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

<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-running-nixos-tests">
 <title>Running Tests interactively</title>

 <para>
  The test itself can be run interactively. This is particularly useful when developing or debugging a test:
<screen>
<prompt>$ </prompt>nix-build nixos/tests/login.nix -A driver
<prompt>$ </prompt>./result/bin/nixos-test-driver
starting VDE switch for network 1
<prompt>&gt;</prompt>
</screen>
  You can then take any Perl statement, e.g.
<screen>
<prompt>&gt;</prompt> startAll
<prompt>&gt;</prompt> testScript
<prompt>&gt;</prompt> $machine->succeed("touch /tmp/foo")
<prompt>&gt;</prompt> print($machine->succeed("pwd")) # Show stdout of command
</screen>
  The function <command>testScript</command> 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>
  To just start and experiment with the VMs, run:
<screen>
<prompt>$ </prompt>nix-build nixos/tests/login.nix -A driver
<prompt>$ </prompt>./result/bin/nixos-run-vms
</screen>
  The script <command>nixos-run-vms</command> starts the virtual machines defined by test.
 </para>

 <para>
  The machine state is kept across VM restarts in <filename>/tmp/vm-state-</filename><varname>machinename</varname>.
 </para>
</section>