Wednesday, May 28, 2008

UNIX shell differences and how to change your shell

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.questions, news.answers
and comp.answers and is additionally available on the world wide web


* 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

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

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

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

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

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
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).


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

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

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 and several other

Rc is available by ftp from,
ftp://ftp.sys.utotonoto.cs/pub/rc and several other places. An FAQ
exists and is posted frequently to and other places.
The Rc mailing list may be subscribed to by sending mail to, 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

Zsh is available by ftp from 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 (announcements), (users discussions) or (zsh hacking and development) with
a subject of "subscribe email-address", there is also an FAQ which
is posted frequently to The manual page, the Z-shell
FAQ and the zsh-list are the main sources of information for this

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 and a paper about es is

Questions on any of the UNIX shells and on shell script programming,
may be posted to the Usenet newsgroup a quick response
can normally be expected, especially on subjects relating to the more
common shells.

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