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The PWB shell (also known as the Mashey shell) was an early Unix shell distributed with some versions of Programmer's Workbench UNIX circa 1975-1977. It was a modified version of the Thompson shell with additional features to increase usability for programming, and was maintained by John Mashey and various others.
Although it was soon superseded by the Bourne shell, several features introduced in the PWB shell remain in many later shells. The if and goto commands were made internal to the shell, and switch and while constructs were introduced. Simple variables could be used, although their names were limited to one letter and some letters were reserved for special purposes. The $ character, used previously for identifying arguments to a shell script, became the marker for dereferencing a variable, and could be used to insert a variable's value into a string in double quotes. (In addition to later shells, this feature would also later appear in the Perl and PHP programming languages.)
These features could not overcome the shortcomings of the Thompson shell, and so a new shell was written from scratch by Stephen Bourne. This Bourne shell was incompatible with the Thompson and PWB shells, but included several of PWB shell's features. After the adoption of the Bourne shell as the standard shell in Version 7 Unix, use of the PWB shell soon died out. (The C shell, developed before the public release of the Bourne shell, also inherited some of the features of the PWB shell.)
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