Monitor a log for a thing which triggers another thing

logmon.sh

#!/bin/sh
#
# You can start the script on boot, in the background, by adding it to /etc/rc.local:
# echo -e "/path/to/logmon.sh &\n" >> /etc/rc.local
#

tail -Fn0 /path/to/that/logfile.log | \
while read line ; do
        echo "$line" | grep "thing"
        if [ $? = 0 ]
        then
                /bin/trigger-sumthin
        fi
done