How to Retrieve a Pull Request's Base Branch Name [GitHub]

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Hello everyone!

If you are doing custom configuration with pull requests, you may run into a case where you need access to the name of the pull request’s base branch. Currently, this is not provided within a CircleCI build but is easily accessible through the GitHub API.

However, within PR builds, we have access to two extra environment variables called CIRCLE_PULL_REQUEST and CIRCLE_PULL_REQUESTS. The first is a link to the main pull request and the second is a list of pull request URLs. We only need CIRCLE_PULL_REQUEST - which we will transform into a GitHub API URL.

We need to mutate https://github.com/<org-name>/<project>/pull/<pr-number> to https://api.github.com/repos/<org-name>/<project>/pulls/<pr-number>. There are many ways this can be done, but we will trim the first 19 characters (https://github.com/) and append the result to https://api.github.com/repos/. After I run it through a sed substitution and replace /pull/ with /pulls/. This could theoretically cause an issue if your organization or project name is pull.

echo https://api.github.com/repos/${CIRCLE_PULL_REQUEST:19} | sed "s/\/pull\//\/pulls\//"

Set an environment variable called GITHUB_TOKEN with your GitHub authorization token. After which we can make the cURL request and retrieve the pull requests base name through jq:

curl -s -H "Authorization: token ${GITHUB_TOKEN}" $pr | jq '.base.ref'

Example usage:

version: 2.1

commands:
  check-pr:
    description: "Run API depending on branch"
    parameters:
      head:
        type: string
      base:
        type: string
      run:
        type: string
    steps:
      - when: 
          condition:
            equal: [ << parameters.head >>, << pipeline.git.branch >> ]
          steps: 
            - run:
                name: Check Pull Request from << parameters.head >> to << parameters.base >>
                command: |
                  if [ -n $CIRCLE_PULL_REQUEST ]; then
                    pr=$(echo https://api.github.com/repos/${CIRCLE_PULL_REQUEST:19} | sed "s/\/pull\//\/pulls\//")

                    base=$(curl -s -H "Authorization: token ${GITHUB_TOKEN}" $pr | jq '.base.ref')

                    if [[ '"<< parameters.base >>"' = "$base" ]]; then
                      << parameters.run >>
                    fi
                  fi

jobs:
  test-dev:
    docker:
    - image: circleci/node:latest
    steps:
      - check-pr:
          base: staging
          head: dev
          run: |
              echo "hello world!"
              # implement features here!

workflows:
  test:
    jobs:
      - test-dev:

Limitations:

  • Ensure that your image has jq installed.
  • If you are trying to catch when a pull request is opened, you must have the Only build pull requests
    feature enabled. If you have this disabled, opening a new pull request will not trigger a new build, because that commit has already been completed.
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