Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!newsfeed.internetmci.com!newsxfer3.itd.umich.edu!news-peer.sprintlink.net!news.sprintlink.net!Sprint!cpk-news-hub1.bbnplanet.com!news.bbnplanet.com!europa.clark.net!dispatch.news.demon.net!demon!gryphon.demon.co.uk!news.looking-glass.org!news.looking-glass.org!not-for-mail
From: Brian Blackmore <bnb@gryphon.demon.co.uk>
Newsgroups: comp.unix.shell,comp.unix.questions,news.answers,comp.answers
Subject: UNIX shell differences and how to change your shell (Monthly Posting)
Supersedes: <shell_866063613@news.looking-glass.org>
Followup-To: comp.unix.shell
Date: 13 Jul 1997 17:41:35 +0100
Organization: None
Sender: bnb@looking-glass.org
Approved: news-answers-request@MIT.Edu
Expires: 24 Aug 1997 16:41:27 GMT
Message-ID: <shell_868812087@news.looking-glass.org>
Reply-To: shell-diff@looking-glass.org
NNTP-Posting-Host: gryphon.looking-glass.org
X-NNTP-Posting-Host: gryphon.demon.co.uk [158.152.42.76]
Lines: 369
Xref: senator-bedfellow.mit.edu comp.unix.shell:51716 comp.unix.questions:130975 news.answers:107262 comp.answers:27074

 The following article answers the frequently asked questions, what
 UNIX shells are available, what are the differences between them and
 how do you change your interactive shell. It is posted monthly to the
 USENET newsgroups comp.unix.shell, comp.unix.questions, news.answers
 and comp.answers and is additionally available on the world wide web
 as http://www.wonderland.org/~eternal/shell.html
 
Contents
 * Modifications since last issue
 * Why change your shell
 * The history of unix shells
 * Deciding on a shell
 * Shell features (table)
 * How to change your shell
 * A warning about changing your shell
 * Further information
 * Copyright and Disclaimer
 
Modifications since last issue
 * Change of authors contact addresses
 
Why change your shell
 The UNIX shell is most people's main access to the UNIX operating
 system and as such any improvement to it can result in considerably
 more effective use of the system, and may even allow you to do things
 you couldn't do before. The primary improvement most of the new
 generation shells give you is increased speed. They require fewer key
 strokes to get the same results due to their completion features, they
 give you more information (e.g. showing your directory in your prompt,
 showing which files it would complete) and they cover some of the more
 annoying features of UNIX, such as not going back up symbolic links to
 directories.
 
A brief history of UNIX shells
 Note, this history is just known to be slightly out of historical
 order, it is in the process of being corrected, but for the moment
 should be taken with a pinch of salt
 
 In the near beginning there was the Bourne shell /bin/sh (written by
 S. R. Bourne). It had (and still does) a very strong powerful
 syntactical language built into it, with all the features that are
 commonly considered to produce structured programs; it has
 particularly strong provisions for controlling input and output and in
 its expression matching facilities. But no matter how strong its input
 language is, it had one major drawback; it made nearly no concessions
 to the interactive user (the only real concession being the use of
 shell functions and these were only added later) and so there was a
 gap for something better.
 
 Along came the people from UCB and the C-shell /bin/csh was born. Into
 this shell they put several concepts which were new, (the majority of
 these being job control and aliasing) and managed to produce a shell
 that was much better for interactive use. But as well as improving the
 shell for interactive use they also threw out the baby with the bath
 water and went for a different input language.
 
 The theory behind the change was fairly good, the new input language
 was to resemble C, the language in which UNIX itself was written, but
 they made a complete mess of implementing it. Out went the good
 control of input and output and in came the bugs. The new shell was
 simply too buggy to produce robust shell scripts and so everybody
 stayed with the Bourne shell for that, but it was considerably better
 for interactive use so changed to the C shell, this resulted in the
 stupid situation where people use a different shell for interactive
 work than for non-interactive, a situation which a large number of
 people still find themselves in today.
 
 After csh was let loose on an unsuspecting world various people
 decided that the bugs really should get fixed, and while they where at
 it they might as well add some extra features. In came command line
 editing, TENEX-style completion and several other features. Out went
 most of the bugs, but did the various UNIX operating system
 manufacturers start shipping tcsh instead of csh? No, most of them
 stuck with the standard C-Shell, adding non-standard features as they
 went along.
 
 Eventually David Korn from AT&T had the bright idea to sort out this
 mess and the Korn shell /bin/ksh made its appearance. This quite
 sensibly junked the C shells language and reverted back to the bourne
 shell language, but it also added in the many features that made the C
 shell good for interactive work (you could say it was the best of both
 worlds), on top of this, it also added a some features from other
 operating. The Korn shell became part of System V but had one major
 problem; unlike the rest of the UNIX shells it wasn't free, you had to
 pay AT&T for it.
 
 It was at about this time that the first attempts to standardize UNIX
 started in the form of the POSIX standard. POSIX specified more or
 less the System V Bourne Shell (by this time the BSD and System V
 versions had got slightly different). Later the standard is upgraded,
 and somehow the new standard managed to look very much like ksh.
 
 Also at about this time the GNU project was underway and they decided
 that they needed a free shell, they also decided that they wanted to
 make this new shell POSIX compatible, thus bash (the Bourne again
 shell) was born. Like the Korn shell bash was based upon the Bourne
 shells language and like the Korn shell, it also pinched features from
 the C shell and other operating systems (in my opinion it put them
 together better; guess which shell I use), but unlike the Korn shell
 it is free. Bash was quickly adopted for LINUX (where it can be
 configured to perform just like the Bourne shell), and is the most
 popular of the free new generation shells.
 
 Meanwhile Tom Duff faced with the problem of porting the Bourne shell
 to Plan 9, revolts and writes rc instead, he publishes a paper on it,
 and Byron Rakitzis reimplements it under UNIX. With the benefit of a
 clean start Rc ended up smalled, simpler, more regular and in most
 peoples opinion a much cleaner shell.
 
 The search for the perfect shell still goes on and the latest entry
 into this arena is zsh. Zsh was written by Paul Falstad while he was a
 student a Princeton and suffers from slight case of feeping
 creaturism. It is based roughly on the bourne shell (although there
 are some minor but important differences) and has so many additional
 features that I don't even think the author even knows all of them.
 
 Additionally rc has been enhanced to produced es, this shell adds the
 ability for the user to redefine low level functions.
 
Deciding on a shell
 Which of the many shells you choose depends on many different things,
 here is what I consider to be the most important, you may think
 differently.
 
 How much time do I have to learn a new shell?
 There is no point in using a shell with a different syntax, or
 a completly different alias system if you havn't the time to
 learn it. If you have the time and are presently using csh or
 tcsh it is worth considering a switch to a Bourne shell
 variant.
 
 What do I wish to be able to do with my new shell?
 The main reason for switching shells is to gain extra
 functionality; its vital you know what you are gaining from the
 switch.
 
 Do I have to be able to switch back to a different shell?
 If you may have to switch back to a standard shell, it is
 fairly important you don't become too dependent on extra
 features and so can't use an older shell.
 
 How much extra load can the system cope with?
 The more advanced shells tend to take up extra CPU, since they
 work in cbreak mode; if you are on an overloaded machine they
 should probably be avoided; this can also cause problems with
 an overloaded network. This only really applies to very old
 systems nowadays.
 
 What support is given for my new shell?
 If your new shell is not supported make sure you have someone
 you can ask if you encounter problems or that you have the time
 to sort them out yourself.
 
 What shell am I using already?
 Switching between certain shells of the same syntax is alot
 easier than switching between shells of a different syntax. So
 if you havn't much time a simple upgrade (eg csh to tcsh) may
 be a good idea.
 
 Can I afford any minor bugs?
 Like most software all shells have some bugs in them
 (especially csh), can you afford the problems that may occur
 because of them.
 
 Do you need to be able to use more than one shell?
 If you use more than one machine you may discover that you need
 to use more than one shell regularly. How different are these
 shells and can you cope with having to switch between these
 shells on a regular basis. It may be to your advantage to
 choose shells that are similar to each other.
 
Shell features
 This table below lists most features that I think would make you
 choose one shell over another. It is not intended to be a definitive
 list and does not include every single possible feature for every
 single possible shell. A feature is only considered to be in a shell
 if in the version that comes with the operating system, or if it is
 available as compiled directly from the standard distribution. In
 particular the C shell specified below is that available on SUNOS 4.*,
 a considerable number of vendors now ship either tcsh or their own
 enhanced C shell instead (they don't always make it obvious that they
 are shipping tcsh.
 sh csh ksh bash tcsh zsh rc es
Job control N Y Y Y Y Y N N
Aliases N Y Y Y Y Y N N
Shell functions Y(1) N Y Y N Y Y Y
"Sensible" Input/Output redirection Y N Y Y N Y Y Y
Directory stack N Y Y Y Y Y F F
Command history N Y Y Y Y Y L L
Command line editing N N Y Y Y Y L L
Vi Command line editing N N Y Y Y(3) Y L L
Emacs Command line editing N N Y Y Y Y L L
Rebindable Command line editing N N N Y Y Y L L
User name look up N Y Y Y Y Y L L
Login/Logout watching N N N N Y Y F F
Filename completion N Y(1) Y Y Y Y L L
Username completion N Y(2) Y Y Y Y L L
Hostname completion N Y(2) Y Y Y Y L L
History completion N N N Y Y Y L L
Fully programmable Completion N N N N Y Y N N
Mh Mailbox completion N N N N(4) N(6) N(6) N N
Co Processes N N Y N N Y N N
Builtin artithmetic evaluation N Y Y Y Y Y N N
Can follow symbolic links invisibly N N Y Y Y Y N N
Periodic command execution N N N N Y Y N N
Custom Prompt (easily) N N Y Y Y Y Y Y
Sun Keyboard Hack N N N N N Y N N
Spelling Correction N N N N Y Y N N
Process Substitution N N N Y(2) N Y Y Y
Underlying Syntax sh csh sh sh csh sh rc rc
Freely Available N N N(5) Y Y Y Y Y
Checks Mailbox N Y Y Y Y Y F F
Tty Sanity Checking N N N N Y Y N N
Can cope with large argument lists Y N Y Y Y Y Y Y
Has non-interactive startup file N Y Y(7) Y(7) Y Y N N
Has non-login startup file N Y Y(7) Y Y Y N N
Can avoid user startup files N Y N Y N Y Y Y
Can specify startup file N N Y Y N N N N
Low level command redefinition N N N N N N N Y
Has anonymous functions N N N N N N Y Y
List Variables N Y Y N Y Y Y Y
Full signal trap handling Y N Y Y N Y Y Y
File no clobber ability N Y Y Y Y Y N F
Local variables N N Y Y N Y Y Y
Lexically scoped variables N N N N N N N Y
Exceptions N N N N N N N Y
Key to the table above.
 Y Feature can be done using this shell.
 
 N Feature is not present in the shell.
 
 F Feature can only be done by using the shells function
 mechanism.
 
 L The readline library must be linked into the shell to enable
 this Feature.
 
Notes to the table above
 1. This feature was not in the orginal version, but has since become
 almost standard.
 2. This feature is fairly new and so is often not found on many
 versions of the shell, it is gradually making its way into
 standard distribution.
 3. The Vi emulation of this shell is thought by many to be
 incomplete.
 4. This feature is not standard but unoffical patches exist to
 perform this.
 5. A version called 'pdksh' is freely available, but does not have
 the full functionality of the AT&T version.
 6. This can be done via the shells programmable completion mechanism.
 7. Only by specifing a file via the ENV environment variable.
 
How to change your shell
 If you ever look at a UNIX manual page it will say that to change your
 shell use chsh or passwd -s; unfortunately it often isn't as simple as
 this, since it requires that your new shell is recognized as a valid
 shell by the system and at present many systems do not recognize the
 newer shells (the normal selection is, /bin/sh, /bin/csh and possibly
 /bin/ksh). You are thus left with having to do some sort of fudge,
 changing your effective login shell without changing your official
 entry in /etc/passwd. You may also be left with the problem that there
 isn't a compiled binary on your system , so you will have to get hold
 of the shell's source and compile it yourself (Its generally best to
 ask around to see if anyones done this already, since it isn't that
 easy). Once done you should add in code to your old shells login file
 so that it overlays your official login shell with your new shell
 (remember to add the login flags to the command line, and with
 csh/tcsh ensure that the overlay doesn't happen recursively since they
 both read the same .login file).
 
 The shell can be recognized as a valid shell if the system
 administrator puts it in the file /etc/shells. If this file does not
 exist, it must be created and should contain all valid shells
 (i.e.don't forget the traditional ones in all their forms).
 
WARNING
 If you do decide to change your shell you must be very careful - if
 handled wrongly it can be almost impossible to correct, and will
 almost certainly cause you a lot of hassle. Never make a new shell a
 login shell until you have tested its new configuration files
 thoroughly and then tested them once again. It is also important that
 you make a full backup of your previous config files onto a floppy
 disk (or a different host if you have a second account) if you have to
 change any of them (which you will probably have to do if you can't
 change your shell entry in /etc/passwd). You should also note that
 your new shell is probably not supported by your system admin, so if
 you have any problems you will probably have to look elsewhere.
 
Further information
 The Bourne shell, the C-Shell and the Korn Shell (if you have it) are
 all distributed as standard with your UNIX operating system,
 information on these should come with your operating system, bug
 reports etc should be sent to your operating system vendor. The free
 version of ksh, pdksh is available as ftp://ftp.cs.mun.ca/pub/pdksh/
 
 A commercial "compiler" (CCsh) is available for the Bourne shell, this
 can provide extra speed on some applications. For more information
 contact Comeau Computing 91-34 120th Street, Richmond Hill, NY, USA
 11418-3214. Email comeau@csanta.attmail.com.
 
 Bash was written and is maintained by the Free Software Foundation,
 the primary source of information for this shell is its manual page.
 Bug reports should be sent to bash-maintainers@ai.MIT.Edu, while
 suggestions and philosophical bug reports may be mailed to
 bug-bash@ai.MIT.Edu or posted to the Usenet newsgroup gnu.bash.bug,
 the source is widely available on many ftp sites, and is subject to
 the GNU copyleft licence.
 
 Tcsh is available by ftp from ftp://ftp.deshaw.com and several other
 places.
 
 Rc is available by ftp from ftp://viz.tamu.edu/pub/rc,
 ftp://ftp.sys.utotonoto.cs/pub/rc and several other places. An FAQ
 exists and is posted frequently to comp.unix.shell and other places.
 The Rc mailing list may be subscribed to by sending mail to
 rc-request@hawkwind.utcs.toronto.edu, this, the manual page and the Rc
 FAQ are the main sources of information for this shell. The original
 paper on rc is available as
 http://plan9.att.com/plan9/plan9doc/7.ps.Z.
 
 Zsh is available by ftp from ftp://ftp.math.gatech.edu/pub/zsh and at
 several mirror sites. Zsh is now maintained by the members of the zsh
 mailing lists, which can be subscribed to by sending email to
 zsh-announce-request@math.gatech.edu (announcements),
 zsh-users-request@math.gatech.edu (users discussions) or
 zsh-workers-request@math.gatech.edu (zsh hacking and development) with
 a subject of "subscribe email-address", there is also an FAQ which
 is posted frequently to comp.unix.shell. The manual page, the Z-shell
 FAQ and the zsh-list are the main sources of information for this
 shell.
 
 ES is available by ftp from ftp://ftp.cs.utoronto/pub/es and at
 several other places, a mailing list can be subscribed to by sending
 mail to es-request@hawkwind.utcs.toronto.edu and a paper about es is
 at http://www.webcom.com/~haahr/es/es-usenix-winter93.html.
 
 Questions on any of the UNIX shells and on shell script programming,
 may be posted to the Usenet newsgroup comp.unix.shell a quick response
 can normally be expected, especially on subjects relating to the more
 common shells.
 
Copyright and Disclaimer
 Copyright to this document is kept by the author, but freedom is
 given to distribute it as long as no money is made from its
 distribution, without the prior concent of the author. The author also
 does not guarantee that the information it contains is correct,
 although every effort is done to ensure that it is.
 
 Email relating to the content of this document should be sent to
 shell-diff@looking-glass.org
 
 Brian Blackmore, bnb@looking-glass.org