Example code to skip tests if source trees are the same.

config.yml

version: 2.1

# this allows you to use CircleCI's dynamic configuration feature
setup: true

parameters:
  tree_sha_path:
    type: string
    default: ".git_tree_sha"
  tree_status_path:
    type: string
    default: ".gh_tree_status.json"

# the continuation orb is required in order to use dynamic configuration
orbs:
  continuation: circleci/continuation@1.0.0
  gh: circleci/github-cli@2.2.0

commands:
  save_previous_tree_status:
    steps:
      - run:
          name: Save previous tree sha
          command: git rev-parse @^^{tree} > << pipeline.parameters.tree_sha_path >>
      # An access token with `statuses:read` permission must be set in the GITHUB_TOKEN environment variable.
      - run:
          name: Save previous commit status as tree status
          command: gh api "/repos/{owner}/{repo}/commits/$(git rev-parse @^)/status" > << pipeline.parameters.tree_status_path >>
      - save_cache:
          key: tree_status_{{ checksum "<< pipeline.parameters.tree_sha_path >>" }}
          paths:
            - << pipeline.parameters.tree_status_path >>
      - run:
          name: Remove previous files
          command: rm << pipeline.parameters.tree_sha_path >> << pipeline.parameters.tree_status_path >>
  restore_current_tree_status:
    steps:
      - run:
          name: Save current tree sha
          command: git rev-parse @^{tree} > << pipeline.parameters.tree_sha_path >>
      - restore_cache:
          key: tree_status_{{ checksum "<< pipeline.parameters.tree_sha_path >>" }}
      - run:
          name: Put empty file if tree status does not exist
          command: |
            if [ ! -e "<< pipeline.parameters.tree_status_path >>" ]; then
              touch << pipeline.parameters.tree_status_path >>
            fi

# our defined job, and its steps
jobs:
  setup:
    executor: continuation/default
    steps:
      - checkout # checkout code
      - gh/install
      - save_previous_tree_status
      - restore_current_tree_status

      - run:
          name: Embed previous status
          command: |
            PREVIOUS_STATE=$(cat << pipeline.parameters.tree_status_path >> | jq -r '.statuses[] | select(.context == "ci/circleci: test") | .state')
            sed -i "s/_PREVIOUS_STATE/$PREVIOUS_STATE/" .circleci/continue_config.yml
      - run: cat .circleci/continue_config.yml

      - continuation/continue:
          configuration_path: .circleci/continue_config.yml

# our single workflow, that triggers the setup job defined above
workflows:
  setup:
    jobs:
      - setup

continue_config.yml

version: 2.1

parameters:
  previous_state:
    type: string
    default: "_PREVIOUS_STATE"

jobs:
  test:
    docker:
      - image: cimg/base:stable
    steps:
      - checkout
      - unless:
          condition:
            equal: [ success, << pipeline.parameters.previous_state >> ]
          steps:
            - run: echo "Run tests"
      - when:
          condition:
            equal: [ success, << pipeline.parameters.previous_state >> ]
          steps:
            - run: echo "Skip tests"

workflows:
  main:
    jobs:
      - test