My brain-dump of random code/configuration.

08 Jan 2023

How to execute NixOS tests interactively for debugging

For complex modules, you may sometimes struggle to understand why a test isn’t behaving properly. To gain more insight, you may want to check the debug output. This leads me to discuss how to execute NixOS tests interactively.

Problem statement

In a standard way of running tests, you can’t interfere with the process to explore what’s gone wrong.

But there’s a trick: you can start the test driver in a python REPL loop, which will provide an interactive shell where you can execute your tests. This is a great way to shorten the feedback loop, as we can execute commands on our VMs. For instance, we can tell a VM to dump logs or to display the contents of files.

So, let’s explore how to run tests interactively.

Running tests interactively

To start the hello-world-server test in the interactive mode, you first need to build the test driver by adding the .driver attribute to the test name and then start it manually by providing the --interactive flag. Here’s how you do it:

# Here we assume that our test machine is running on `x86_64-linux`, adjust this to your own architecture)

$ nix build .#checks.x86_64-linux.hello-world-server.driver

This will write out result symlink (all files are created in the nix store and we don’t want to copy them outside) pointing to the test driver. We can run the test driver like this:

./result/bin/nixos-test-driver --interactive

Note: Usually when running tests there’s no Internet access because you want things to be reproducible and self-contained. Running NixOS tests this way will allow the VM to access the Internet, which will make some services work that didn’t work previously in the nix build sandbox. Therefore, some tests will pass that were failing previously.

Inside the REPL, you can type out the Python commands to test your module. For example:

>>> node1.wait_for_unit("hello-world-server")

Direct shell access

The API of the test driver gives you direct shell access with <yourmachine>.shell_interact(), so you can access the shell running inside the guest machine.

To try it out, let’s replace the placeholder with the name of the VM defined in the test — node1:

>>> node1.shell_interact()
node1: Terminal is ready (there is no initial prompt):
$ hostname


For complex modules, you may need to execute certain tests and only then inspect the virtual machine. In such case, you can use the breakpoint() function in your test script and run the test-driver without the --interactive flag:

# shortened example ./tests/hello-world-server.nix from above

(import ./lib.nix) {

 # ...

 testScript = ''


   output = node1.succeed("curl localhost:8000/index.html")

   # The test will stop at this line, giving you control over execution.


   assert "Hello world" in output, f"'{output}' does not contain 'Hello world'"


Here, we stopped the test flow and are looking at the value of output and checking the status of the module with systemctl.

$ nix build .#checks.x86_64-linux.hello-world-server.driver
$ ./result/bin/nixos-test-driver
>>> print(output)
>>> node1.execute("systemctl status hello-world-server")


In this article, we showed how you can interactively execute NixOS tests for easier troubleshooting and debugging. In short, you can do so using either the --interactive flag or breakpoints in your test script. In comparison to running tests in a sandbox, you can get immediate feedback and code completion, and look at the intermediate results.

By employing these techniques, you can improve the quality and reliability of your NixOS modules and ensure that they are functioning correctly.


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