in reply to existing file is modified or not
I want to know is this the right way to find whether the existing file is modified or not ?
Only if the exact difference is of any use. Otherwise simply storing a 128 bit MD5 hash would be the typical approach as it is very economical on space.
There is lots of software that monitor change in directories. Tripwire springs to mind but there are many others. However it appears to me that you might do well to look at CVS as this does all sorts of useful stuff like versioning, *diffs*, rollback, detecting and merging changes where possible, generating conflict files otherwise...... rsync might also be an option.
It really depends on the problem you are trying to solve. But when it comes to syncronising directories of files there has been a *lot* of good work done that you may find very useful.