# Writing Tests {#sec-writing-nixos-tests}
A NixOS test is a Nix expression that has the following structure:
```nix
import ./make-test-python.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 =
''
Python code…
'';
}
```
The attribute `testScript` is a bit of Python 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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/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/simple.nix`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/nfs/simple.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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/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`](https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/qemu-vm.nix).
The test script is a sequence of Python 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` if this is also the identifier of the machine in the declarative
config. If you didn\'t specify multiple machines using the `nodes`
attribute, it is just `machine`. The following example starts the
machine, waits until it has finished booting, then executes a command
and checks that the output is more-or-less correct:
```py
machine.start()
machine.wait_for_unit("default.target")
if not "Linux" in machine.succeed("uname"):
raise Exception("Wrong OS")
```
The first line is actually unnecessary; machines are implicitly started
when you first execute an action on them (such as `wait_for_unit` or
`succeed`). If you have multiple machines, you can speed up the test by
starting them in parallel:
```py
start_all()
```
## Machine objects {#ssec-machine-objects}
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.
`get_screen_text_variants`
: Return a list of different interpretations of what is currently
visible on the machine\'s screen using optical character
recognition. The number and order of the interpretations is not
specified and is subject to change, but if no exception is raised at
least one will be returned.
::: {.note}
This requires passing `enableOCR` to the test attribute set.
:::
`get_screen_text`
: Return a textual representation of what is currently visible on the
machine\'s screen using optical character recognition.
::: {.note}
This requires passing `enableOCR` to the test attribute set.
:::
`send_monitor_command`
: 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.
`send_key`
: Simulate pressing keys on the virtual keyboard, e.g.,
`send_key("ctrl-alt-delete")`.
`send_chars`
: Simulate typing a sequence of characters on the virtual keyboard,
e.g., `send_chars("foobar\n")` will type the string `foobar`
followed by the Enter key.
`execute`
: Execute a shell command, returning a list `(status, stdout)`.
If the command detaches, it must close stdout, as `execute` will wait
for this to consume all output reliably. This can be achieved by
redirecting stdout to stderr `>&2`, to `/dev/console`, `/dev/null` or
a file. Examples of detaching commands are `sleep 365d &`, where the
shell forks a new process that can write to stdout and `xclip -i`, where
the `xclip` command itself forks without closing stdout.
Takes an optional parameter `check_return` that defaults to `True`.
Setting this parameter to `False` will not check for the return code
and return -1 instead. This can be used for commands that shut down
the VM and would therefore break the pipe that would be used for
retrieving the return code.
`succeed`
: Execute a shell command, raising an exception if the exit status is
not zero, otherwise returning the standard output. Commands are run
with `set -euo pipefail` set:
- If several commands are separated by `;` and one fails, the
command as a whole will fail.
- For pipelines, the last non-zero exit status will be returned
(if there is one, zero will be returned otherwise).
- Dereferencing unset variables fail the command.
- It will wait for stdout to be closed. See `execute` for the
implications.
`fail`
: Like `succeed`, but raising an exception if the command returns a zero
status.
`wait_until_succeeds`
: Repeat a shell command with 1-second intervals until it succeeds.
`wait_until_fails`
: Repeat a shell command with 1-second intervals until it fails.
`wait_for_unit`
: Wait until the specified systemd unit has reached the "active"
state.
`wait_for_file`
: Wait until the specified file exists.
`wait_for_open_port`
: Wait until a process is listening on the given TCP port (on
`localhost`, at least).
`wait_for_closed_port`
: Wait until nobody is listening on the given TCP port.
`wait_for_x`
: Wait until the X11 server is accepting connections.
`wait_for_text`
: Wait until the supplied regular expressions matches the textual
contents of the screen by using optical character recognition (see
`get_screen_text` and `get_screen_text_variants`).
::: {.note}
This requires passing `enableOCR` to the test attribute set.
:::
`wait_for_console_text`
: Wait until the supplied regular expressions match a line of the
serial console output. This method is useful when OCR is not
possibile or accurate enough.
`wait_for_window`
: Wait until an X11 window has appeared whose name matches the given
regular expression, e.g., `wait_for_window("Terminal")`.
`copy_from_host`
: Copies a file from host to machine, e.g.,
`copy_from_host("myfile", "/etc/my/important/file")`.
The first argument is the file on the host. The file needs to be
accessible while building the nix derivation. The second argument is
the location of the file on the machine.
`systemctl`
: Runs `systemctl` commands with optional support for
`systemctl --user`
```py
machine.systemctl("list-jobs --no-pager") # runs `systemctl list-jobs --no-pager`
machine.systemctl("list-jobs --no-pager", "any-user") # spawns a shell for `any-user` and runs `systemctl --user list-jobs --no-pager`
```
`shell_interact`
: Allows you to directly interact with the guest shell. This should
only be used during test development, not in production tests.
Killing the interactive session with `Ctrl-d` or `Ctrl-c` also ends
the guest session.
To test user units declared by `systemd.user.services` the optional
`user` argument can be used:
```py
machine.start()
machine.wait_for_x()
machine.wait_for_unit("xautolock.service", "x-session-user")
```
This applies to `systemctl`, `get_unit_info`, `wait_for_unit`,
`start_job` and `stop_job`.
For faster dev cycles it\'s also possible to disable the code-linters
(this shouldn\'t be commited though):
```nix
import ./make-test-python.nix {
skipLint = true;
machine =
{ config, pkgs, ... }:
{ configuration…
};
testScript =
''
Python code…
'';
}
```
This will produce a Nix warning at evaluation time. To fully disable the
linter, wrap the test script in comment directives to disable the Black
linter directly (again, don\'t commit this within the Nixpkgs
repository):
```nix
testScript =
''
# fmt: off
Python code…
# fmt: on
'';
```
## Failing tests early {#ssec-failing-tests-early}
To fail tests early when certain invariables are no longer met (instead of waiting for the build to time out), the decorator `polling_condition` is provided. For example, if we are testing a program `foo` that should not quit after being started, we might write the following:
```py
@polling_condition
def foo_running():
machine.succeed("pgrep -x foo")
machine.succeed("foo --start")
machine.wait_until_succeeds("pgrep -x foo")
with foo_running:
... # Put `foo` through its paces
```
`polling_condition` takes the following (optional) arguments:
`seconds_interval`
:
specifies how often the condition should be polled:
```py
@polling_condition(seconds_interval=10)
def foo_running():
machine.succeed("pgrep -x foo")
```
`description`
:
is used in the log when the condition is checked. If this is not provided, the description is pulled from the docstring of the function. These two are therefore equivalent:
```py
@polling_condition
def foo_running():
"check that foo is running"
machine.succeed("pgrep -x foo")
```
```py
@polling_condition(description="check that foo is running")
def foo_running():
machine.succeed("pgrep -x foo")
```