Here is the code that worked for me:

<script src=/SiteAssets/jquery.SPServices-2013.02a.js type=text/javascript></script>

<script src=/SiteAssets/jquery.js type=text/javascript></script>



<script type=text/javascript>

  var userid= _spPageContextInfo.userId;

  var requestUri = _spPageContextInfo.webAbsoluteUrl + /_api/web/getuserbyid( + userid + );

  var requestHeaders = { accept : application/json;odata=verbose };

  $.ajax({

    url : requestUri,

    contentType : application/json;odata=verbose,

    headers : requestHeaders,

    success : onSuccess,

    error : onError

  });



  function onSuccess(data, request){

    var loginName = data.d.Title;

    alert(loginName);

  }



  function onError(error) {

    alert(error);

  }

</script>