configuration files (for example, ".vimrc"), is a remnant of the RUNCOM ancestry of Unix shells. The PWB shell or Mashey shell, sh, was an upward-compatible version of the Thompson shell, augmented by John Mashey and others and distributed with the Programmer's Workbench UNIX, circa 1975–1977. It focused on making shell programming practical, especially in large shared computing centers. It added shell variables (precursors of environment variables, including the search path mechanism that evolved into $PATH), user-executable shell scripts, and interrupt-handling. Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, and while/end/break/continue. As shell programming became widespread, these external commands were incorporated into the shell itself for performance. But the most widely distributed and influential of the early Unix shells were the Bourne shell and the C shell. Both shells have been used as the coding base and model for many derivative and work-alike shells with extended feature sets. Bourne shell The Bourne shell, sh, was a new Unix shell by Stephen Bourne at Bell Labs. Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the later Unix shells, including here documents, command substitution, more generic variables and more extensive builtin control structures. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68. Traditionally, the Bourne shell program name is and its path in the Unix file system hierarchy is . But a number of compatible work-alikes are also available with various improvements and additional features. On many systems, sh may be a symbolic link or hard link to one of these alternatives: Almquist shell (ash): written as a BSD-licensed replacement for the Bourne Shell; often used in resource-constrained environments. The sh of FreeBSD, NetBSD (and their derivatives) are based on ash that has been enhanced to be POSIX conformant. Busybox: a set of Unix utilities for small and embedded systems, which includes 2 shells: ash, a derivative of the Almquist shell; and hush, an independent implementation of a Bourne shell. Debian Almquist shell (dash): a modern replacement for ash in Debian and Ubuntu Bourne-Again shell (bash): written as part of the GNU Project to provide a superset of Bourne Shell functionality. This shell can be found installed and is the default interactive shell for users on most Linux systems. KornShell (ksh): written by David Korn based