Get Jenkins environment variable as input

[Thilina Madhusanka]

Hi 

I need to get input from user for a file name. if user enter file name like  "nameoffile$BUILD_NUMBER" 

how can i resolve this $BUILD_NUMBER to jenkins build number? 

i see it is been done in most of the plugins. like ext-email. 

can some one tell how to achieve this ?  

Thanks. 

[Tomas Bjerre]

I do that in the expand method here:

https://github.com/jenkinsci/violation-comments-to-stash-plugin/blob/master/src/main/java/org/jenkinsci/plugins/jvcts/perform/JvctsPerformer.java#L122