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How-to: Set branch for build depending on whether it's a push or PR event

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Use case

You want to run the same build for both push and PR events, but for PR events you need to run the build on the target branch. During a PR triggered build CF_BRANCH points to the source branch, so you can't just use the exact same variable reference for both cases.

Solution:

You can set up some freestyle steps to conditionally choose either CF_BRANCH or CF_PULL_REQUEST_TARGET, depending on if this is a PR build or a push build. You can either export this branch name to a third variable or overwrite one of these existing branch variables, whichever is easier for your implementation.

For example, this step will overwrite the value of CF_BRANCH with that of CF_PULL_REQUEST_TARGET, if CF_PULL_REQUEST_TARGET exists (which would only be the case for a PR event). Then you should be able to just continue using CF_BRANCH in the rest of your build.

steps:
  set_branch:
    title: "Set branch variable"
    image: "alpine:latest"
    commands:
      - cf_export CF_BRANCH=${{CF_PULL_REQUEST_TARGET}}
    when:
      condition:
        all:
          whenPR: 'includes("${{CF_PULL_REQUEST_TARGET}}", "{{CF_PULL_REQUEST_TARGET}}") == false'

You could also do something in a similar vein to export to a third variable instead. This example exports the branch name to a custom variable BUILD_BRANCH depending on whether the current build is a PR build.

steps:
  set_push_branch:
    title: "Set branch variable for push"
    image: "alpine:latest"
    commands:
      - cf_export BUILD_BRANCH=${{CF_BRANCH}}
    when:
      condition:
        all:
          whenPR: 'includes("${{CF_PULL_REQUEST_TARGET}}", "{{CF_PULL_REQUEST_TARGET}}") == true'
  set_pr_branch:
    title: "Set branch variable for PR"
    image: "alpine:latest"
    commands:
      - cf_export BUILD_BRANCH=${{CF_PULL_REQUEST_TARGET}}
    when:
      condition:
        all:
          whenPR: 'includes("${{CF_PULL_REQUEST_TARGET}}", "{{CF_PULL_REQUEST_TARGET}}") == false'

Notes This condition works because if a variable is not set, it is simply replaced with the literal ${{}} variable reference string, so we are checking whether the reference is getting us the literal string or some other actual value.

Read more

How-to: Check environment variable value or existence in conditionals Documentation for cf_export command

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