nixpkgs/nixos/doc/manual/from_md/development/running-nixos-tests-interactively.section.xml

<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-running-nixos-tests-interactively">
  <title>Running Tests interactively</title>
  <para>
    The test itself can be run interactively. This is particularly
    useful when developing or debugging a test:
  </para>
  <programlisting>
$ nix-build nixos/tests/login.nix -A driverInteractive
$ ./result/bin/nixos-test-driver --interactive
starting VDE switch for network 1
&gt;
</programlisting>
  <para>
    You can then take any Python statement, e.g.
  </para>
  <programlisting language="python">
&gt; start_all()
&gt; test_script()
&gt; machine.succeed(&quot;touch /tmp/foo&quot;)
&gt; print(machine.succeed(&quot;pwd&quot;)) # Show stdout of command
</programlisting>
  <para>
    The function <literal>test_script</literal> executes the entire test
    script and drops you back into the test driver command line upon its
    completion. This allows you to inspect the state of the VMs after
    the test (e.g. to debug the test script).
  </para>
  <para>
    You can re-use the VM states coming from a previous run by setting
    the <literal>--keep-vm-state</literal> flag.
  </para>
  <programlisting>
$ ./result/bin/nixos-test-driver --interactive --keep-vm-state
</programlisting>
  <para>
    The machine state is stored in the
    <literal>$TMPDIR/vm-state-machinename</literal> directory.
  </para>
</section>