how can I tell incrontab to run this command when there has been any modification done under the /web
directory?
/web IN_CREATE,IN_DELETE,IN_MODIFY /bin/echo "this is a test..."
Here is the incrontab record syntax taken from this page:
<directory> <file change mask> <command or action> options
So, your command should be OK. You need to pay attention to two points:
/bin/echo
will not be connected to any terminal so the output goes nowhere (but you will get a info line that the action ran in the syslog).