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

**URL:** <https://discuss.circleci.com/t/how-to-retrieve-a-pull-requests-base-branch-name-github/36911>\
**Category:** Tips, Tricks, and Hacks\
**Tags:** pull-requests, github, circle-yml, configuration\
**Created:** [July 31, 2020, 8:37pm UTC](https://discuss.circleci.com/t/how-to-retrieve-a-pull-requests-base-branch-name-github/36911 "2020-07-31T20:37:32Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Jaryt](https://sea2.discourse-cdn.com/circleci/user_avatar/discuss.circleci.com/jaryt/32/24066_2.png) [@Jaryt](https://discuss.circleci.com/u/Jaryt)\
**Post date:** [July 31, 2020, 8:37pm UTC](https://discuss.circleci.com/t/how-to-retrieve-a-pull-requests-base-branch-name-github/36911/1 "2020-07-31T20:37:32Z")

</div>

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`.

```auto
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`:

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

```

**Example usage** :

```auto
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.