Search, by just one user field - fullName?
There's a nice list in the Craft docs with lot of examples for supported search syntaxes. Here's two that probably do what you are after:


  body:salty → where the ‘body’ field contains “salty”.
  body::"salty dog" → where the ‘body’ field is set to “salty dog” and nothing more.


Combine it with your variable like so:

craft.users.search('fullName::"' ~ getauth ~ '"')