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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. |
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發(fā)表于 2006-03-30 08:20
and C shell to define a standard set of features which all compliant shells must have. 
