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. First appearing in Version 7 Unix, the Bourne shell superseded the Mashey shell.
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. 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 ALGOL 68C compiler that he had been working on at Cambridge University. In addition to the style in which the program was written, Bourne reused portions of ALGOL 68's if ~ then ~ elif ~ then ~ else ~ fi, case ~ in ~ esac and for/while ~ do ~ od" (using done instead of od) clauses in the common Unix Bourne shell syntax. Moreover, – although the v7 shell is written in C – Bourne took advantage of some macros to give the C source code an ALGOL 68 flavor. These macros (along with the finger command distributed in Unix version 4.2BSD) inspired the International Obfuscated C Code Contest (IOCCC).
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, SystemIII, 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: