I forgot to add
~$ diff folder1 folder2

will do the exact same thing, just you don't have the options you get with
ls such as listing extra details to tell which file in a folder is most
recent (at least not that I know how to do).

also , I see there is something called "rsync" which is supposed to
automatically copy over files that do not exist in the target folder. see
this website for details:
unstableme.blogspot.com/.../...s-using-diff-bash.h\
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