Scripting support

[Marco Gerards <metgerards@student.han.nl>]

Hi,

Last days I have been thinking about scripting support.  With my last
patch it should be easy to implement.  I hope Vladimir is still
interested in this.

The most important is the general design.  I think we shouldn't put
too much in the core and not even in normal mode.  This is the same
approach bash has taken.  I think the core functionality should be:

- variables (we have that)
- functions
    Functions should work *exactly* like normal GRUB commands.  In
    that case we can easily mix GRUB commands and functions.
- Return values
    All commands should be able to return a value. That's also what
    bash has. This value can be stored in a variable or simply
    returned, I am not too sure about that.  Just check what bash
    does.
- Conditional statements like if and case
- loops: until, while and for

Whenever a command line should be executed or when a script file is
executed, it's the same.  The block of code is just parsed and
executed by a function (the parser, I guess :)).

It's important to see that GRUB does not do anything like
calculations.  All this can be moved into a command.  When making a
bash script you have the same, there is the expr command.  So we have
to keep the core functionality minimal and should offer the commands
in command/ like we do now.

First the lexer.  I think it should work like this:

normal/parser.c should contain the lexer.  The prototypes etc. can
just be added to include/grub/parser.h.

There are just a few tokens we have to recognize.  Think about stuff
like a string of text, if, elif, case, ;, [], etc.

The parser can be quite simple.  It has to support all kinds of loops,
etc.  I am not sure how to deal with that.  perhaps we have to feed
the block within a for loop (for example) to the parser until the for
loop exits.  Another possibility is creating some kind of P-code and
parse that later on.  I wonder what the best approach is.

For the parser we should use bison, I think.  Writing a handwritten
parser is acceptable if some standard approach is taken and it's
really really clean and well documented.

Any idea is welcome.  I hope we can discuss the implementation in
detail so we can have scripting support soon.

Thanks,
Marco