Isso fará isso por você:
$ cat /etc/lsb-release | grep -i desc
DISTRIB_DESCRIPTION="Ubuntu 14.04 LTS"
$ apt-cache show run-one
....
Description-en: run just one instance of a command and its args at a time
This utility will run just one instance at a time of some command and
unique set of arguments (useful for cronjobs, eg).