Cloud build: substitutions in `substitutions` section?

Question

2019-07-04: Update

Ended up using

options:
   env:

Not perfect since doesn't allow variable in build steps but atleast environments are covered.

Problem

GCP's Cloud build is not substituting variables in substitutions section.

Is there a way to have those substitutions applied?

Example snippet

substitutions:
  _HUGO_VERSION: "0.55.6"
  _HUGO_IMG: gcr.io/$PROJECT_ID/hugo:$_HUGO_VERSION

Expectation

To have $_HUGO_VERSION and $PROJECT_ID replaced with values, to get:

_HUGO_IMG=gcr.io/foo/hugo:0.55.6

Actual

But the value for _HUGO_IMG is taken literally:

_HUGO_IMG=gcr.io/$PROJECT_ID/hugo:$_HUGO_VERSION

user_5144299 · Answer

Your expected behavior is now supported. Please see the documentation: Using payload bindings and bash-style string operations in substitutions

options:
    dynamic_substitutions: true
substitutions:
    _HUGO_IMG: gcr.io/${PROJECT_ID}/hugo:${_HUGO_VERSION}

user_10615860 · Answer

Thanks for posting this!

Could you post the full config, when I try this I get an error for using = instead of : when declaring the substitutions.

The behavior I see instead is:

hugo version: 0.55.6
hugo img: gcr.io//hugo:

To interpolate default or custom substitutions, I suggest using env variables instead

steps:
- id: 'checking sub values'
  name: 'gcr.io/cloud-builders/docker'
  entrypoint: bash
  args:
  - '-c'
  - |
    echo 'hugo version: '${_HUGO_VERSION} # hugo version: 0.55.6
    echo 'hugo img: '${_HUGO_IMG} # hugo img: gcr.io//hugo:
    echo 'env hugo img: '$$HUGO_IMG # env hugo img: 'gcr.io/my-project/hugo:0.55.6'
substitutions:
    _HUGO_VERSION: "0.55.6"
    _HUGO_IMG: 'gcr.io/$PROJECT_ID/hugo:$_HUGO_VERSION'
options:
    env:
    - HUGO_IMG='gcr.io/$PROJECT_ID/hugo:$_HUGO_VERSION'

Note that env vars need to be used with $$