The variables f and fshift are MATLAB arrays which represent the frequency components of a signal, where the array f varies within the interval [0,Fs] and fshift varies within the interval [-Fs/2,Fs/2]. While performing fftshift, we shift the zero-frequency component to the center of the spectrum. As a result, the spectrum is now centered around zero. We can choose different variable names for them as well.
When you mention show the expression y=fft(x), I understand you want to show the values in the y-array containing the FFT values. You can either print them out using disp
You can also plot the magnitude using the plot function
f = (0:length(y)-1)*fs/length(y);
Hope this helps!