sift extract keypoints

edit

i want to know how sift extract keypoints from image? or how these lines work?

% Call keypoints executable
if isunix
   command = '!./sift ';
else
   command = '!siftWin32 ';%this on
end
command = [command ' <tmp.pgm >tmp.key'];
eval(command);%fill data of tmp.pgm in tmp.key