What's GNU: Bash—The GNU Shell

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Conclusion of an article started last month. While originally written by Brian Fox of the Free Software Foundation, bash is now maintained by Chet Ramey. In this article, Chet explains the history of shells and then goes on to explore features specific to bash.
Reflections and Lessons Learned

The lesson that has been repeated most often during bash development is that there are dark corners in the Bourne Shell, and people use all of them. In the original description of the Bourne shell, quoting and the shell grammar are both poorly specified and incomplete; subsequent descriptions have not helped much. The grammar presented in Bourne's paper describing the shell distributed with the Seventh Edition of Unix is so far off that it does not allow the command who|wc. In fact, as Tom Duff states:

“Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help.”1

The POSIX.2 standard includes a yacc grammar that comes close to capturing the Bourne shell's behavior, but it disallows some constructs which sh accepts without complaint-and there are scripts out there that use them. It took a few versions and several bug reports before bash implemented sh-compatible quoting, and there are still some “legal” sh constructs which bash flags as syntax errors. Complete sh compatibility is a tough nut.

The shell is bigger and slower than I would like, though the current version is faster than previously.

The readline library could stand a substantial rewrite.

A hand-written parser to replace the current yacc-gener-ated one would probably result in a speedup, and would solve one glaring problem: the shell could parse commands in “$(...)” constructs as they are entered, rather than reporting errors when the construct is expanded.

As always, there is some chaff to go with the wheat. Areas of duplicated functionality need to be cleaned up. There are several cases where bash treats a variable specially to enable functionality available another way ($notify vs. set -o notify and $nolinks vs.

set -o physical, for instance); the special treatment of the variable name should probably be removed. A few more things could stand removal; the $allow_null_ glob_expansion and $glob_dot_filenames variables are of particularly questionable value. The $[...] arithmetic evaluation syntax is redundant now that the POSIX-mandated $((...)) construct has been implemented, and could be deleted. It would be nice if the text output by the help builtin were external to the shell rather than compiled into it. The behavior enabled by $command_oriented_history, which causes the shell to attempt to save all lines of a multi-line command in a single history entry, should be made the default and the variable removed.

Availability

As with all other GNU software, bash is available for anonymous FTP from prep.ai.mit.edu:/pub/gnu and from other GNU software mirror sites. The current version is in bash-1.13.5.tar.gz in that directory. Use archie to find the nearest archive site. The latest version is always available for FTP from bash.CWRU.Edu:/pub/ dist. bash documentation is available for FTP from bash.CWRU.Edu:/pub/bash.

The Free Software Foundation sells tapes and CD-ROMs containing bash; send electronic mail to gnu@prep.ai.mit.edu or call +1-617-876-3296 for more information. bash is also distributed with several versions of Unix-compatible systems. It is included as /bin/sh and /bin/bash on several Linux distributions and as contributed software in BSDI's BSD/386 and FreeBSD.

Conclusion

bash is a worthy successor to sh. It is sufficiently portable to run on nearly every version of Unix from 4.3 BSD to SVR4.2, and several Unix workalikes. It is robust enough to replace sh on most of those systems, and provides more functionality. It has several thousand regular users, and their feedback has helped to make it as good as it is today-a testament to the benefits of free software.

1 Tom Duff, “Rc-A Shell for Plan 9 and UNIX systems”, Proc. of the Summer 1990 EUUG Conf., London, July, 1990, pp. 21-33

2 BSD/386 is a trademark of Berkeley Software Design, Inc.

Chet Ramey (chet@po.cwru.edu) is a programmer at Case Western Reserve University and volunteer at the Free Software Foundation.

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