Complex Pipelines are hard to write and maintain within the text area of the
Pipeline configuration page. To make this easier, Pipeline can also be written
in a text editor and checked into source control as a
Jenkins can load via the Pipeline Script from SCM option.
To do this, select Pipeline script from SCM when defining the Pipeline.
With the Pipeline script from SCM option selected, you do not enter any Groovy code in the Jenkins UI; you just indicate by specifying a path where in source code you want to retrieve the pipeline from. When you update the designated repository, a new build is triggered, as long as the Pipeline is configured with an SCM polling trigger.
The first line of a