What are the differences among various shells?


  
  1. A brief History of Unix Shells
  2. A Checklist before Deciding on Your Shell
  3. Comparison of Shell Features

Available shells in CEE UCL related to this topic:

  • sh (/bin/sh)
  • csh (/bin/csh)
  • ksh (/bin/ksh)
  • bash (/usr/local/bin/bash)
  • tcsh (/usr/local/bin/tcsh)
  • zsh (/usr/local/bin/zsh)

A brief History of Unix Shells

  

If you wish to be happy for one hour, get drunk.
If you wish to be happy for three days, get married.
If you wish to be happy for a month, kill your pig and eat it.
If you wish to be happy forever, learn to fish.
- Chinese Proverb

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.

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 shell expression matching functionalities.

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 (University of California, Berkeley) 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 and several other features - Tc shell (/usr/local/bin/tcsh). 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 others. 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 [Portable Operating System - Unix] standard. POSIX specified more or less the System V Bourne Shell [by this time the BSD (Berkeley Software Distribution) 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 the Bourne again shell (/usr/local/bin/bash) 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. Bash was originally written by Brian Fox of the Free Software Foundation. The current developer and maintainer is Chet Ramey of Case Western Reserve University.

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 Z shell (/usr/local/bin/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 kernel functions.

A Checklist before Deciding on Your 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 (i.e., bash or zsh).

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 be able to switch back to a different shell?

If you have to switch back and forth to a current/standard shell, it is quite important you don't become too dependent on extra features only available on the new shell you switched. In other word, switching shell should make your life easier, not more difficult.

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. However, this only really applies to very old systems nowadays. (Go ahead and knock yourself out with experimenting with different shells. CEE UCL can handle all of them quite nicely. )

What support is given for my new shell?

If your new shell is not supported by OCCS USG, 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.

Comparison of Different Shell Features

  

Table below lists most features that you can use to 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.



 Y  Yes Feature can be done using this shell
 N  No Feature is not present in the shell
 F  Function Feature can only be done by using the shells function mechanism
 L  Library The readline library must be linked into the shell to enable this Feature

  Feature   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
       (shell category)
  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  
  Feature   sh csh ksh bash tcsh zsh rc es  

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.