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1991-10-31
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TECHINCAL NOTES FOR CSHELL
==========================
THE PARSER
----------
The heart of CShell is the parser. It was, I suppose, originally taken
from a UNIX csh. It does its job in two steps:
First comes the preformatting, which is done while copying the original
line to a temporary buffer. It is important to make a difference between
quoted and and unquoted characters. Earlier shell versions of the shell
set the most significant bit of quoted chars, but this made the use of
international character sets impossible. Now it's done the other way
round: Every _unquoted_ char with special meaning (e.g. the asterisk) is
replaced by a number between 128 and 159. Ascii code 160 is used as word
separator in strings (so words may contain blanks).
After the preformatting is done, the parser's main task is building up the
argument array av[i] for the internal and external commands. Memory is
allocated for every argument and the argument is put together from one or
more tokens (e.g. $a$b). After av[0] has been completed, certain flags
are set in order to do things like preventing wild card expinsion of
arguments to 'dir'.
Then, with the argument line is complete, the aliases list is searched for
a name that matches av[0]. If it exists, the parser is called recursively
with the alias's command line. Recursive calls of the parser (using the
function execute()) have no more side effects at all since version 5.1 and
can therefore used to do things like automatic cd and class features.
In order to speed up this process with many allocations and deallocations,
I have written a custom storage allocator. It allocates groups of
1000-byte-blocks (called frames) using malloc() and then distibutes them
until they're used up. Of course, deallocation of single allocations is
not possible, only the frame as a whole can be deallocated. But that's
okay, since all temporaries are deallocated when the parser is left.
THE BUILTINS
------------
The other side of the shell are the built in commands. They look much like
CLI commands in C, but they have lots of support functions for their needs.
To write a builtin command for CShell, you use the global variables av and
ac instead of argv and argc, and use printf() for output. That's it. Any
other changes, if necessary, will be done by me.
Every builtin command has its equivalent as a C function with the prefix
do_ (example: do_addbuffers). The are no hard coded jumps to any of those
functions, instead there is a table with the descriptions of all functions
(like name, usage, minimum arguments and, last but not least, a pointer to
that function).
WHY BUILT IN COMMANDS?
----------------------
The reasons why C-Shell still relies heavily on internal commands:
- They can take advantage from each other (e.g. class recognition in the
'dir' command, use of 'search' command in quick cd)
- They have a fast calling sequence
- They need no hard disk seeks, so the startup can be accelerated
significantly
- They are shorter than external commands
- And finally, if you don't like them, don't use them. The wasted resources
become more and more negligible nowadays... CShell uses 2% of my memory.
SOURCES
-------
The source for CShell is available, if you don't have it, request it from
me. It consists of 10 modules, together 200K. It is compilable under SAS
(Lattice) & Manx C, although the executables produced by Manx are not
residentable. If you want to modify it, ask me for an up-to-date version
first. You may not release modified versions (imagine the chaos if
everbody releases his private csh), but if you send them to me, you have a
good chance that I'll build it in. You'll be, of course, be mentioned in
the docs.
Even if you don't intend to modify it, I still encourage you to get the
source, because there are so many things you can look up there once you
need them (did *you* know how to set the system date?). I apologize for my
poor to nonexistent comments, I'll go through again when I've got the time.
Ah, yes, that famous Dillon-formatting is gone, now it's traditionally
formatted.