concourse/tasks/get-last-stable-tag.yaml

---
platform: linux

image_resource:
  type: registry-image
  source:
    repository: google/cloud-sdk
    tag: alpine

inputs:
- name: repo

outputs:
- name: last-stable-tag

run:
  path: sh
  args:
  - -exc
  - |
    commit=$(cat repo/.git/refs/tags/stable)
    date=$(cd repo/.git/refs/tags; grep -l $commit stable-2*)
    [[ -z $date ]] && exit 1
    echo $date | sed 's/\..*//' | sed 's/stable-//' | tee last-stable-tag/date
    echo stable-`date +%Y%m%d` | tee last-stable-tag/stable-today
    echo stable > last-stable-tag/stable