Question

Can I get the requested URL from ajaxStart ? I want to execute a common action for all ajax request accept some few requests.

Answer 1

No, within ajaxStart you do not have access to the jqXHR object nor the ajaxOptions:

// Watch for a new set of requests
if ( s.global && jQuery.active++ === 0 ) {
  jQuery.event.trigger( "ajaxStart" );
}

As you can see, there are no arguments being passed to ajaxStart. Contrast this with ajaxSend:

// Send global event
if ( fireGlobals ) {
  globalEventContext.trigger( "ajaxSend", [ jqXHR, s ] );
}

Where the jqXHR object is being passed as an argument along with the settings:

$(document).ajaxSend(function(evt, request, settings) {
  alert("Starting request at " + settings.url + ".");
});

Also see this answer for a better understanding of this design.