Need advice on how to use Exec in App Engine to move files and to change directory

I am working on an App Engine that is creating a fairly large set of files to send to a vendor. I have successfully encrypted the files and ftp'd them over to another server, but now we want to create a tar file of all the files before we send them.
I wanted to cd to the directory of the files before issuing the tar command and then move it to the directory where it will be sent from. I tried issuing a tar command with the entire path and that resulted in an empty file, so I am trying to cd to the directory before issuing the tar command.