Originally based on BSD RCP protocol, SCP (Secure copy) is a mechanism that allows you to transfer a file between two end points over a secure SSH connection. However, as a simple secure copy protocol, SCP does not understand range-request or partial transfer like HTTP does. As such, popular SCP implementations like the scp command line tool cannot resume aborted downloads from lost network connections.
If you want to resume an interrupted SCP transfer, you need to rely on other programs which support range requests. One popular such program is rsync. Similar to scp, rsync can also transfer files over SSH.
Suppose you were trying to download a file (bigdata.tgz) from a remote host remotehost.com using scp, but the SCP transfer was stopped in the middle due to a stalled SSH connection. You can use the following rsync command to easily resume the stopped transfer. Note that the remote server must have rsync installed as well.
$ rsync -P --rsh=ssh firstname.lastname@example.org:bigdata.tgz ./bigdata.tgz
The "-P" option is the same as "--partial --progress", allowing rsync to work with partially downloaded files. The "--rsh=ssh" option tells rsync to use ssh as a remote shell.
Once the command is invoked, rsync processes on local and remote hosts compare a local file (./bigdata.tgz) and a remote file (email@example.com:bigdata.tgz), determine among themselves what portion of the file is not the same, and transfer the discrepancy to either end. In this case, missing bytes in the partially downloaded local file is downloaded from a remote host.
If the above rsync session itself gets interrupted, you can resume it as many time as you want by typing the same command. rsync will automatically restart the transfer where it left off.
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