When I arrived at Bell Labs a number of people were looking at ways to add programming capabilities such as variables and control flow primitives to the original shell. One day [mid 1975?] Dennis [Ritchie] and I came out of a meeting where somebody was proposing yet another variation by patching over some of the existing design decisions that were made in the original shell that Ken wrote. And so I looked at Dennis and he looked at me and I said "you know we have to re-do this and re-think some of the original design decisions that were made because you can't go from here to there without changing some fundamental things". So that is how I got started on the new shell.

Was there a particular problem that the language aimed to solve?

The primary problem was to design the shell be a fully programmable scripting language that could also serve as the interface to users typing commands interactively at a terminal.

First of all, it needed to be compatible with the existing usage that people were familiar with. There were two usage modes. One was scripting and even though it was very limited there were already many scripts people had written. Also, the shell or command interpreter reads and executes the commands you type at the terminal. And so it is constrained to be both a command line interpreter and a scripting language. As the Unix command line interpreter, for example, you wouldn't want to be typing commands and have all the strings quoted like you would in C, because most things you type are simply uninterpreted strings. You don't want to type ls directory and have the directory name in string quotes because that would be such a royal pain. Also, spaces are used to separate arguments to commands. The basic design is driven from there and that determines how you represent strings in the language, which is as un-interpreted text. Everything that isn't a string has to have something in front of it so you know it is not a string. For example, there is $ sign in front of variables. This is in contrast to a typical programming language, where variables are names and strings are in some kind of quote marks. There are also reserved words for built-in commands like for loops but this is common with many programming languages.

So that is one way of saying what the problem was that the Bourne Shell was designed to solve. I would also say that the shell is the interface to the Unix system environment and so that's its primary function: to provide a fully functional interface to the Unix system environment so that you could do anything that the Unix command set and the Unix system call set will provide you. This is the primary purpose of the shell.