GitLab rules:

https://docs.gitlab.com/ee/ci/yaml/#rules

myjob:
   rules:
       - if: $CI_MERGE_REQUEST_ID
         when: always
       - when: never
   script:
       - echo "Test"

Official example (https://docs.gitlab.com/ee/ci/yaml/#rules):

docker build:
  script: docker build -t my-image:$CI_COMMIT_REF_SLUG .
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
      changes:
        - Dockerfile
      when: manual
      allow_failure: true

!= 
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See also

• GitLab CI, variables, templates, executors, .gitlab-ci.yml, .gitlab/, Credential helpers, notifications, pipeline editor, Keyword reference, Changelog, ContainersNotReady, GitLab environments