Shell script issue when added to cron

I have written a shell script in order to monitor Nagios service status continuously (See this question), which is as follows:

#!/bin/bash

RCPT="abc@xyz.com"

service nagios status | grep [0-9]

if [ $? -eq 0 ]
then
        exit 0
else
        service nagios start
        echo "Hello, Nagios Service has been started, please confirm." | mail -s "Nagios Service Stopped" $RCPT
fi

When above script is executed from command-line, it would work as expected - it would send an email if PID is not found in the service status output, else exits. But, when I added this script in crontab, it would dispatch a notification every time it is executed.

What might be the problem and how to overcome it?