RFE: Allow ignition_file to append to a file

[nhi-vanye]

It would be nice to allow appending content to files rather than over-writing the existing content

The use case is to add environment variables to /etc/profile.env to set the HTTP_PROXY variable so the newly deployed OS can access the internet.

[IvanovOleg]

Ignition works only during the first VM startup. That's why even you change the content of the file and suppress the recreation of the VM (due to the custom data change) you will never get an expected result.

[captn3m0]

The current ignition version (since 2.2.0) supports appending to files:

append (boolean): whether to append to the specified file. Creates a new file if nothing exists at the path. Cannot be set if overwrite is set to true.

Spec: https://coreos.com/ignition/docs/latest/configuration-v2_3.html