American Scientist

To the Editors:

In "Qwerks of History" (Computing Science, January–February), Brian Hayes makes many interesting points worth pondering, but the article contains a historical error that should be corrected before it propagates any further.

Hayes states, "My candidate for the most important single innovation introduced by Unix is the hierarchical file system. I also consider it the feature most desperately in need of a better idea."

The hierarchical file system was not introduced by Unix. Dennis Ritchie and Ken Thompson participated for Bell Labs in the design and implementation of Multics, a joint project among MIT, General Electric and Bell Labs. One of the many features in Multics was a hierarchical file system of unlimited depth. That feature, in turn, was an outgrowth of experience with a strictly two-level hierarchical file system in the Compatible Time-Sharing System. This limited hierarchy was visible primarily to the system itself, but it was so evidently useful that when planning for Multics began in 1964, a user-navigable hierarchy was a top priority. And it was an operational feature in the first, limping version of Multics in 1967.

Between 1960 and 1967 several organizations undertook projects to create time-sharing systems. Because it was apparent by 1964 that it might be a good idea, it is entirely possible that one of those other projects had a hierarchical file system running before Multics, but I don't recall seeing anyone else make a clear claim of priority.

Ritchie and Thompson went on to make many other significant innovations in the design of Unix, examples including the C language and pipes. But many of the features of Unix, including the hierarchical file system, are of greater antiquity.

Jerry Saltzer
Waban, Massachusetts