Driver Log Levels

The NixOS test driver emits a lot of output by default: progress lines for every driver action, informational notes, warnings, and error messages. During development that is useful, but in CI — or when you just want to see what actually went wrong — the noise can bury the relevant lines.

This feature has been recently upstreamed

Since PR #509867, the driver supports a configurable log level.

If the attribute logLevel doesn't exist in your NixOS version, please upgrade to the latest nixpkgs.

Available Levels

The driver distinguishes three levels. Each level includes itself and every more severe level above it:

Level	Includes	Typical use
info	info, warning, error	Default. Good for local development.
warning	warning, error	Quieter runs; hides routine progress messages.
error	error only	Minimal output; only failure-relevant lines.

Serial console output from the VMs is controlled separately and is not affected by the log level.

Setting the Log Level

In a test definition

Use the top-level logLevel option in your test. It is documented in the NixOS manual.

test.nix

{
  name = "example-test";

  logLevel = "warning";

  nodes.machine = { ... }: {
    # configuration
  };

  testScript = ''
    start_all()
  '';
}

The default is "info".

On the command line

The interactive driver accepts --log-level <level>:

./result/bin/nixos-test-driver --log-level warning

This is handy when you want to temporarily quieten a run without editing the test or need more debug output.

When to Use Which Level

• info (default)

  ---

  Keep the default while you are developing a test. Progress messages make it obvious where the driver is spending time.

• warning in CI

  ---

  Trim routine progress lines so CI logs stay focused on problems. Warnings and errors are still surfaced.

• error for minimal output

  ---

  Useful for large test matrices where you only want to see which tests failed and why.