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scp is a great way to securely transfer files from computer to computer, but wouldn’t it be nice if you could just automatically send files over the existing SSH connection you’ve already opened?

Back in the days of modem-based BBSes and dial-up machine access, file transfers were forced to run over the same TTY as your interaction with the system. A number of different solutions evolved for this, starting with the grandfather of transfer solutions, XModem. Other transfer protocols evolved, some starting from the ground up like Kermit, while YModem and ZModem build on the foundation of XModem.