Example from markdownlint-cli-action of linting only changed files.

lint-changed-action.yml

# source: https://github.com/DavidAnson/markdownlint-cli2-action/blob/main/.github/workflows/changed.yml
on: [push, pull_request]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v4
      with:
        fetch-depth: 0
    - uses: tj-actions/changed-files@v45
      id: changed-files
      with:
        files: '**/*.md'
        separator: ","
    - uses: DavidAnson/markdownlint-cli2-action@v17
      if: steps.changed-files.outputs.any_changed == 'true'
      with:
        globs: ${{ steps.changed-files.outputs.all_changed_files }}
        separator: ","