<P> 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 . </P> <P> The Bourne shell, sh, was a complete rewrite 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 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 sh and its path in the Unix file system hierarchy is / bin / sh . 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: </P> <Ul> <Li> 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 for the occasion . </Li> <Li> 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 and macOS systems . </Li> <Li> Debian Almquist shell (dash): a modern replacement for ash in Debian and Ubuntu </Li> <Li> Korn shell (ksh): written by David Korn based on the Bourne shell sources while working at Bell Labs </Li> <Li> Public domain Korn shell (pdksh) </Li> <Li> MirBSD Korn shell (mksh): a descendant of the OpenBSD / bin / ksh and pdksh, developed as part of MirOS BSD </Li> <Li> Z shell (zsh): a relatively modern shell that is backward compatible with bash </Li> <Li> 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 . </Li> </Ul> <Li> 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 for the occasion . </Li>

What is the default log in shell for the linux operating system