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 In Version 7 Unix (1979), the PWB shell was superseded by the Bourne shell. The PWB shell was the standard shell for PWB/UNIX, circa 1975–78., but did not run on any edition of Research Unix, as it required a new system call udata(2) that let login(1) set login name, login directory ($s) and TTY ($t) so sh(1) to obtain them. Notable features Several features were introduced in the PWB shell that remain in many later shells. The if and goto commands were made internal to the shell, and extended to allow if-then-else-endif, and switch and while constructs were introduced, as well as onintr to ignore interrupts or catch them to perform cleanup. Simple variables could be used, although their names were limited to one letter and some letters were reserved for special purposes, of which some are the precursors of the environment variables found in all Unix systems from Version 7 onward. For example, The $s variable is the ancestor of $HOME, used to avoid hard-coding pathnames. The $p variable is the ancestor of $PATH, which let users search for commands in their own choice of directories. Unlike most of the UNIX systems of the time, the original PWB/UNIX computer center was shared by multiple programming groups who could not change the contents of /bin or /usr/bin, but wanted to create their own sets of shared commands. In addition, the shell's command-searching was enhanced to allow shell procedures to be invoked like binary commands, i.e., if the shell found a non-binary file marked executable, it would fork another shell instance to read that file as a shell script. Thus people could type command arguments rather than sh pathname/command arguments. All this behavior was packaged as the function pexec, which is the ancestor of execvp, to allow any program to invoke commands