How do I kill a backgrounded/detached ssh session?

I am using the program synergy together with an ssh tunnel

It works, i just have to open an console an type these two commands:

ssh -f -N -L localhost:12345:otherHost:12345 otherUser@OtherHost
synergyc localhost

because im lazy i made an Bash-Script which is run with one mouseclick on an icon:

#!/bin/bash
ssh -f -N -L localhost:12345:otherHost:12345 otherUser@OtherHost
synergyc localhost

the Bash-Script above works as well, but now i also want to kill synergy and the ssh tunnel via one mouseclick, so i have to save the PIDs of synergy and ssh into file to kill them later:

#!/bin/bash

mkdir -p /tmp/synergyPIDs || exit 1
rm -f /tmp/synergyPIDs/ssh || exit 1
rm -f /tmp/synergyPIDs/synergy || exit 1

[ ! -e /tmp/synergyPIDs/ssh ] || exit 1
[ ! -e /tmp/synergyPIDs/synergy ] || exit 1

ssh -f -N -L localhost:12345:otherHost:12345 otherUser@OtherHost
echo $! > /tmp/synergyPIDs/ssh
synergyc localhost
echo $! > /tmp/synergyPIDs/synergy

But the files of this script are empty.

How do I get the PIDs of ssh and synergy?
(I try to avoid ps aux | grep ... | awk ... | sed ... combinations, there has to be an easier way.)

Quick summary: Will not work.

My first idea is that you need to start the processes in the background to get their PIDs with $!.

A pattern like

some_program &
some_pid=$!
wait $some_pid

might do what you need... except that then ssh won't be in the foreground to ask for passphrases any more.

Well then, you might need something different after all. ssh -f probably spawns a new process your shell can never know from invoking it anyway. Ideally, ssh itself would offer a way to write its PID into some file.

With all due respect to the users of pgrep, pkill, ps | awk, etc, there is a much better way.

Consider that if you rely on ps -aux | grep ... to find a process you run the risk of a collision. You may have a use case where that is unlikely, but as a general rule, it's not the way to go.

SSH provides a mechanism for managing and controlling background processes. But like so many SSH things, it's an "advanced" feature, and many people (it seems, from the other answers here) are unaware of its existence.

In my own use case, I have a workstation at home on which I want to leave a tunnel that connects to an HTTP proxy on the internal network at my office, and another one that gives me quick access to management interfaces on co-located servers. This is how you might create the basic tunnels, initiated from home:

$ ssh -fNT -L8888:proxyhost:8888 -R22222:localhost:22 officefirewall
$ ssh -fNT -L4431:www1:443 -L4432:www2:443 colocatedserver

These cause ssh to background itself, leaving the tunnels open. But if the tunnel goes away, I'm stuck, and if I want to find it, I have to parse my process list and home I've got the "right" ssh (in case I've accidentally launched multiple ones that look similar).

Instead, if I want to manage multiple connections, I use SSH's ControlMaster config option, along with the -O command-line option for control. For example, with the following in my ~/.ssh/config file,

host officefirewall colocatedserver
    ControlMaster auto
    ControlPath ~/.ssh/cm_sockets/%r@%h:%p

the ssh commands above, when run, will leave spoor in ~/.ssh/cm_sockets/ which can then provide access for control, for example:

$ ssh -O check officefirewall
Master running (pid=23980)
$ ssh -O exit officefirewall
Exit request sent.
$ ssh -O check officefirewall
Control socket connect(/home/ghoti/.ssh/cm_socket/ghoti@192.0.2.5:22): No such file or directory

And at this point, the tunnel (and controlling SSH session) is gone, without the need to use a hammer (kill, killall, pkill, etc).

Bringing this back to your use-case...

You're establishing the tunnel through which you want syngergyc to talk to syngergys on TCP port 12345. For that, I'd do something like the following.

Add an entry to your ~/.ssh/config file:

Host otherHosttunnel
    HostName otherHost
    User otherUser
    LocalForward 12345 otherHost:12345
    RequestTTY no
    ExitOnForwardFailure yes
    ControlMaster auto
    ControlPath ~/.ssh/cm_sockets/%r@%h:%p

Note that the command line -L option is handled with the LocalForward keyword, and the Control{Master,Path} lines are included to make sure you have control after the tunnel is established.

Then, you might modify your bash script to something like this:

#!/bin/bash

if ! ssh -f -N otherHosttunnel; then
    echo "ERROR: couldn't start tunnel." >&2
    exit 1
else
    synergyc localhost
    ssh -O exit otherHosttunnel
fi

The -f option backgrounds the tunnel, leaving a socket on your ControlPath to close the tunnel later. If the ssh fails (which it might due to a network error or ExitOnForwardFailure), there's no need to exit the tunnel, but if it did not fail (else), synergyc is launched and then the tunnel is closed after it exits.

You might also want to look in to whether the SSH option LocalCommand could be used to launch synergyc from right within your ssh config file.

just came across this thread and wanted to mention the "pidof" linux utility:

$ pidof init
1

You can use lsof to show the pid of the process listening to port 12345 on localhost:

lsof -t -i @localhost:12345 -sTCP:listen

Examples:

PID=$(lsof -t -i @localhost:12345 -sTCP:listen)
lsof -t -i @localhost:12345 -sTCP:listen >/dev/null && echo "Port in use"

well i dont want to add an & at the end of the commands as the connection will die if the console wintow is closed ... so i ended up with an ps-grep-awk-sed-combo

ssh -f -N -L localhost:12345:otherHost:12345   otherUser@otherHost
echo `ps aux | grep -F 'ssh -f -N -L localhost' | grep -v -F 'grep' | awk '{ print $2 }'` > /tmp/synergyPIDs/ssh
synergyc localhost
echo `ps aux | grep -F 'synergyc localhost' | grep -v -F 'grep' | awk '{ print $2 }'` > /tmp/synergyPIDs/synergy

(you could integrate grep into awk, but im too lazy now)

You can drop the -f, which makes it run it in background, then run it with eval and force it to the background yourself.

You can then grab the pid. Make sure to put the & within the eval statement.

eval "ssh -N -L localhost:12345:otherHost:12345 otherUser@OtherHost & " 
tunnelpid=$!

This is more of a special case for synergyc (and most other programs that try to daemonize themselves). Using $! would work, except that synergyc does a clone() syscall during execution that will give it a new PID other than the one that bash thought it has. If you want to get around this so that you can use $!, then you can tell synergyc to stay in the forground and then background it.

synergyc -f -n mydesktop remoteip &
synergypid=$!

synergyc also does a few other things like autorestart that you may want to turn off if you are trying to manage it.

Another option is to use pgrep to find the PID of the newest ssh process

ssh -fNTL 8073:localhost:873 otherUser@OtherHost
tunnelPID=$(pgrep -n -x ssh)
synergyc localhost
kill -HUP $tunnelPID

Based on the very good answer of @ghoti, here is a simpler script (for testing) utilising the SSH control sockets without the need of extra configuration:

#!/bin/bash
if ssh -fN -MS /tmp/mysocket -L localhost:12345:otherHost:12345 otherUser@otherHost; then
    synergyc localhost
    ssh -S /tmp/mysocket -O exit otherHost
fi

synergyc will be only started if tunnel has been established successfully, which itself will be closed as soon as synergyc returns. Albeit the solution lacks proper error reporting.

You could look out for the ssh proceess that is bound to your local port, using this line:

netstat -tpln | grep 127\.0\.0\.1:12345 | awk '{print $7}' | sed 's#/.*##'

It returns the PID of the process using port 12345/TCP on localhost. So you don't have to filter all ssh results from ps.

If you just need to check, if that port is bound, use:

netstat -tln | grep 127\.0\.0\.1:12345 >/dev/null 2>&1

Returns 1 if none bound or 0 if someone is listening to this port.

There are many interesting answers here, but nobody mentioned that the manpage of SSH does describe this exact case! (see TCP FORWARDING section). And the solution they offer is much simpler:

ssh -fL 12345:localhost:12345 user@remoteserver sleep 10
synergyc localhost

Now in details:

1. First we start SSH with a tunnel; thanks to -f it will initiate the connection and only then fork to background (unlike solutions with ssh ... &; pid=$! where ssh is sent to background and next command is executed before the tunnel is created). On the remote machine it will run sleep 10 which will wait 10 seconds and then end.
2. Within 10 seconds, we should start our desired command, in this case synergyc localhost. It will connect to the tunnel and SSH will then know that the tunnel is in use.
3. After 10 seconds pass, sleep 10 command will finish. But the tunnel is still in use by synergyc, so SSH will not close the underlying connection until the tunnel is released (i.e. until synergyc closes socket).
4. When synergyc is closed, it will release the tunnel, and SSH in turn will terminate itself, closing a connection.

The only downside of this approach is that if the program we use will close and re-open connection for some reason then SSH will close the tunnel right after connection is closed, and the program won't be able to reconnect. If this is an issue then you should use an approach described in @doak's answer which uses control socket to properly terminate SSH connection and uses -f to make sure tunnel is created when SSH forks to the background.