Shellscript to monitor a log file if keyword triggers then execute a command?

Question

Is there a cheap way to monitor a log file like tail -f log.txt, then if something like [error] appears, execute a command?

Thank you.

Answer 1

tail -fn0 logfile | \ while read line ; do         echo "$line" | grep "pattern"         if [ $? = 0 ]         then                 ... do something ...         fi done