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Port forwarding is something a router does.  

Port triggering is basically port forwarding that's enabled when a connection attempt is detected and then automatically disabled.  That's also handled on a router.

A shell is simply a program that takes interactive or batch command input and is used to start other programs, or run small scripts that usually end up invoking other programs to do something.

So shells are really not related to port forwarding or port triggering.  You can tell your shell to start a program to generate traffic on a specific IP and port, which may enable some port triggering, but that's about it.