Quick-start tutorial with LXD

In this tutorial, we will create our first cloud-init user data script and deploy it into an LXD container.

Why LXD?

We’ll be using LXD for this tutorial because it provides first class support for cloud-init user data, as well as systemd support. Because it is container based, it allows us to quickly test and iterate upon our user data definition.

How to use this tutorial

In this tutorial, the commands in each code block can be copied and pasted directly into the terminal. Omit the prompt ($) before each command, or use the “copy code” button on the right-hand side of the block, which will copy the command for you without the prompt.

Each code block is preceded by a description of what the command does, and followed by an example of the type of output you should expect to see.

Install and initialise LXD

If you already have LXD set up, you can skip this section. Otherwise, let’s install LXD:

$ sudo snap install lxd

If you don’t have snap, you can install LXD using one of the other installation options.

Now we need to initialise LXD. The minimal configuration will be enough for the purposes of this tutorial. If you need to, you can always change the configuration at a later time.

$ lxd init --minimal

Define our user data

Now that LXD is set up, we can define our user data. Create the following file on your local filesystem at /tmp/my-user-data:

  - echo 'Hello, World!' > /var/tmp/hello-world.txt

Here, we are defining our cloud-init user data in the #cloud-config format, using the runcmd module to define a command to run. When applied, it will write Hello, World! to /var/tmp/hello-world.txt (as we shall see later!).

Launch a LXD container with our user data

Now that we have LXD set up and our user data defined, we can launch an instance with our user data:

$ lxc launch ubuntu:focal my-test --config=user.user-data="$(cat /tmp/my-user-data)"

Verify that cloud-init ran successfully

After launching the container, we should be able to connect to our instance using:

$ lxc shell my-test

You should now be in a shell inside the LXD instance.

Before validating the user data, let’s wait for cloud-init to complete successfully:

$ cloud-init status --wait

Which provides the following output:

status: done

Verify our user data

Now we know that cloud-init has been successfully run, we can verify that it received the expected user data we provided earlier:

$ cloud-init query userdata

Which should print the following to the terminal window:

  - echo 'Hello, World!' > /var/tmp/hello-world.txt

We can also assert the user data we provided is a valid cloud-config:

$ cloud-init schema --system --annotate

Which should print the following:

Valid schema user-data

Finally, let us verify that our user data was applied successfully:

$ cat /var/tmp/hello-world.txt

Which should then print:

Hello, World!

We can see that cloud-init has received and consumed our user data successfully!

Tear down

Exit the container shell (by typing exit or pressing ctrl-d). Once we have exited the container, we can stop the container using:

$ lxc stop my-test

We can then remove the container completely using:

$ lxc rm my-test

What’s next?

In this tutorial, we used the runcmd module to execute a shell command. The full list of modules available can be found in our modules documentation. Each module contains examples of how to use it.

You can also head over to the examples page for examples of more common use cases.