Specify the full path to the service command, e.g.:

system("/usr/sbin/service jboss blah blah blah");
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You can find out the full path to service command like this:

# which service
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If this works your problem is most probably the PATH environment variable. From section 5 of Debian crontab manpage: "Note in particular that if you want a PATH other than "/usr/bin:/bin", you will need to set it in the crontab file."

Update: You should also search PM for something like "crontab path" to get further advice.