Useful Nuke Expressions
Set a write node based on the read node at the top of the pipe
[file dirname [knob [topnode].file]]/jpg/[file rootname [file tail [knob [topnode].file]]].jpg
The [topnode] is the key word. I use [file rootname] in this case just to strip the extension. More complex operations would require a [split].
Links:
Heavyimage Nuke python / tcl snippits