Jenkins multibranch pipeline Scan without execution

Also, you can do it programatically

import jenkins.branch.*
import jenkins.model.Jenkins


for (f in Jenkins.instance.getAllItems(jenkins.branch.MultiBranchProject.class)) {
  if (f.parent instanceof jenkins.branch.OrganizationFolder) {
    continue;
  }
  for (s in f.sources) {
    def prop = new jenkins.branch.NoTriggerBranchProperty();
    def propList = [prop] as jenkins.branch.BranchProperty[];
    def strategy = new jenkins.branch.DefaultBranchPropertyStrategy(propList);
    s.setStrategy(strategy);
  }

  f.computation.run()
}

This is a Groovy snippet you can execute in Jenkins, it's gonna do the scanning but will not start new "builds" for all discovered branches.

If you are using job-dsl you could simply do this and it will scan everything without actually running the build the first time you index.

organizationFolder('Some folder name') {
  buildStrategies {
    skipInitialBuildOnFirstBranchIndexing()
  }
}

To add to @Stqs's answer, you could also set noTriggerBranchProperty it using Job DSL plugin, e.g.:

multibranchPipelineJob('example') {
  ...
  branchSources {
    branchSource {
      ...
      strategy {
        defaultBranchPropertyStrategy {
          props {
            // Suppresses the normal SCM commit trigger coming from branch indexing
            noTriggerBranchProperty()
            ...
          }
        }
      }
    }
  }
  ...
}

In your Branch Sources section you can add a Property named Suppress automatic SCM triggering.

This prevents Jenkins from building everything with an Jenkinsfile.