global string query;
global string Email;
global string Contact;

public Batch_Class (String b, String q, String a){

    Email = b;
    query = q;
    Contact = a;

}
global database.querylocator start(Database.BatchableContext BC){
    query = 'Select id,b from a where b != null';
    return Database.getQueryLocator(query);
}
global void execute(Database.BatchableContext BC, List<a> scope){
    List<a> cons = new List<a>();
    for (a s : scope){
        s.b= s.b +'.test';
        System.debug(s);
        cons.add(s);
    }
    update cons;
}
global void finish(Database.BatchableContext BC){
}