A BRIEF HISTORY OF UNIX SHELLS
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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. 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 was 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, publishes a paper on it, and Byron Rakitzis reimplements it under UNIX. Rc ended up smaller, simpler, more regular and in most people's 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 that 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. 1) 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 completely 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. 2) What do I wish to be able to do with my new shell? The main reason for switching shells is to gain extra functionality; it's vital you know what you are gaining from the switch. 3) 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. 4) 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. 5) 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. 6) 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. 7) Can I afford any minor bugs? All shells have some bugs in them (especially csh) can you afford the problems that may occur because of them. SHELL FEATURES (0) 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 N Y Y N 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 Can avoid user startup files N N N Y N Y N Can specify start up file N N Y Y N N N 0 This table does not include every single possible feature for every single possible shell, but it does include all the features that I think would make you choose one shell over another. It is *not* a definitive list. 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. HOW TO CHANGE YOUR SHELL If you ever look a UNIX manual 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 anyone has 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 [email protected], while Suggestions and `philosophical' bug reports may be mailed to [email protected] 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 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 protected], 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 [email protected] 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.