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

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 th 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