Bourne shell

The Bourne shell (sh) is a shell command-line interpreter for computer operating systems.

The Bourne shell was the default shell for Version 7 Unix. 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.

Developed by Stephen Bourne at Bell Labs, it was a replacement for the Thompson shell, whose executable file had the same name—sh. It 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.

History

Origins

Work on the Bourne shell initially started in 1976.^[1] First appearing in Version 7 Unix,^[2] the Bourne shell was preceded by the Mashey shell.

Some of the primary goals of the shell were:^[3]

• 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
  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
• 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.
• Contains strong provisions for controlling input and output and in its expression matching facilities.

The Bourne shell also was the first to feature the convention of using file descriptor 2> for error messages, allowing much greater programmatic control during scripting by keeping error messages separate from data.

Stephen Bourne's coding style was influenced by his experience with the

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.[6]

Features of the Bourne shell versions since 1979 include:^[7]

• 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 (

Bill Joy, the author of the C shell, criticized the Bourne shell as being unfriendly for interactive use,^[10] 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,^[11] and Tom Christiansen also criticized C shell as being unsuitable for scripting and programming.^[12]

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.
• 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.

Usage

The Bourne shell was once standard on all branded

• Comparison of command shells
• Unix shell

References

External links

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Wikibooks has a book on the topic of: Bourne Shell Scripting

• The individual members of "The Traditional Bourne Shell Family"
• "Characteristical common properties of the traditional Bourne shells"
• Historical C source code for the Bourne shell using mac.h macros from 1979
• Original Bourne Shell documentation from 1978
• A port of the "heirloom" SVR4 Bourne shell from OpenSolaris to some other Unix-like systems
• Migrating from the System V (SVR4) Shell to the POSIX Shell
• Bourne Shell Tutorial (syntax)
• Faqs shell differences
• Howard Dahdah, The A–Z of Programming Languages: Bourne shell, or sh – An in-depth interview with Steve Bourne, creator of the Bourne shell, or sh, Computerworld, 5 March 2009.