Getting base branch SHA on pull request in Github Action Workflow

Question

In GitHub action on pull request, I need to run some code in the context of the "current master", and later re-run the same code in the context of the PR branch.

I can check out compare a pull request to the base it is being PR-ed against. How would I find the SHA of the base branch (e.g. current master if PR is against the master)?

jobs:
  job_on_base:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with:
          ref: "${{ github.base_ref }}"
      - run: |
          # Seems like I can get it here with $(git log -1 --format="%H")
          echo "My current SHA is ... ?"

  job_on_pr:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with:
          ref: "${{ github.ref }}"
      - run: |
          echo "My current SHA is $GITHUB_SHA"
          echo "The BASE SHA is ?"

score 14 · Answer 1 · answered Jun 21 '20 at 19:09

14

If the job runs on a pull_request event the base sha is available as ${{ github.event.pull_request.base.sha }}

answered Jun 21 '20 at 19:09

Jacob Rask

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9

and if anyone is looking here for branch's sha it's: `${{github.event.pull_request.head.sha}}` https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit – Danny Varod Dec 14 '21 at 12:32

score 6 · Answer 2 · answered Aug 29 '22 at 21:35

What worked for me was: github.event.workflow_run.head_sha.

The top-voted answer suggesting github.event.pull_request.head.sha didn't work for me.

I found this by examining all the possible data in the github.context object using the method suggested here - https://stackoverflow.com/a/70107953

score 3 · Answer 3 · answered Apr 24 '20 at 06:12

This turned out to be a git question, rather than Github actions. The actions/checkout@v2 creates a shallow --depth=1 clone, so to get PR's parent one can parse git cat-file -p output as described here. The first (base) parent could be accessed with

git cat-file -p <SHA> | awk 'NR > 1 {if(/^parent/){print $2; exit}}'

The better approach turned out to be using fetch-depth: 2 parameter. It allows just one job to handle both pull request and master merge cases, and can also be used with HEAD^1 to get to the parent.

steps:
  - uses: actions/checkout@v2
    with:
      fetch-depth: 2

Getting base branch SHA on pull request in Github Action Workflow

3 Answers3