UNIX shell differences and how to change your shell
Brian Blackmore, firstname.lastname@example.org
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 email@example.com,
while any other email should be sent to firstname.lastname@example.org.
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
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
- 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
- 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
- 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
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
- 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
- The Vi emulation of this shell is thought by many to be
- This feature is not standard but unoffical patches exist to perform
- 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
- A library can be linked into the shell to provide this feature.
- This can be done via the shells prompt function.
- 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
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
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
email@example.com, 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
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
Brian Blackmore, firstname.lastname@example.org
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