OpenStackCLI: Access `--property`'s from cloudinit script

Using the OpenStack CLI, I am creating server instances and need to pass custom properties (--property) into the init script referenced by the --user-data parameter.

So, my invocation looks something like this:

openstack server create myServer \
  ...other args...
  --user-data ./initScript.sh \
  --property "foo=bar"

In initScript.sh how do I get access to the foo property and it's value?

Elements defined with --property go into the meta element of the metadata. [source]

If you run

curl http://169.254.169.254/openstack/2018-08-27/meta_data.json

You'll get

{
   "hostname": "test.novalocal",
   …
   "meta": {
      "foo": "bar",
      …
   },
   …
}

And to parse out the value in a shell script, you can use for example:

$ curl http://169.254.169.254/openstack/2018-08-27/meta_data.json \
    | jq -r '.meta.foo'
bar

See documentation for the Metadata Service for more information on the format and various methods available to retrieve this information.