The A-Z of programming languages: Bourne shell

Computerworld Australia - Computerworld Australia is undertaking a series of investigations into the most widely used programming languages. Previously we have spoken to Alfred v. Aho of AWK fame, S. Tucker Taft on the Ada 1995 and 2005 revisions, Microsoft about its server-side script engine ASP, Chet Ramey about his experiences maintaining Bash, Bjarne Stroustrup of C++ fame and to Charles H. Moore about the design and development of Forth.

We've also had a chat with the irreverent Don Woods about the development and uses of INTERCAL, as well as Stephen C. Johnson on YACC, Luca Cardelli on Modula-3, Walter Bright on D, Simon Peyton-Jones on Haskell and more recently, Larry Wall, creator of the Perl programming language.

On this occasion we speak to Steve Bourne, creator of the Bourne shell, or sh. In the early 1970s Bourne was at the Computer Laboratory in Cambridge, England working on a compiler for ALGOL68 as part of his PhD work in dynamical astronomy. This work paved the way for him to travel to IBM's T.J. Watson Research Center in New York in 1973, in part to undertake research into compilers. Through this work, and via a series of connections and circumstance, Bourne got to know people at Bell Labs who then offered him a job in the Unix group in 1975. It was during this time Bourne developed sh.

What prompted the creation of the Bourne shell? The original shell wasn't really a language; it was a recording -- a way of executing a linear sequence of commands from a file, the only control flow primitive being goto a label. These limitations to the original shell that Ken Thompson wrote were significant. You couldn't, for example, easily use a command script as a filter because the command file itself was the standard input. And in a filter the standard input is what you inherit from your parent process, not the command file.

The original shell was simple but as people started to use Unix for application development and scripting, it was too limited. It didn't have variables, it didn't have control flow and it had very inadequate quoting capabilities.

My own interest, before I went to Bell Labs, was in programming language design and compilers. At Cambridge I had worked on the language ALGOL68 with Mike Guy. A small group of us wrote a compiler for ALGOL68 that we called ALGOL68C. We also made some additions to the language to make it more usable. As an aside we boot strapped the compiler so that it was also written in ALGOL68C.

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.