Transition_linkedIssue.groovy

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.workflow.IssueWorkflowManager

def issueManager = ComponentAccessor.issueManager
def issueService = ComponentAccessor.issueService
def issueWorkflowManager = ComponentAccessor.getComponent(IssueWorkflowManager)
def user = ComponentAccessor.jiraAuthenticationContext.loggedInUser

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def parentLink = customFieldManager.getCustomFieldObjectByName('Parent Link')

def customFieldValue = issue.getCustomFieldValue(parentLink)
def parentKey = customFieldValue.key
def parentIssue = issueManager.getIssueByCurrentKey(parentKey)
def startProgressAction = issueWorkflowManager.getAvailableActions(issue, user).find{it.name == "Start Progress"}
def inputParams = issueService.newIssueInputParameters()
def validationResult = issueService.validateTransition(user, parentIssue.id, startProgressAction.id, inputParams)
if (validationResult.valid) {
 def issueResult = issueService.transition(user, validationResult)
 if (issueResult.valid) {
 log.debug "Successfully transitioned $parentIssue.key"
 } else {
 log.error("Could not transistion $parentIssue.key to in progress following $issue.key")
 log.error(issueResult.errorCollection?.errorMessages)
 log.error(issueResult.warningCollection?.warnings)
 }
} else {
 log.error("Could not transistion $parentIssue.key to in progress following $issue.key")
 log.error(validationResult.errorCollection?.errorMessages)
 log.error(validationResult.warningCollection?.warnings)
}

The script can trigger a transition on the "parent issue" (coming from portfolio) of an existing issue. This script can be part of the script listener and any issue updates on an issue can be used to trigger relevant updates on the parent.

Issue workflow manager is important to use, instead of hard-coding transition ids
def startProgressAction = issueWorkflowManager.getAvailableActions(issue, user).find{it.name == "Start Progress"}