To download all-2.0.tar.gz , you first need a from the offline Nessus machine and your activation key .
WORKDIR="/opt/nessus_offline_sync" OUTFILE="$WORKDIR/nessus-update-plugins-all-2.0.tar.gz" LOG="$WORKDIR/sync.log" download nessus-update-plugins all-2.0.tar.gz