My code looks like this :

clc;
clear;
visaString = 'GPIB0::16::0::INSTR';

% Create a VISA-GPIB object.
vi = instrfind('Type', 'visa-gpib', 'RsrcName', visaString, 'Tag', '');
%vi = instrfind;

if isempty(vi)                           
     vi =  visa('agilent',visaString);
else
    fclose(vi);
    vi = vi(1);
end
..........