correct my understanding in FFT

fft()
fftshift()

sampling rate

http://www.mathworks.com/matlabcentral/fileexchange/25473-why-use-fftshiftfftfftshiftx-in-matlab-instead-of-fftx

f=200;
Fs=1000;
t=1:(1/Fs):10;
x=sin(2*pi*f*t);

fftlen=512;
figure; plot(Fs*(-256:255)/512,tmp(1:512))
fftshift(abs(fft(y,fftlen)))