Do notice the Issue List is rather long and with some old problems still open

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.customfields.manager.OptionsManager
 
def componentManager = ComponentManager.instance
def optionsManager = componentManager.getComponentInstanceOfType(OptionsManager.class)
def cf = customFieldManager.getCustomFieldObjects(issue).find {it.name == 'Task Type'}
def fieldConfig = cf.getRelevantConfig(issue)
def optionClone = optionsManager.getOptions(fieldConfig).find {it.value == "Clone"}
issue.setCustomFieldValue(cf, optionClone)

issue.assignee == transientVars["issue"].assignee;
issue.setAssignee(null);
issue.setDescription("A new Description");
issue.setSummary("Unit Test - " + transientVars["issue"].key);

import com.atlassian.jira.ComponentManager;
import com.atlassian.jira.issue.fields.CustomField;
import com.atlassian.jira.issue.CustomFieldManager;
CustomFieldManager customFieldManager = ComponentManager.getInstance().getCustomFieldManager();
CustomField UseCustomer = customFieldManager.getCustomFieldObject(10600);

System.out.println("Custom field value: " + issue.getCustomFieldValue(UseCustomer));

String userName="pho"
def userManager = ComponentAccessor.getUserManager()
def user = userManager.getUserObject(userName)
issue.setAssignee(user)

//Update the issue - may not be nessesary
ComponentManager.getInstance().getIssueManager().updateIssue(ComponentManager.getInstance().jiraAuthenticationContext?.user, issue, EventDispatchOption.DO_NOT_DISPATCH, false)
ComponentManager.getInstance().getIndexManager().reIndex(issue);

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.security.Permissions
import com.atlassian.jira.event.type.EventDispatchOption
import com.atlassian.jira.util.ImportUtils
 
//Examples, 8 is my subtask issue type id. Will be different for others
updateAssignee("PRJ-1485", "user1")
 
//Method to do all the work
def updateAssignee(parentId, userName) {
def issue = ComponentManager.getInstance().getIssueManager().getIssueObject(parentId)
def userManager = ComponentAccessor.getUserManager()
def user = userManager.getUserObject(userName)
issue.setAssignee(user)
//Update the issue
ComponentManager.getInstance().getIssueManager().updateIssue(ComponentManager.getInstance().jiraAuthenticationContext?.user, issue, EventDispatchOption.DO_NOT_DISPATCH, false)
ComponentManager.getInstance().getIndexManager().reIndex(issue);
}

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.label.LabelManager
import org.springframework.util.StringUtils
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

labelManager = ComponentManager.getComponentInstanceOfType(LabelManager.class)
CustomFieldManager customFieldManager = ComponentManager.getInstance().getCustomFieldManager();


CustomField UseCustomer = customFieldManager.getCustomFieldObject(10600);
CustomField Customer = customFieldManager.getCustomFieldObject(10900);


labelManager.removeLabelsForCustomField(10900)


Set labelSet = new HashSet();
labelSet.add(issue.getCustomFieldValue(UseCustomer));


def componentManager = ComponentManager.getInstance()
def authContext = componentManager.getJiraAuthenticationContext()
def user = authContext.getUser()


// issue.getCustomFieldValue(UseCustomer)
// Does not work - the "labelSet" is not a valid set - https://answers.atlassian.com/questions/332748/set-labels-in-jira
labelManager.setLabels(user, issue.id,labelSet, false,false)

The

workflowTransitionUtil.setIssue(issueLink.getDestinationObject());

should be

workflowTransitionUtil.setIssue(issueLink.getSourceObject());

Going "the other way". There is also an getInwardLinks(issue.id) collection.

import com.atlassian.jira.issue.Issue;
import com.atlassian.jira.ComponentManager;
User = ComponentManager.getInstance().getJiraAuthenticationContext().getUser().getName();

<% out << issue.getCustomFieldValue(componentManager.getCustomFieldManager().getCustomFieldObjectByName("Mail Body")) %>
<br><br>
Best Regards,
<br><br>
GService Desk
<br>
<a href="http://sd.mydomain.dk">Service Desk</a>
<br>
You cant reply to this email.

// This script makes a requirement: if the Custom Field "OprStatusShow" is set to public, Incident Start and Resolved must be set.

import com.opensymphony.workflow.InvalidInputException
import com.atlassian.jira.ComponentManager
import com.atlassian.jira.project.ProjectManager
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.fields.CustomField

//Project "Support" Id = 10130
if (issue.getProjectObject().getId() == 10130) {

ComponentManager componentManager = ComponentManager.getInstance();
CustomFieldManager customFieldManager = componentManager.getCustomFieldManager();
CustomField IncidentResolvedField = customFieldManager.getCustomFieldObject("customfield_12224");
CustomField IncidentStartField = customFieldManager.getCustomFieldObject("customfield_10091");
CustomField OprStatusShowField = customFieldManager.getCustomFieldObject("customfield_11820");


String IncidentResolved = (String)issue.getCustomFieldValue(IncidentResolvedField);
String IncidentStart = (String)issue.getCustomFieldValue(IncidentStartField);
String OprStatusShow = (String)issue.getCustomFieldValue(OprStatusShowField);

if (OprStatusShow == "Yes") {

	if (IncidentStart == null || IncidentResolved == null) {
		InvalidInputException e= new InvalidInputException();
		e.addError("Incident Start And Incident Resolved must not be empty");
	 	throw e;
		}
	}
}