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stdout on one CentOS server needs to be piped to stdin on another CentOS server. Is this possible?
ScottPack, MikeyB and jofel all have valid answers. I awarded the answer to Scott because, even though my question didn't specify security as a requirement, it's always nice to be safe. However, the other two fellows' suggestions will also work.
This is an unabashed yes.
When one uses ssh to execute a command on a remote server it performs some kind of fancy internal input/output redirection. In fact, I find this to be one of the subtly nicer features of OpenSSH. Specifically, if you use ssh to execute an arbitrary command on a remote system, then ssh will map STDIN and STDOUT to that of the command being executed.
For the purposes of an example, let's assume you want to create a backup tarball, but don't want to, or can't, store it locally. Let's have a gander at this syntax:
$ tar -cf - /path/to/backup/dir | ssh remotehost "cat - > backupfile.tar"
We're creating a tarball, and writing it to STDOUT, normal stuff. Since we're using ssh to execute a remote command, STDIN gets mapped to the STDIN of cat. Which we then redirect to a file.
netcat on both ends makes for a great simple, easy communication channel. tar cf - /path/to/dir | nc 1.2.3.4 5000 on one server, nc -l -p 5000 > backupfile.tar on the other.
A convenient way of piping data between hosts when you don't need to worry about security over the wire is using netcat on both ends on the connection.
This also lets you set them up asynchronously:
On the "receiver" (really, you'll have two-way communication, but it's easier to think of it like this), run:
nc -l -p 5000 > /path/to/backupfile.tar
And on the "sender", run:
tar cf - /path/to/dir | nc 1.2.3.4 5000
A very powerful tool for creating uni- and bidirectional connections is socat. For a short look at the possibilities, look at the examples in its manpage.
It replaces netcat and similar tools completely and has support for ssl encrypted connections. For beginners, it might be not simple enough, but it is at least good to know that it exists.
Things only get slightly more complicated when you have a bastion server that must be used.
You can pass ssh as the command to ssh like so:
cat local_script.sh | ssh -A usera@bastion ssh -A userb@privateserver "cat > remote_copy_of_local_script.sh; bash remote_copy_of_local_script.sh"Beware of pseudo-terminals
Note that the point of key importance here is that ssh, like most tools, just treats stdout and stdin correct by default.
However, when you start to see option like Disable pseudo-terminal allocation. and Force pseudo-terminal allocation. you may need to do a little trial and error. But, as a general rule you don't want to alter tty behavior unless you are trying to fix garbled/binary junk in a terminal emulator (what a human types in).
For example, I tend to use -At so that my workstation's ssh-agent gets forwarded, and so that running tmux remotely doesn't barf binary (like so ssh -At bastion.internal tmux -L bruno attach). And, for docker too (like so sudo docker exec -it jenkins bash).
However, those two -t flags cause some hard to track down data corruption when I try to do something like this:
# copy /etc/init from jenkins to /tmp/init in testjenkins running as a container
ssh -A bastion.internal \
ssh -A jenkins.internal \
sudo tar cf - -C /etc init | \
sudo docker exec -i testjenkins \
bash -c 'tar xvf - -C /tmp'
# note trailing slashes to make this oneliner more readable.
Try to put your ssh public key in another host just by one command
ssh [email protected] 'cat >> .ssh/authorized_keys' < .ssh/id_rsa.pub
I find this to be the easiest, after setting up no password handshaking between servers for the user you are running the command as:
Uncompressed
tar cf - . | ssh servername "cd /path-to-dir && tar xf -"
Compression on the fly
tar czf - . | ssh servername "cd /path-to-dir && tar xzf -"
ssh is already configured for compression.
Compression could be set in any of the config files, a quick check for that is to see if ssh -v localhost exit 2>&1 | fgrep -i compress gives any output (AFAIK there is no option to dump the configuration as ssh has read it in).
tar has a -C path flag that works for both the c and x commands. You don't have to put a separate cd command in there. (But it is good to note that you can run more than a single command.)
Feb 21, 2017 at 16:07
-C is a GNU extension (though now also supported by bsdtar and schily tar)
Feb 21, 2017 at 16:34
For moving some docker images around, or VPN .conf files; sometimes I do this:
docker save alpine:latest | ssh remote-host "cat - | docker import - alpine:latest"
docker save alpine:lates export or save filessh remote-host ssh to remote-hostcat - | read from STDIN and pipe itdocker import - alpine:latest read from STDIN and save itThen alpine:latest will be available on remote-host
docker import - getmeili/meilisearch < <(ssh remote-host docker save getmeili/meilisearch)
ssh remote-host ssh to remote-hostdocker save getmeili/meilisearch run this command<(CMD) redirect the output back to STDIN< read from STDINdocker import - getmeili/meilisearch read from STDINThen getmeili/meilisearch will be available on local host.
cat - < <(ssh remote-host-1 "cat /home/pf/configs/shf.conf") | ssh remote-host-2 "cat - > /tmp/shf.conf"
In this style, I download shf.conf file from the remote-host (1) and without write it to desk sent it to another remote-host (2).
[ remote-1 ] => [ local (mypc)] => (remote-2)
Actually we can omit cat - < and use:
ssh remote-host-2 "cat - > /tmp/shf.conf" < <(ssh remote-host-1 "cat /home/pf/configs/shf.conf")
And this also sends newline if we be prompt for password , so the third version is better:
ssh remote-host-1 "cat /home/pf/configs/shf.conf" | ssh remote-host-2 "cat - > /tmp/shf.conf"
