How to process gitlab-like reference tags with yq?

[sigurdblueface]

I have a following yaml file, utilizing format somewhat similar to reference tags in gitlab.

ports:
  - name: http
    port: 8080
    hosts:
      - hostname: "!reference[.placeholders, ingressHostname]"
...
placeholders:
  ingressHostname: xyz.com

My goal is to substitute these reference tags with values from placeholders like that.

ports:
  - name: http
    port: 8080
    hosts:
      - hostname: xyz.com

Couldn't find something on this in documentation.

[mikefarah]

You can use the eval operator to dynamically evaluate expressions - it gets a little complex for what you're trying to do, but if I change the format to this:

ports:
  - name: http
    port: 8080
    hosts:
      - hostname: !!reference .placeholders.ingressHostname
placeholders:
  ingressHostname: xyz.com

Where !!reference is the custom tag; and the value is a valid yq expression, then you can:

yq '. as $i | (.. | select(tag == "!!reference")) |= (. as $x | $i | eval($x)) | del(.placeholders)' data.yaml

and get

ports:
  - name: http
    port: 8080
    hosts:
      - hostname: xyz.com

Explanation:

• . as $i saves a pointer to the root doc, we'll need this later
• (.. | select(tag == "!!reference")) matches all the nodes with tag !!reference
• This is where it gets a little tricky. We want to call eval against the root document, passing in the current value of the node as the expression. . as $x saves the current value as $x, then we pipe $i into eval, with $x as the param
• Finally, we can delete 'placeholders'