The PWB shell (also known as the Mashey shell) was a Unix shell.
History
The PWB shell was a modified (and generally constrained to be upward-compatible) version of the Thompson shell with additional features to increase usability for programming. It was maintained by John Mashey and various others (Dick Haight, Alan Glasser).
PWB/UNIX started with Research Unix 4th Edition in mid-October 1973, and was frequently updated over the next few years, as the PWB department tracked Research Unix changes and added a few features. The PWB shell was released in mid-1975, and remained available through Version 6 Unix-based PWB/UNIX. In Version 7 Unix (1979), the PWB shell was superseded by the Bourne shell.
Notable features
Several features were introduced in the PWB shell that remain in many later shells. The and commands were made internal to the shell, and extended to allow ---, and and constructs were introduced, as well as to ignore interrupts or catch them to perform cleanup.