Get Second Last commit SHA of a branch using GitHub Actions

Question

I am trying to create a simple rollback workflow that will get the previous SHA of 'main' branch and deploy a docker image with that tag. In my local machine this command works -

git rev-parse HEAD~1

And returns previous SHA. So I created a workflow around it to set output of a variable but now I am getting this error-

Run echo "prev_sha=$(git rev-parse HEAD~1)" >> "$GITHUB_OUTPUT"
fatal: ambiguous argument 'HEAD~1': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

This is the workflow -

on: workflow_dispatch

jobs:
  get_prev_sha:
    name: get previous sha of main
    runs-on: ubuntu-latest
    outputs:
      prev_sha: ${{ steps.prev_sha.outputs.prev_sha }}
    steps:      
      - name: Checkout
        uses: actions/checkout@v3
        with:
          ref: 'main'

      - name: Get last commit SHA
        id: prev_sha
        run: |
          echo "prev_sha=$(git rev-parse HEAD~1)" >> "$GITHUB_OUTPUT"

My only question is, how do I get the previous (2nd last) hash of a branch using GitHub Actions and set is as output? Any help is appreciated.

Answer 1

According to the description of actions/checkout@v4:

Only a single commit is fetched by default, for the ref/SHA that triggered the workflow. Set fetch-depth: 0 to fetch all history for all branches and tags.

and, here's the usage of fetch-depth:

# Number of commits to fetch. 0 indicates all history for all branches and tags.
# Default: 1
fetch-depth: ''

So, you can use fetch-depth to fetch the last two commits only i.e. fetch-depth: 2 and git rev-parse HEAD~1 should return the correct commit hash.