#!groovy

// imports
import jenkins.model.Jenkins

// parameters
def systemMessage = '''
<h1><span style="background-color: #ff0000">Scheduled maintanence: Jenkins won't be available for use tonight @7pm CST</span></h1>
'''

// get Jenkins instance
Jenkins jenkins = Jenkins.getInstance()

// set Jenkins system message
jenkins.setSystemMessage(systemMessage)

// save current Jenkins state to disk
jenkins.save()