A NixOS test is a Nix expression that has the following structure: import ./make-test.nix { # Either the configuration of a single machine: machine = { config, pkgs, ... }: { configuration… }; # Or a set of machines: nodes = { machine1 = { config, pkgs, ... }: { … }; machine2 = { config, pkgs, ... }: { … }; … }; testScript = '' Perl code… ''; } The attribute testScript is a bit of Perl code that executes the test (described below). During the test, it will start one or more virtual machines, the configuration of which is described by the attribute machine (if you need only one machine in your test) or by the attribute nodes (if you need multiple machines). For instance, login.nix only needs a single machine to test whether users can log in on the virtual console, whether device ownership is correctly maintained when switching between consoles, and so on. On the other hand, nfs.nix, which tests NFS client and server functionality in the Linux kernel (including whether locks are maintained across server crashes), requires three machines: a server and two clients. There are a few special NixOS configuration options for test VMs: virtualisation.memorySize The memory of the VM in megabytes. virtualisation.vlans The virtual networks to which the VM is connected. See nat.nix for an example. virtualisation.writableStore By default, the Nix store in the VM is not writable. If you enable this option, a writable union file system is mounted on top of the Nix store to make it appear writable. This is necessary for tests that run Nix operations that modify the store. For more options, see the module qemu-vm.nix. The test script is a sequence of Perl statements that perform various actions, such as starting VMs, executing commands in the VMs, and so on. Each virtual machine is represented as an object stored in the variable $name, where name is the identifier of the machine (which is just machine if you didn’t specify multiple machines using the nodes attribute). For instance, the following starts the machine, waits until it has finished booting, then executes a command and checks that the output is more-or-less correct: $machine->start; $machine->waitForUnit("default.target"); $machine->succeed("uname") =~ /Linux/; The first line is actually unnecessary; machines are implicitly started when you first execute an action on them (such as waitForUnit or succeed). If you have multiple machines, you can speed up the test by starting them in parallel: startAll; The following methods are available on machine objects: start Start the virtual machine. This method is asynchronous — it does not wait for the machine to finish booting. shutdown Shut down the machine, waiting for the VM to exit. crash Simulate a sudden power failure, by telling the VM to exit immediately. block Simulate unplugging the Ethernet cable that connects the machine to the other machines. unblock Undo the effect of block. screenshot Take a picture of the display of the virtual machine, in PNG format. The screenshot is linked from the HTML log. sendMonitorCommand Send a command to the QEMU monitor. This is rarely used, but allows doing stuff such as attaching virtual USB disks to a running machine. sendKeys Simulate pressing keys on the virtual keyboard, e.g., sendKeys("ctrl-alt-delete"). sendChars Simulate typing a sequence of characters on the virtual keyboard, e.g., sendKeys("foobar\n") will type the string foobar followed by the Enter key. execute Execute a shell command, returning a list (status, stdout). succeed Execute a shell command, raising an exception if the exit status is not zero, otherwise returning the standard output. fail Like succeed, but raising an exception if the command returns a zero status. waitUntilSucceeds Repeat a shell command with 1-second intervals until it succeeds. waitUntilFails Repeat a shell command with 1-second intervals until it fails. waitForUnit Wait until the specified systemd unit has reached the “active” state. waitForFile Wait until the specified file exists. waitForOpenPort Wait until a process is listening on the given TCP port (on localhost, at least). waitForClosedPort Wait until nobody is listening on the given TCP port. waitForX Wait until the X11 server is accepting connections. waitForWindow Wait until an X11 window has appeared whose name matches the given regular expression, e.g., waitForWindow(qr/Terminal/).