UNIX distribution, most notably John Mashey, began modifying the Thompson shell to make it more suitable for programming. 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 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 as part of several Ancient Unix source distributions, and has been ported to modern Unices as a historical exhibit. See also Comparison of command shells References External links Manual page for the Thompson shell in Unix 1st Edition. The Evolution of the Unix Time-Sharing System - describes the early development of the shell Origins of the Bourne shell - manual pages for the 3rd, 4th, and 6th edition Thompson shells, and other resources on the early shells Sixth Edition Thompson Shell Port - the Thompson shell and its associated utilities ported to modern Unix systems Unix shells