Monitoring and updating a text file

Hello

I'm looking for a tool that will monitor and update live a text file. That file would be open and an external software will write some log in it.

Any idea how to do that?
Martin

If the file is going to be in use it may be locked so hard to do

Did installed NotePad ++. It has a function to do that and another function to let the window always displayed on the screen

Martin