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Subject: UNIX shell differences and how to change your shell (Monthly Posting)
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Archive-name: unix-faq/shell/shell-differences
Version: 1.05

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.

Contents
========

 o Modifications since last issue
 o Why change your shell
 o The history of unix shells
 o Deciding on a shell
 o Shell features (table)
 o How to change your shell
 o A warning about changing your shell
 o Further information
 o Copyright and Disclaimer

Modifications since last issue
==============================

 o Fixed entry for bash under history completion, it does it (M-^)
 o Added 2 new entries in shell features table for Has
   non-interactive startup file and has non-login startup file.

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 a lot less 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
==============================

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, they stuck with the standard C-Shell.

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 faced with the problem of porting the Bourne shell to Plan 9,
Tom Duff revolts and writes rc, he publishes a paper on it, and Byron
Rakitzis reimplements it under UNIX. Rc ended up smalled, simpler,
more regular and in most peoples opinion a much better programmed
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 is a feature packed shell which has so many features that I
don't even think the he even knows all of them.

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.
 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.

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.

                                        sh   csh   ksh  bash tcsh zsh  rc
Job control                             N    Y     Y    Y    Y    Y    N
Aliases                                 N    Y     Y    Y    Y    Y    N
Shell functions                         Y(1) N     Y    Y    N    Y    Y
"Sensible" Input/Output redirection     Y    N     Y    Y    N    Y    Y
Directory stack                         N    Y     Y    Y    Y    Y    N
Command history                         N    Y     Y    Y    Y    Y    N(7)
Command line editing                    N    N     Y    Y    Y    Y    N(7)
Vi Command line editing                 N    N     Y    Y    Y(3) Y    N(7)
Emacs Command line editing              N    N     Y    Y    Y    Y    N(7)
Rebindable Command line editing         N    N     N    Y    Y    Y    N(7)
User name look up                       N    Y     Y    Y    Y    Y    N(7)
Login/Logout watching                   N    N     N    N    Y    Y    N
Filename completion                     N    Y(1)  Y    Y    Y    Y    N(7)
Username completion                     N    Y(2)  Y    Y    Y    Y    N(7)
Hostname completion                     N    Y(2)  Y    Y    Y    Y    N(7)
History completion                      N    N     N    Y    Y    Y    N(7)
Fully programmable Completion           N    N     N    N    Y    Y    N
Mh Mailbox completion                   N    N     N    N(4) N(6) N(6) N
Co Processes                            N    N     Y    N    N    Y    N
Builtin artithmetic evaluation          N    Y     Y    Y    Y    Y    N
Can follow symbolic links               N    N     Y    Y    Y    Y    N
Periodic command execution              N    N     N    N    Y    Y    N
Custom Prompt (easily)                  N    N     Y    Y    Y    Y    Y
Sun Keyboard Hack                       N    N     N    N    N    Y    N
Spelling Correction                     N    N     N    N    Y    Y    N
Process Substitution                    N    N     N    Y(2) N    Y    Y
Underlying Syntax                       sh   csh   sh   sh   csh  sh   rc
Freely Available                        N    N     N(5) Y    Y    Y    Y
Checks Mailbox                          N    Y     Y    Y    Y    Y    N(8)
Tty Sanity Checking                     N    N     N    N    Y    Y    N
Can cope with large argument lists      Y    N     Y    Y    Y    Y    Y
Has non-interactive startup file        N    Y     Y(9) Y(9) Y    Y    N
Has non-login startup file              N    Y     Y(9) Y    Y    Y    N
Can avoid user startup files            N    N     N    Y    N    Y    Y
Can specify startup file                N    N     Y    Y    N    N    N

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. A library can be linked into the shell to provide this feature.
 8. This can be done via the shells prompt function.
 9. 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.

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 [e-mail], while
suggestions and philosophical bug reports may be mailed to
[e-mail] 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.

Rc is available by ftp from ftp:viz.tamu.edu/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
[e-mail], this, the manual page and the Rc
FAQ are the main sources of information for this shell.

Zsh is now maintained by the zsh mailing list, which can be subscribed to
by sending email to [e-mail] containing subscribe
zsh-list, 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.

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
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.

of Kent at Canterbury, [e-mail]