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I have an already established ssh connection between two machines.

Is there a way to send commands to the remote machine from a shell script that is run on the local machine, using the already open connection, and without starting another ssh session?

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I suppose that you need to use the existing connection? Do you need to use the existing shell session? –  ulidtko Mar 6 '12 at 13:46
@ulidtko: Yes, i'll edit the question. –  SWeko Mar 6 '12 at 13:59

2 Answers 2

up vote 18 down vote accepted

It's very simple with recent enough versions of OpenSSH if you plan in advance.

Open a master connection the first time. For subsequent connections, route slave connections through the existing master connection. In your ~/.ssh/config, set up connection sharing to happen automatically:

ControlMaster auto
ControlPath ~/.ssh/control:%h:%p:%r

If you start an ssh session to the same (user, port, machine) as an existing connection, the second session will be tunneled over the first. Establishing the second connection requires no new authentication and is very fast.

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It looks the second one will not be registered in /var/log/secure or /var/log/auth.log. However, will the second one be registered in /var/log/wtmp? –  Xichen Li Sep 6 '13 at 15:21
@XichenLi /var/log/secure and /var/log/auth.log log SSH connections; a slave connection does not appear there because it piggybacks on an existing connection. If your ssh session allocates a terminal (i.e. ssh somehost with no command supplied, or ssh -t), that is (normally) logged in wtmp, regardless of how that terminal appeared (sshd whatever method was used to establish the connection, terminal emulator application, …). –  Gilles Sep 6 '13 at 15:51

That's pretty easy to achieve using the nc tool and ssh tunnels.

1. Open ssh tunnel

In your ssh session, type ~C on a new line. You will get the ssh "service console" prompt which looks like this:


Type in the local forward command to open an ssh tunnel:

ssh> -L22000:targethost:22001
Forwarding port.

Where targethost is the hostname or IP address of the machine you are connected to.

Now, assuming the ssh server on the target machine wasn't configured to forbid tunnels, you have the desired connection forwarding: ssh client on your machine listens to port 22000, and it will forward any traffic sent to it to the 22001 port on targethost.

2. Start a network server on the remote machine

This is as simple as entering into your already open ssh session the following command:

nc -l localhost 22001 | sh

This will start a TCP server listening on port 22001 – which is the target port of our ssh tunnel – and route the received data (presumably, shell commands) to a targethost shell instance.

3. Send your script over the tunnel

cat yourscript.sh | nc localhost 22000

This will send the script's body to your ssh tunnel and will end up being executed in a shell on the targethost. You will see script's output in your terminal with ssh session.

I'll also note that ssh tunnel (step 1.) in this scenario isn't strictly required; you could as well start the server open and connect to it directly over the internet. However, you will need to use the tunnel if the target host can't be reached directly (e.g. is behind a NAT), or ssh encryption is desired.

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Great answer. Technically, to get to the service console, the ~ character must come after a newline, so LF ~ C is probably a better sequence. –  Alexios Mar 6 '12 at 14:54
@Alexios, correct. –  ulidtko Mar 6 '12 at 16:22
Amazing trick! For the record, ssh man page shows some more information about it (scroll down to section called "ESCAPE CHARACTERS") –  Carles Sala Feb 18 at 16:07
@CarlesSala also good to know that less, the common default pager, supports search which can save you some scrolling if you know your keywords: just type man ssh /ESCAPE and you're there. –  ulidtko Feb 18 at 20:12
@ulidtko sure, but as far as I know, one can not link a CLI command into a SO comment, so I found the html man page a better option ;-) –  Carles Sala Feb 19 at 12:30

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