# Writing Tests {#sec-writing-nixos-tests}

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

```nix
import ./make-test-python.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 =
    ''
      Python code…
    '';
}
```

The attribute `testScript` is a bit of Python 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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/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/simple.nix`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nfs/simple.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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix).

The test script is a sequence of Python 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` if this is also the identifier of the machine in the declarative
config. If you didn\'t specify multiple machines using the `nodes`
attribute, it is just `machine`. The following example starts the
machine, waits until it has finished booting, then executes a command
and checks that the output is more-or-less correct:

```py
machine.start()
machine.wait_for_unit("default.target")
if not "Linux" in machine.succeed("uname"):
  raise Exception("Wrong OS")
```

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

```py
start_all()
```

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.

`get_screen_text_variants`

:   Return a list of different interpretations of what is currently
    visible on the machine\'s screen using optical character
    recognition. The number and order of the interpretations is not
    specified and is subject to change, but if no exception is raised at
    least one will be returned.

    ::: {.note}
    This requires passing `enableOCR` to the test attribute set.
    :::

`get_screen_text`

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

    ::: {.note}
    This requires passing `enableOCR` to the test attribute set.
    :::

`send_monitor_command`

:   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.

`send_key`

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

`send_chars`

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

`execute`

:   Execute a shell command, returning a list `(status, stdout)`.
    Takes an optional parameter `check_return` that defaults to `True`.
    Setting this parameter to `False` will not check for the return code
    and return -1 instead. This can be used for commands that shut down
    the VM and would therefore break the pipe that would be used for
    retrieving the return code.

`succeed`

:   Execute a shell command, raising an exception if the exit status is
    not zero, otherwise returning the standard output. Commands are run
    with `set -euo pipefail` set:

    -   If several commands are separated by `;` and one fails, the
        command as a whole will fail.

    -   For pipelines, the last non-zero exit status will be returned
        (if there is one, zero will be returned otherwise).

    -   Dereferencing unset variables fail the command.

`fail`

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

`wait_until_succeeds`

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

`wait_until_fails`

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

`wait_for_unit`

:   Wait until the specified systemd unit has reached the "active"
    state.

`wait_for_file`

:   Wait until the specified file exists.

`wait_for_open_port`

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

`wait_for_closed_port`

:   Wait until nobody is listening on the given TCP port.

`wait_for_x`

:   Wait until the X11 server is accepting connections.

`wait_for_text`

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

    ::: {.note}
    This requires passing `enableOCR` to the test attribute set.
    :::

`wait_for_console_text`

:   Wait until the supplied regular expressions match a line of the
    serial console output. This method is useful when OCR is not
    possibile or accurate enough.

`wait_for_window`

:   Wait until an X11 window has appeared whose name matches the given
    regular expression, e.g., `wait_for_window("Terminal")`.

`copy_from_host`

:   Copies a file from host to machine, e.g.,
    `copy_from_host("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`

    ```py
    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`
    ```

`shell_interact`

:   Allows you to directly interact with the guest shell. This should
    only be used during test development, not in production tests.
    Killing the interactive session with `Ctrl-d` or `Ctrl-c` also ends
    the guest session.

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

```py
machine.start()
machine.wait_for_x()
machine.wait_for_unit("xautolock.service", "x-session-user")
```

This applies to `systemctl`, `get_unit_info`, `wait_for_unit`,
`start_job` and `stop_job`.

For faster dev cycles it\'s also possible to disable the code-linters
(this shouldn\'t be commited though):

```nix
import ./make-test-python.nix {
  skipLint = true;
  machine =
    { config, pkgs, ... }:
    { configuration…
    };

  testScript =
    ''
      Python code…
    '';
}
```

This will produce a Nix warning at evaluation time. To fully disable the
linter, wrap the test script in comment directives to disable the Black
linter directly (again, don\'t commit this within the Nixpkgs
repository):

```nix
  testScript =
    ''
      # fmt: off
      Python code…
      # fmt: on
    '';
```