How to scp with a second remote host

I don't know of any way to copy the file directly in one single command, but if you can concede to running an SSH instance in the background to just keep a port forwarding tunnel open, then you could copy the file in one command.

Like this:

# First, open the tunnel
ssh -L 1234:remote2:22 -p 45678 user1@remote1
# Then, use the tunnel to copy the file directly from remote2
scp -P 1234 user2@localhost:file .

Note that you connect as user2@localhost in the actual scp command, because it is on port 1234 on localhost that the first ssh instance is listening to forward connections to remote2. Note also that you don't need to run the first command for every subsequent file copy; you can simply leave it running.

Double ssh

Even in your complex case, you can handle file transfer using a single command line, simply with ssh ;-)
And this is useful if remote1 cannot connect to localhost:

ssh user1@remote1 'ssh user2@remote2 "cat file"' > file

tar

But you loose file properties (ownership, permissions...).

However, tar is your friend to keep these file properties:

ssh user1@remote1 'ssh user2@remote2 "cd path2; tar c file"' | tar x

You can also compress to reduce network bandwidth:

ssh user1@remote1 'ssh user2@remote2 "cd path2; tar cj file"' | tar xj

And tar also allows you transferring a recursive directory through basic ssh:

ssh user1@remote1 'ssh user2@remote2 "cd path2; tar cj ."' | tar xj

ionice

If the file is huge and you do not want to disturb other important network applications, you may miss network throughput limitation provided by scp and rsync tools (e.g. scp -l 1024 user@remote:file does not use more than 1 Mbits/second).

But, a workaround is using ionice to keep a single command line:

ionice -c2 -n7 ssh u1@remote1 'ionice -c2 -n7 ssh u2@remote2 "cat file"' > file

Note: ionice may not be available on old distributions.

This will do the trick:

scp -o 'Host remote2' -o 'ProxyCommand ssh user@remote1 nc %h %p' \ 
    user@remote2:path/to/file .

To SCP the file from the host remote2 directly, add the two options (Host and ProxyCommand) to your ~/.ssh/config file (see also this answer on superuser). Then you can run:

scp user@remote2:path/to/file .

from your local machine without having to think about remote1.

With openssh version 7.3 and up it is easy. Use ProxyJump option in the config file.

# Add to ~/.ssh/config 
Host bastion
    Hostname bastion.client.com
    User userForBastion
    IdentityFile ~/.ssh/bastion.pem

Host appMachine
    Hostname appMachine.internal.com
    User bastion
    ProxyJump bastion                   # openssh 7.3 version new feature ProxyJump
    IdentityFile ~/.ssh/appMachine.pem. #no need to copy pem file to bastion host  

Commands to run to login or copy

ssh appMachine   # no need to specify any tunnel. 
scp helloWorld.txt appMachine:.   # copy without intermediate jumphost/bastion host copy.** 

ofcourse you can specify bastion Jump host using option "-J" to ssh command, if not configured in config file.

Note scp does not seems to support "-J" flag as of now. (i could not find in man pages. However above scp works with config file setting)

There is a new option in scp that add recently for exactly this same job that is very convenient, it is -3.

TL;DR For the current host that has authentication already set up in ssh config files, just do:

scp -3 remote1:file remote2:file

Your scp must be from recent versions.

All other mentioned technique requires you to set up authentication from remote1 to remote2 or vice versa, which not always is a good idea.
Argument -3 means you want to move files from two remote hosts by using current host as intermediary, and this host actually does the authentication to both remote hosts, so they don't have to have access to each other.
You just have to setup authentication in ssh config files, which is fairly easy and well documented, and then just run the command in TL;DR

The source for this answer is https://superuser.com/a/686527/713762