connections.

`waitForText`

  Wait until the supplied regular expressions matches the textual contents of the screen by using optical character recognition (see `getScreenText`).

  This requires passing `enableOCR` to the test attribute set.

`waitForWindow`

  Wait until an X11 window has appeared whose name matches the given regular expression, e.g., `waitForWindow(qr/Terminal/)`.

`copyFileFromHost`

  Copies a file from host to machine, e.g., `copyFileFromHost("myfile", "/etc/my/important/file")`.

  The first argument is the file on the host. The file needs to be accessible while building the nix derivation. The second argument is the location of the file on the machine.

`systemctl`

  Runs `systemctl` commands with optional support for `systemctl --user`

              $machine->systemctl("list-jobs --no-pager"); // runs `systemctl list-jobs --no-pager`
              $machine->systemctl("list-jobs --no-pager", "any-user"); // spawns a shell for `any-user` and runs `systemctl --user list-jobs --no-pager`

To test user units declared by `systemd.user.services` the optional `$user` argument can be used:

        $machine->start;
        $machine->waitForX;
        $machine->waitForUnit("xautolock.service", "x-session-user");

This applies to `systemctl`, `getUnitInfo`, `waitForUnit`, `startJob` and `stopJob`.