make a cronjob wait for previous rsync job to finish

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http://serverfault.com – I'm using rsync to backup some data from one server to another. All works fine, but it might take longer to finish depending on how much data there is to transfer. Is there any gauranteed way to ensure that an rsync command doesn't start before the previous one finished using a cronjob? For example, every hour I run the rsync command, but its possible the transfer takes more than 1 hour to complete, so the next one would start before the previous one finishes. (HowTos)