run a command as soon as it occurs in a log file

search-for-term.sh

tail -f file.log | grep --line-buffered "search pattern" | while read line
do
  echo $line
done

# the line --line-buffered is key here, the read will fail without it

search-for-term2.sh

tail -f file.log | while read line; do if [[ $line == *search pattern* ]]; then
    mycommand
fi; done

search-for-term3.sh

tail -f -n 0 logfile.out | nawk '/search-pattern/ {system("echo whatever")}'

# thet n -0 is essential, so that only new occurences will be matched