Let’s say we have three machines: A, B and C.
Today I have to copy a huge file from A to B.
Currently I would need to copy it first from A to C and then from C to B. Is there a way to connect or pipe the scp to stream the incoming data to the target machine without need to save in the middle?
man scp writes (with my emphasis)
-3Copies between two remote hosts are transferred through the local host. Without this option the data is copied directly between the two remote hosts. [...]
So what you need is a copy command that routes via your local system (I almost always include -p, to preserve timestamps and permissions, so I've done that here too):
scp -3p serverA:path/to/file(s) serverB:/path/to/destination Very recent versions of scp have -3 enabled by default since 9 August 2021, and to disable it one must now specify -R. Using this third party copy feature will disable prompts for passphrases and passwords as scp cannot ask for them for both hosts.
scp user1@A:from_file user2@B:to_file should do the trick. Read man scp.
ssh user1@A "scp from_file user2@B:to_file" scp use -3 mode by default, in which case this should work.