How to set.dir for logfile only?

I would like to have mothur save its logfile in a specific folder (that is backed-up automatically), different from the folder where it saves the output files (and that is not backed-up automatically, due to space limitations). I have not managed to use set.dir to that purpose. How could this be possible (on Linux)?

I think you could set.dir at the beginning of running mothur and then use the relative paths to files within each command.