Do stuff only in a main repo and not forks

base-check.yml

on: push
name: Workflow
jobs:
  work:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@master
      - id: base_check
        name: check if repo is base
        run: |
          if [ "$GITHUB_REPOSITORY" = "owner/repo" ]; then
            echo "::set-output name=base::true"
          fi
      - name: do stuff
        if: steps.base_check.outputs.base == 'true'
        run: echo 'I am not a fork!'
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}