I would like to copy a set of files from directory A to directory B, with the caveat that if a file in directory A is identical to a file in directory B, that file should not be copied (and thus its modification time should not be updated). Is there a way to do that with existing tools, without writing my own script to do it?
To elaborate a bit on my use-case: I am autogenerating a bunch of
.c files in a temporary directory (by a method that has to generate all of them unconditionally), and when I re-generate them, I'd like to copy only the ones that have changed into the actual source directory, leaving the unchanged ones untouched (with their old creation times) so that
make will know that it doesn't need to recompile them. (Not all the generated files are
.c files, though, so I need to do binary comparisons rather than text comparisons.)
(As a note: This grew out of the question I asked on http://stackoverflow.com/questions/8981552/speeding-up-file-comparions-with-cmp-on-cygwin/8981762#8981762, where I was trying to speed up the script file I was using to do this operation, but it occurs to me that I really should ask if there's a a better way to do this than writing my own script -- especially since any simple way of doing this in a shell script will invoke something like
cmp on every pair of files, and starting all those processes takes too long.)