The only real utter absolute difference is that rsync uses a vastly
more brilliant algorithm to update the remote end, which has the
consequence of making the updates go faster when the network
connection between the ends is appreciably slower than local disk
access. This is described in the rsync paper, which is included in
the src tarballs. That's it.
There are other differences, but they're partly historical and
partly cosmetic, and none of them are as fundamental.
rdist has a different user interface, I personally greatly prefer
wrestling with the complexities of baroque rsync cmdlines over rdist
config files. That is a matter of taste and nothing else.
rsync has always transported itself easily over ssh; support for
transports other than rsh was only added to rdist later in its life
--- after the versions frozen into some vendor OS code bases got
set, last I heard.
rsync supports a separate, private protocol for an anonymous rsync
server; I don't know of an equivalent for rdist.
-Bennett
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