Many other shells have been written including the Bourne Again shell (Bash), Korn Shell (ksh), the C Shell (csh), and variations such as tcsh. What is your opinion on them?

I believe that Bash is an open source clone of the Bourne shell. And it may have some additional things in it, I am not sure. It was driven (I'm sure everybody knows this) from the open source side of the world because the Unix licence tied up the Unix intellectual property (source code) so you had to get the licence in order to use it.

The C shell was done a little after I did the Bourne shell -- I talked to Bill Joy about it at the time. He may have been thinking about it at the same time as I was writing sh but anyway it was done in a similar time frame. Bill was interested in some other things that at the time I had less interest in. For example, he wanted to put in the history feature and job control so he went ahead and wrote the C shell. Maybe in retrospect I should have included some things like history and job control in the Unix shell. But at the time I thought they didn't really belong in there ... when you have a window system you end up having some of those functions anyway.

I don't recall exactly when the Korn shell was written. The early 80s I suspect. At the time I had stopped adding "features" to sh and people wanted to continue to add things like better string processing. Also POSIX was being defined and a number of changes were being considered in the standard to the way sh was being used. I think ksh also has some csh facilities such as job control and so on. My own view, as I have said, was that the shell had reached the limits of features that could be included without making it rather baroque and certainly more complex to understand.

Why hasn't the c shell (and its spawn) dropped off the edge of the planet? Is that actually happening?