UNIX shell differences and how to change your shell
Contents
I have a new email address, and from around the end of July my old address
will be no more. Email relating to the content of this document should
now be sent to shell-diff@gryphon.demon.co.uk, while any other email should
be sent to bnb@gryphon.demon.co.uk.
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.
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.
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.
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How much time do I have to learn a new shell?
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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.
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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.
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What support is given for my new shell?
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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.
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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.
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
-
This feature was not in the orginal version, but has since become almost
standard.
-
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.
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The Vi emulation of this shell is thought by many to be incomplete.
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This feature is not standard but unoffical patches exist to perform this.
-
A version called 'pdksh' is freely available, but does not have the full
functionality of the AT&T version.
-
This can be done via the shells programmable completion mechanism.
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A library can be linked into the shell to provide this feature.
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This can be done via the shells prompt function.
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Only by specifing a file via the ENV environment variable.
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.
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 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.
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 rc-request@hawkwind.utcs.toronto.edu,
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 Majordomo@sterling.com 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 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@gryphon.demon.co.uk.
Brian Blackmore, bnb@gryphon.demon.co.uk
html-ified by P. Christias, christia@theseas.ntua.gr
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