差分チェックスクリプト

scripts/circleci/is_changed_directory.sh

#!/bin/bash
# See: https://blog.hatappi.me/entry/2018/10/08/232625

if [ "${CIRCLE_BRANCH}" = "master" ]; then
  DIFF_TARGET="HEAD^ HEAD"
else
  DIFF_TARGET="origin/master"
fi

DIFF_FILES=(`git diff ${DIFF_TARGET} --name-only --relative=${1}`)

if [ ${#DIFF_FILES[@]} -eq 0 ]; then
  exit 1
else
  exit 0
fi

`.circleci/config.yml`

..circleci/config.yml

script_setup: &script_setup
  command: |
    echo 'export PATH=$PATH:${CIRCLE_WORKING_DIRECTORY}/scripts/circleci' >> $BASH_ENV

jobs:
  sample_job:
    steps:
      - checkout
      - run:
          <<: *script_setup
      - run:
          name: 後続の job 実行継続判定
          command: |
            if ! is_changed_directory.sh "foo/bar" -a "${CIRCLE_BRANCH}" != "master"; then
              echo "foo/bar に変更がありません"
              circleci step halt
            fi

参考

CircleCIで特定のディレクトリ配下にコミットされた時にjobを実行したい
CircleCI | Orbs FAQ Failing Builds