Use this script in the Script Console to update the value of a system field for all issues returned by a JQL query.
Example
As a project manager, I want to modify the description of a set of similar issues in a project. With this script, I can easily bulk change all of these issue descriptions automatically, saving me time and reducing the risk of human error.
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.search.SearchProvider
import com.atlassian.jira.jql.parser.JqlQueryParser
import com.atlassian.jira.web.bean.PagerFilter
import com.atlassian.jira.issue.search.SearchQuery
// The issues returned from that JQL will get alteredfinal searchQuery = "project = TEST"
// Get some components
def jqlQueryParser = ComponentAccessor.getComponent(JqlQueryParser)
def searchProvider = ComponentAccessor.getComponent(SearchProvider)
def loggedInUser = ComponentAccessor.jiraAuthenticationContext.loggedInUser
def issueService = ComponentAccessor.issueService
// Perform the search
def query = jqlQueryParser.parseQuery(searchQuery)
def searchResults = searchProvider.search(SearchQuery.create(query, loggedInUser), PagerFilter.unlimitedFilter)
// Iterate all the results to update each issue
searchResults.results.each { documentIssue ->
// Define the new params (a new description)
def issueInputParameters = issueService.newIssueInputParameters()
issueInputParameters.setDescription("A new description")
// Update the issue
def issueId = documentIssue.document.fields.find { it.name() == "issue_id" }.stringValue().toLong()
def updateValidationResult = issueService.validateUpdate(loggedInUser, issueId, issueInputParameters)
assert updateValidationResult.valid : updateValidationResult.errorCollection
// Validate the update
def issueResult = issueService.update(loggedInUser, updateValidationResult)
assert issueResult.valid : issueResult.errorCollection
}