How to redirect command output to log4j in a groovy script

groovy-logger.groovy

#!/usr/bin/env groovy
@Grapes(
  @Grab('log4j:log4j:1.2.17')
)
import org.apache.log4j.Level
import org.apache.log4j.Logger

def redirectLog = { Level level ->
    return { String s ->
      if (s.trim())
        Logger.rootLogger.log(level, s)
    } as Appendable
}

Logger.rootLogger.level = Level.DEBUG
// Optional : configure appenders
//Logger.rootLogger.removeAllAppenders()
//Logger.rootLogger.addAppender new ConsoleAppender([layout: new PatternLayout('%m%n'), writer: System.out.newWriter()])
//Logger.rootLogger.addAppender new FileAppender(new TTCCLayout(), 'myscript.log')

Process proc = ['ls', '-l'].execute()
proc.waitForProcessOutput(redirectLog(Level.INFO), redirectLog(Level.ERROR))