nixpkgs/nixos/doc/manual/development/writing-nixos-tests.section.md

Writing Tests

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

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 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, 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 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:

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:

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). If the command detaches, it must close stdout, as execute will wait for this to consume all output reliably. This can be achieved by redirecting stdout to stderr >&2, to /dev/console, /dev/null or a file. Examples of detaching commands are sleep 365d &, where the shell forks a new process that can write to stdout and xclip -i, where the xclip command itself forks without closing 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.

• It will wait for stdout to be closed. See execute for the implications.

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

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:

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):

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):

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