Someone posted this to a forum... as an attachment...

install_plugins.init.groovy

import jenkins.model.*
import java.util.logging.Logger
def logger = Logger.getLogger("02_installPlugins.groovy")
def installed = false
def initialized = false
def fileSeperator=File.separator;
println("Plugins Count Before Installation: "+Jenkins.instance.pluginManager.plugins.size())
def filepath="/home/lvadmin/.jenkins/init.groovy.d"+fileSeperator+"pluginslist.cfg";
println "pluginslist.cfg FilePath : $filepath"
def pluginslist = []
def instance = Jenkins.getInstance()
def pm = instance.getPluginManager()
def uc = instance.getUpdateCenter()

if(new File( filepath).exists()){
    new File( filepath ).eachLine { line ->
        pluginslist << line
    }
	logger.info("Pluginslist count: "+pluginslist.size())
	logger.info("Pluginslist to be installed: "+pluginslist)
	
	if(pluginslist.size()>0){
	pluginslist.each {
	  logger.info "-------------->>>>> Installing Plugin "+it.trim();
	  if (!pm.getPlugin(it)) {
		    logger.info("Looking UpdateCenter for " + it)
		    if (!initialized) {
		      uc.updateAllSites()
		      initialized = true
		    }
		    def plugin = uc.getPlugin(it)
		    if (plugin) {
		      logger.info("Installing " + it)
		        def installFuture = plugin.deploy()
		      while(!installFuture.isDone()) {
		        logger.info("Waiting for plugin install: " + it)
		        sleep(3000)
		      }
		      installed = true
		    }
		  }
		}
		println("Plugins Count after Installation: "+Jenkins.instance.pluginManager.plugins.size())
	}
	else{
		println("Zero Plugins specified in pluginslist.cfg file, skipping installation step")
	}
}
else{
	println("File Not Present @ Path: "+filepath)
}

if (installed) {
  logger.info("Plugins installed, initiating jenkins restart...!")
  instance.save()
  instance.restart()
}