Get commit SHA in Github actions

In a Github action you can get the commit SHA using ${GITHUB_SHA}, which is a default env variable.. However, this commit SHA seems to be a merge commit!? which does not equal the commit SHA displayed on PR's Github UI. Any thoughts on how I can get the SHA that is displayed in PRs (on Github UI)?

To understand what happens

Reference: Github community post with the weide-zhou (Github Partner) answer.

When you can create a pull request, github will execute workflow based on a fake merge branch: refs/pull/:prNumber/merge, the merge_commit_sha doesn’t exist on base or head branch, but points to that surrogate merge commit, and there is a mergeable key to show the status of the test commit.

Therefore, here, the github.sha stands for the actual merge commit.

Github Variables

Tip: you can print the GitHub variables using the following step:

- name: Show GitHub context
  env:
    GITHUB_CONTEXT: ${{ toJson(github) }}
  run: echo "$GITHUB_CONTEXT"

It seems that what you want here is the ${{ github.event.pull_request.head.sha }} value.

In the case of pull_request, the hash of the latest commit can be found in the ${{ github.event.pull_request.head.sha }} variable, whereas ${{ github.sha }} refers to the PR merge commit.

Note that if the pull_request has been opened for a fork repo, the github.event.pull_request variable will be empty (don't know if it's a bug or something they are working on).