$ watchfile index.html diff index.html index_old.htmlThis command example will watch file index.html for changes. Once detected run a command diff index.html index_old.html. A command to run can be anything.
The watcher continues to run untill you Ctrl-C or kill it.
To make it ran at the backgroud all the time example use:
$ nohup watchfile index.html diff index.html index._old.html &this will make it immunune to bash terminal closure (nohup) and shall continue to run the it at the background (&).
Download the required binary here: https://github.com/biosckon/watchfile/releases/
Alternatively for go programmers... you know what to do.