in the same way as the shell. 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.) Descendants 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 equivalents of most of the PWB shell's features, but done from scratch, rather than incrementally, with much discussion among the various participants. In particular, environment variables and related machinery were designed by Stephen Bourne, John Mashey, and Dennis Ritchie as a general mechanism to replace the earlier, more limited features. After the adoption of the Bourne shell as the standard shell in Version 7 Unix, use of the PWB shell was phased out, although for a while, there was an internal Bell Labs course called Bourne Shell Programming for Mashey Shell Programmers. The C shell, developed before the public release of the Bourne shell, also inherited some of the features of the PWB shell. See also Comparison of command shells References External links The Traditional Bourne Shell Family: History and Development (Sven Mascheck) Manual page for the PWB shell, 31 May 1977 Article by Mashey on the PWB shell's development and influence Using a Command Language as a High-Level Programming Language Unix shells Text-oriented programming languages Scripting languages