The file is DIR1/SUB1/file.txt and I want to copy it to DIR1/SUB2/file.txt
Using cat. Not that pussy mv crap.
First off, "mv" is for moving/renaming, not copying. For copying, you want "cp". Secondly, why the hell do you want to use cat?
cat DIR1/SUB1/file.txt >> DIR1/SUB2/file.txt
Realize that this will NOT give you the ownership and permissions you need, so you will have to go back and use chown and chmod, respectively, to set those. If you want to preserve permissions, use "cp -p".