Check process of a single SSH tunnel in a shell script?

My thoughts:

1. Pid files. I think this is a general approach.
2. XY problem, use autossh.

(Some other answers may elaborate these ideas.)

3. There are two scripts, right? (not exactly complying with DRY). Give them different names. Your script waits for its ssh to exit. The additional instance, instead of checking pid(s) of ssh, should check pid(s) of its own name. Exactly one pid two pids mean no previous script is still running:

   [ $(pidof -x scriptname | wc -w) -eq 2 ] && createTunnel
   

   Why two? Because $() starts a subshell that also counts. Quite dirty, oh well.

4. Somewhat outside-the-box, I think. Create two symlinks with unique names, like:

   ln -s /usr/bin/ssh ssh-foo
   ln -s /usr/bin/ssh ssh-bar
   

   Let the first script run ssh-foo, let the second run ssh-bar. Their pidof invocations should target ssh-foo or ssh-bar respectively as well. This way they won't mix. As a bonus, additional ssh (possibly run for a completely different reason in the future) won't affect them.

And finally:

5. You don't have to check anything.

   /usr/bin/ssh -o ExitOnForwardFailure=yes -N -R :2222:localhost:22 [email protected]
   

   If the tunnel already exists, port forwarding will fail for sure and ssh will exit. I guess you can run this line directly from a crontab, no script is required.