Daniel Beardsley - all posts

SSH in the background

2012 / 03 / 27 - San Luis Obispo, CA

I found myself wanting to launch and control a port-forwarding ssh session from a shell script today and I eventually figured out how to do it.

The tricky part was that I also wanted the script to do other things (start a Selenium server) and kill off the ssh connection when it was done.

The Code

# Start an ssh port-forwarding command in the background
ssh user@host.com -TN -R $remote_port:localhost:$localport >/dev/null /2>&1 &

# Get pid of ssh background process -- tricky

# kill the ssh process on exit
trap "kill $ssh_pid" EXIT

# Do other stuff that uses the forwarded port.
# In my case, start Selenium
java -jar selenium-server-standalone* 

How it works

The SSH command is the trickiest bit, so here it is, more in depth.

ssh user@host.com

   # Disable tty allocation (needed for backgrounding)

   # Dont run a remote command (i.e. only does port forwarding)

   # Setup remote port to forward to local port
   -R $remote_port:localhost:$localport

   # Redirect stdin and stdout to /dev/null
   # and put the process in the background (the &)
   >/dev/null /2>&1 &
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