a=getCookie("username3");
 c_start=document.cookie.indexOf("username3=");
 if(c_start == -1){
  $("#login_form").show();
     $("#logined").hide();
 }
 else{
  $("#login_form").hide();
     $("#logined").show();
     $("#ustr").html(a);
 }
 

<script language=javascript>
 //獲得coolie 的值
function cookie(name){   
   var cookieArray=document.cookie.split("; "); //得到分割的cookie名值對   
   var cookie=new Object();   
   for (var i=0;i<cookieArray.length;i++){   
      var arr=cookieArray[i].split("=");       //將名和值分開   
      if(arr[0]==name)return unescape(arr[1]); //如果是指定的cookie，則返回它的值   
   }
   return "";
}

function delCookie(name)//刪除cookie
{
   document.cookie = name+"=;expires="+(new Date(0)).toGMTString();
}

function getCookie(objName){//獲取指定名稱的cookie的值
    var arrStr = document.cookie.split("; ");
    for(var i = 0;i < arrStr.length;i ++){
        var temp = arrStr[i].split("=");
        if(temp[0] == objName) return unescape(temp[1]);
   }
}

function addCookie(objName,objValue,objHours){      //添加cookie
    var str = objName + "=" + escape(objValue);
    if(objHours > 0){                               //為時不設定過期時間，瀏覽器關閉時cookie自動消失
        var date = new Date();
        var ms = objHours*3600*1000;
        date.setTime(date.getTime() + ms);
        str += "; expires=" + date.toGMTString();
   }
   document.cookie = str;
}

function SetCookie(name,value)//兩個參數，一個是cookie的名子，一個是值
{
    var Days = 30; //此 cookie 將被保存 30 天
    var exp = new Date();    //new Date("December 31, 9998");
    exp.setTime(exp.getTime() + Days*24*60*60*1000);
    document.cookie = name + "="+ escape (value) + ";expires=" + exp.toGMTString();
}

function getCookie(name)//取cookies函數       
{
    var arr = document.cookie.match(new RegExp("(^| )"+name+"=([^;]*)(;|$)"));
    if(arr != null) return unescape(arr[2]); return null;
}

function delCookie(name)//刪除cookie
{
    var exp = new Date();
    exp.setTime(exp.getTime() - 1);
    var cval=getCookie(name);
    if(cval!=null) document.cookie= name + "="+cval+";expires="+exp.toGMTString();
}
</script>