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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-writing-nixos-tests">
<title>Writing Tests</title>
<para>
A NixOS test is a Nix expression that has the following structure:
</para>
<programlisting language="bash">
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…
'';
}
</programlisting>
<para>
The attribute <literal>testScript</literal> 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 <literal>machine</literal> (if you need
only one machine in your test) or by the attribute
<literal>nodes</literal> (if you need multiple machines). For
instance,
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/login.nix"><literal>login.nix</literal></link>
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,
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nfs/simple.nix"><literal>nfs/simple.nix</literal></link>,
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.
</para>
<para>
There are a few special NixOS configuration options for test VMs:
</para>
<variablelist>
<varlistentry>
<term>
<literal>virtualisation.memorySize</literal>
</term>
<listitem>
<para>
The memory of the VM in megabytes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>virtualisation.vlans</literal>
</term>
<listitem>
<para>
The virtual networks to which the VM is connected. See
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nat.nix"><literal>nat.nix</literal></link>
for an example.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>virtualisation.writableStore</literal>
</term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
For more options, see the module
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix"><literal>qemu-vm.nix</literal></link>.
</para>
<para>
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 <literal>name</literal> if this is also the
identifier of the machine in the declarative config. If you didn't
specify multiple machines using the <literal>nodes</literal>
attribute, it is just <literal>machine</literal>. 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:
</para>
<programlisting language="python">
machine.start()
machine.wait_for_unit("default.target")
if not "Linux" in machine.succeed("uname"):
raise Exception("Wrong OS")
</programlisting>
<para>
The first line is actually unnecessary; machines are implicitly
started when you first execute an action on them (such as
<literal>wait_for_unit</literal> or <literal>succeed</literal>). If
you have multiple machines, you can speed up the test by starting
them in parallel:
</para>
<programlisting language="python">
start_all()
</programlisting>
<para>
The following methods are available on machine objects:
</para>
<variablelist>
<varlistentry>
<term>
<literal>start</literal>
</term>
<listitem>
<para>
Start the virtual machine. This method is asynchronous — it
does not wait for the machine to finish booting.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>shutdown</literal>
</term>
<listitem>
<para>
Shut down the machine, waiting for the VM to exit.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>crash</literal>
</term>
<listitem>
<para>
Simulate a sudden power failure, by telling the VM to exit
immediately.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>block</literal>
</term>
<listitem>
<para>
Simulate unplugging the Ethernet cable that connects the
machine to the other machines.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>unblock</literal>
</term>
<listitem>
<para>
Undo the effect of <literal>block</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>screenshot</literal>
</term>
<listitem>
<para>
Take a picture of the display of the virtual machine, in PNG
format. The screenshot is linked from the HTML log.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>get_screen_text_variants</literal>
</term>
<listitem>
<para>
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.
</para>
<note>
<para>
This requires passing <literal>enableOCR</literal> to the
test attribute set.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>get_screen_text</literal>
</term>
<listitem>
<para>
Return a textual representation of what is currently visible
on the machine's screen using optical character recognition.
</para>
<note>
<para>
This requires passing <literal>enableOCR</literal> to the
test attribute set.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>send_monitor_command</literal>
</term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>send_key</literal>
</term>
<listitem>
<para>
Simulate pressing keys on the virtual keyboard, e.g.,
<literal>send_key("ctrl-alt-delete")</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>send_chars</literal>
</term>
<listitem>
<para>
Simulate typing a sequence of characters on the virtual
keyboard, e.g.,
<literal>send_chars("foobar\n")</literal> will type
the string <literal>foobar</literal> followed by the Enter
key.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>execute</literal>
</term>
<listitem>
<para>
Execute a shell command, returning a list
<literal>(status, stdout)</literal>. If the
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