Shell cmd to join files using cat

I've written a simple shell script command to join two files together using cat. From the command line I would write something like:

cat fileA.txt >> fileB.txt

My shell command, which compiles, is:

RegisterGroovy( "AddToFile( addfrom ; addto )" ; "String[] cmd = ["cat", addfrom, ">>", addto];¶

Process process = Runtime.getRuntime().exec( cmd );¶

String processResult = process.getInputStream().getText();¶

process.waitFor();¶

exitValue = process.exitValue();¶

return processResult;" )

However, this merely returns the contents of each file and does not add the contents of fileA.txt to fileB.txt.

Any thoughts?