How to do something with Bash when a text line appears in a file

Question

I want to run a command as soon as a certain text appears in a log file. How do I do that in Bash?

Answer 1

Use command

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

The --line-buffered is the key here, otherwise the read will fail.

Answer 2

Using only tail:

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