How to export a pipeline using REST api?

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All of the Data Collector APIs are accessible from the UI by going to Help / RESTful API. Scroll down to the store section and you'll see GET /v1/pipeline/{pipelineId}/export. You can supply a pipeline id there to try it out, or, from curl:

curl -u admin:admin http://localhost:18630/rest/v1/pipeline/{pipelineId}/export -H "X-Requested-By:sdc"

You'll receive the JSON pipeline in response.