How do I access a configuration variable passed into grails from the commandline -

I am trying to send a variable like the command line to my Geral app:

 < Code> grails -Dmy.build.number = 33 prod war root.war   
 I tried to access it as a  grailsApplication.config.my.build.number  But I get an empty map. It seems weird that the variable is defined, but what I send is not set.  
 How do I get a command-line property?  
 I plan to install some of my assets (CSS, JS, image etc.) to work as a CDN in CloudFront. So in my construction phase I want to use the build number as the cash buster and want to set my property to point to  mycloudfronturl / assets / $ {buildnumber} /script.js  . If there is a better attitude, then I am also open for this.   
 
 
  system getProperty ("my.build.number");