The Bourne shell (sh) is a shell command-line interpreter for computer operating systems. It first appeared on Version 7 Unix, as its default shell. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link or hard link to a compatible shell—even when other shells are used by most users.
The Bourne shell was once standard on all branded Unix systems, although historically BSD-based systems had many scripts written in csh. As the basis of POSIX sh syntax, Bourne shell scripts can typically be run with Bash or dash on Linux or other Unix-like systems; Bash itself is a free clone of Bourne.
Contents
History
Origins
Work on the Bourne shell initially started in 1976. Developed by Stephen Bourne at Bell Labs, it was a replacement for the Thompson shell, whose executable file had the same name—sh. The Bourne shell was also preceded by the Mashey shell. Bourne was released in 1979 in the Version 7 Unix release distributed to colleges and universities. Although it is used as an interactive command interpreter, it was also intended as a scripting language and contains most of the features that are commonly considered to produce structured programs.
It gained popularity with the publication of The Unix Programming Environment by Brian Kernighan and Rob Pike—the first commercially published book that presented the shell as a programming language in a tutorial form.
Some of the primary goals of the shell were:
To allow shell scripts to be used as filters
To provide programmability including control flow and variables
Control over all input/output file descriptors
Control over signal handling within scripts
No limits on string lengths when interpreting shell scripts
Rationalize and generalize string quoting mechanism
The environment mechanism. This allowed context to be established at startup and provided a way for shell scripts to pass context to sub scripts (processes) without having to use explicit positional parameters
Features of the original version
Features of the Version 7 UNIX Bourne shell include:
Scripts can be invoked as commands by using their filename
May be used interactively or non-interactively
Allows both synchronous and asynchronous execution of commands
Supports input and output redirection and pipelines
Provides a set of built-in commands
Provides flow control constructs and quotation facilities
Typeless variables
Provides local and global variable scope
Scripts do not require compilation before execution
Does not have a goto facility, so code restructuring may be necessary
Command substitution using backquotes: `command`
Here documents using << to embed a block of input text within a script
for ~ do ~ done loops, in particular the use of $* to loop over arguments, as well as for ~ in ~ do ~ done loops for iterating over lists
case ~ in ~ esac selection mechanism, primarily intended to assist argument parsing
sh provided support for environment variables using keyword parameters and exportable variables
Features introduced after 1979
Over the years, the Bourne shell was enhanced at AT&T. The various variants are thus called like the respective AT&T Unix version it was released with (some important variants being Version7, System III, SVR2, SVR3, SVR4). As the shell was never versioned, the only way to identify it was testing its features.
Features of the Bourne shell versions since 1979 include:
Built-in test command – System III shell (1981)
# as comment character – System III shell (1981)
Colon in parameter substitutions "${parameter:=word}" – System III shell (1981)
continue with argument – System III shell (1981)
cat <<-EOF for indented here documents – System III shell (1981)
Functions and the return builtin – SVR2 shell (1984)
Built-ins unset, echo, type – SVR2 shell (1984)
Source code de-ALGOL68-ized – SVR2 shell (1984)
Modern "$@" – SVR3 shell (1986)
Built-in getopts – SVR3 shell (1986)
Cleaned up parameter handling allows recursively callable functions – SVR3 shell (1986)
8-bit clean – SVR3 shell (1986)
Job control – SVR4 shell (1989)
Multi-byte support – SVR4 shell (1989)
Variants
DMERT shell
Duplex Multi-Environment Real-Time (DMERT) is a hybrid time-sharing/real-time operating system developed in the 1970s at Bell Labs Indian Hill location in Naperville, Illinois uses a 1978 snapshot of Bourne Shell "VERSION sys137 DATE 1978 Oct 12 22:39:57". The DMERT shell runs on 3B21D computers still in use in the telecommunications industry.
Korn shell
The Korn shell (ksh), written by David Korn based on the original Bourne Shell source code, was a middle road between the Bourne shell and the C shell. Its syntax was chiefly drawn from the Bourne shell, while its job control features resembled those of the C shell. The functionality of the original Korn Shell (known as ksh88 from the year of its introduction) was used as a basis for the POSIX shell standard. A newer version, ksh93, has been open source since 2000 and is used on some Linux distributions. A clone of ksh88 known as pdksh is the default shell in OpenBSD.
Schily Bourne Shell
Jörg Schilling's Schily-Tools includes three Bourne Shell derivatives.
Relationship to other shells
C shell
Bill Joy, the author of the C shell, criticized the Bourne shell as being unfriendly for interactive use, a task for which Stephen Bourne himself acknowledged C shell's superiority. Bourne stated, however, that his shell was superior for scripting and was available on any Unix system, and Tom Christiansen also criticized C shell as being unsuitable for scripting and programming.
Almquist shells
Due to copyright issues surrounding the Bourne Shell as it was used in historic CSRG BSD releases, Kenneth Almquist developed a clone of the Bourne Shell, known by some as the Almquist shell and available under the BSD license, which is in use today on some BSD descendants and in low-memory situations. The Almquist Shell was ported to Linux, and the port renamed the Debian Almquist shell, or dash. This shell provides faster execution of standard sh (and POSIX-standard sh, in modern descendants) scripts with a smaller memory footprint than its counterpart, Bash. Its use tends to expose bashisms – bash-centric assumptions made in scripts meant to run on sh.
Other shells
Bash (the Bourne-Again shell) was developed in 1989 for the GNU project and incorporates features from the Bourne shell, csh, and ksh. It is meant to be POSIX-compliant.
rc was created at Bell Labs by Tom Duff as a replacement for sh for Version 10 Unix. It is the default shell for Plan 9 from Bell Labs. It has been ported to UNIX as part of Plan 9 from User Space.
Z shell, developed by Paul Falstad in 1990, is an extended Bourne shell with a large number of improvements, including some features of Bash, ksh, and tcsh.