Editing Thompson shell (section)

==Decline and replacements==
As a result of the simplistic design, by the 1975 release of [[Version 6 Unix]], it was becoming clear that the Thompson shell was inadequate for most serious programming tasks.

At this time, the developers of the [[PWB/UNIX|Programmer's Workbench UNIX]] distribution, most notably [[John Mashey]], began modifying the Thompson shell to make it more suitable for programming.<ref name=Mashey1976-10-13/>
The result, known as the [[PWB shell]] or the Mashey shell, included more advanced flow-control mechanisms and introduced shell variables, but remained limited by the necessity to remain compatible with the Thompson shell.

Finally, the Thompson shell was replaced as the main Unix shell by the [[Bourne shell]] in [[Version 7 Unix]] and the [[C shell]] in [[Berkeley Software Distribution#2BSD .28PDP-11.29|2BSD]], both released in 1979. Since virtually all modern Unix and [[Unix-like]] systems are descended from V7 and 2BSD, the Thompson shell is generally no longer used. It is, however, available as [[open-source software|open-source]] as part of several [[Ancient UNIX|Ancient Unix]] source distributions, and has been ported to modern Unices as a historical exhibit.