Retrieve Previous successful build SHA in pipelines

[federico]

Hello,

I am using NX in my angular projected, and I would like to deploy/test only affected items during my PRs.

I created the following pipeline :


ds

# This is an example Starter pipeline configuration
# Use a skeleton to build, test and deploy using manual and parallel steps
# -----
# You can specify a custom docker image from Docker Hub as your build environment.
image: node:12

definitions:
  caches:
   bundler: vendor/bundle

pipelines:
  pull-requests:
    - step: 
      name: 'Installation'
      caches:
        - node
      script:
        - npm install
      artifacts:
        - node_modules/**

    - parallel:
       - step:
         name: 'Test'
         script:
           - npm run affected:test -- --base=origin/develop
       - step:
         name: 'Lint'
         script:
           - npm run affected:lint -- --base=origin/develop
       - step:
         name: 'Stylelint'
         script:
           - npm run affected:stylelint -- --base=origin/develop

   branches:
     develop:
       -step:
          name: 'Deploy Staging'
          deployment: staging
          script:
            - npm run affected:build -- --base=origin/develop

Now, the last line don't work because, when the pipeline execute, the branch is already merged in develop so

 npm run affected:build -- --base=origin/develop

 Will do nothing as it is comparing to itself.

I would like to replace the --base=origin/develop with --base=$previousBuildSHA. But I am quite confused on how to retrieve it.