A GitHub Action for establishing which HCL directories have been modified, setting up Packer, and running packer fmt against the templates

packer_fmt.yml

name: Packer Format
on:
  pull_request:
    types: [opened, synchronize]
    paths:
      - '**/*'

permissions:
  contents: read
  pull-requests: write

env:
  PRODUCT_VERSION: "1.9.4"

jobs:
  find_hcl:
    runs-on: ubuntu-latest
    if: github.event.pull_request.state == 'open'
    outputs:
      run_fmt: ${{ steps.find-modified-hcl.outputs.run_fmt }}
      hcl_dirs: ${{ steps.find-modified-hcl.outputs.hcl_dirs }}
    steps:
      - name: Checkout
        uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Find modified HCL
        id: find-modified-hcl
        shell: bash -x -e -o pipefail {0}
        env:
          BASE_REF: ${{ github.base_ref }}
          HEAD_REF: ${{ github.head_ref }}
        run: |
          MODIFIED_FILES=($(git diff --name-only origin/${BASE_REF} origin/${HEAD_REF}))
          [[ -n "$MODIFIED_FILES" ]] && MODIFIED_DIRS=($(xargs -n 1 dirname <<< "${MODIFIED_FILES[@]}" | sort | uniq)) || true
          [[ -n "$MODIFIED_DIRS" ]] && MODIFIED_HCL_DIRS=($(for f in "${MODIFIED_FILES[@]}"; do [[ "$f" == *.hcl ]] && dirname "$f" || true; done | sort | uniq)) || true

          if [[ -z "$MODIFIED_HCL_DIRS" ]]; then
            echo "No HCL files changed in this PR. Skipping Packer fmt."
            echo "run_fmt=false" >> $GITHUB_OUTPUT
            echo "hcl_dirs=[]" >> $GITHUB_OUTPUT
          else
            echo "Running checks on the following Packer components:"
            printf -- "- %s\n" "${MODIFIED_HCL_DIRS[@]}"
            echo "run_fmt=true" >> $GITHUB_OUTPUT
            echo "hcl_dirs=$(IFS=,; printf '["%s"]\n' "${MODIFIED_HCL_DIRS[*]}" | sed 's/,/", "/g')" >> $GITHUB_OUTPUT
          fi

  packer:
    runs-on: ubuntu-latest
    needs: find_hcl
    if: github.event.pull_request.state == 'open' && needs.find_hcl.outputs.run_fmt == 'true'
    strategy:
      fail-fast: false
      matrix:
        dir: ${{ fromJson(needs.find_hcl.outputs.hcl_dirs) }}

    steps:
      - name: Checkout
        uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Setup `packer`
        uses: hashicorp/setup-packer@main
        id: setup
        with:
          version: ${{ env.PRODUCT_VERSION }}

      - name: Run `packer fmt`
        id: packer-fmt
        run: |
          hcl_dirs="${{ matrix.dir }}"
          for dir in $hcl_dirs; do
            cd $dir
            packer fmt .
          done