inotify-tools and then you can wait for a file to change using:
inotifywait -m thefile.txt
-m flag monitors the file, instead of exiting on the first event. You can use
-e to wait for a particular event--see the man page.
But you probably want to do something when that happens. You can pipe the output of the command to a bash while loop:
inotifywait -m thefile.txt | while read file; do echo $file; done
file in this case will have the file name and event name, but you don't need to use it obviously.