Over the years, many different Unix shells have been created. Why is this necessary? Why not just have one main shell that can be enhanced from time to time? With the Bourne shell family (Bash, Korn...

Over the years, many different Unix shells have been created. Why is this necessary? Why not just have one main shell that can be enhanced from time to time?

With the Bourne shell family (Bash, Korn shell), what command do you use to make a shell variable part of the environment?