depot/third_party/nixpkgs/nixos/doc/manual/development/running-nixos-tests-interactively.section.md

Running Tests interactively

The test itself can be run interactively. This is particularly useful when developing or debugging a test:

$ nix-build nixos/tests/login.nix -A driverInteractive
$ ./result/bin/nixos-test-driver --interactive
starting VDE switch for network 1
>

You can then take any Python statement, e.g.

> start_all()
> test_script()
> machine.succeed("touch /tmp/foo")
> print(machine.succeed("pwd")) # Show stdout of command

The function test_script 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).

You can re-use the VM states coming from a previous run by setting the --keep-vm-state flag.

$ ./result/bin/nixos-test-driver --interactive --keep-vm-state

The machine state is stored in the $TMPDIR/vm-state-machinename directory.