What's GNU: Bash—The GNU Shell
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.
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 firstname.lastname@example.org 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.
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 (email@example.com) is a programmer at Case Western Reserve University and volunteer at the Free Software Foundation.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide