Reusable CircleCI command to halt a job if target files are not changed

circleci_config_commands.yml

# .circleci/config.yml
commands:
  check-diff-files-to-halt:
    description: "halt CircleCI step if there is any files matched with a pattern"
    parameters:
      pattern:
        description: "regular expression to filter changed files"
        type: string
    steps:
      - run: |
          # `|| :` is used to avoid exit when there is no changed file which matches the pattern.
          changed_files=$(bash ./ci/get_changed_files.sh "<< parameters.pattern >>" || :)
          num_changed_files=$(echo "$changed_files" | grep -v -e '^\s*$' | wc -l || :)
          if [[ $num_changed_files -eq 0 ]] ; then
            echo "Halt due to no changed file"
            circleci step halt
          else
            echo "Changed files match the pattern << parameters.pattern >>"
            echo $changed_files
          fi

circleci_config_jobs.yml

jobs:
  test-basics:
    docker:
      - image: python3.8
    steps:
      - checkout
      # Halt the job, if any file in `./src/` is not changed
      - check-diff-files-to-halt:
          pattern: "^src/"
      - run:
          name: Run unit tests
          command: pytest -v -s ./src/tests/