Re: How to implement Variable Substitution / Interpolation

[Hans Aberg]

On 17 Sep 2009, at 19:52, Kyle Brandt wrote:

> I am just learning flex / bison and I am writing my own shell with  
> it.  I am
> trying to figure out a good way to do variable interpolation.

You might inquiry in the Usenet newsgroup comp.compilers, which can  
give better advice on general parsing problems. There are probably  
examples out there.

> My initial
> approach to this was to have flex scan for something like ~ for my  
> home
> directory, or $myVar , and then set what the yyval.string to what is
> returned using a look up function. My problem is that this doesn't  
> help me
> when text appears as one token:
>
> kbsh:/home/kbrandt% echo ~
> /home/kbrandt
> kbsh:/home/kbrandt% echo ~/foo
> /home/kbrandt /foo
> kbsh:/home/kbrandt%

I think the normal thing, though, would be to be letting the lexer  
just parse the tokens, and hand that over to the parser, which in turn  
builds the actions. So a grammar perhaps like:
%token TILDE
%token SLASH

%%
command_line:
  command args
  ;

args:
    /* empty */
  | args arg
  ;

arg:
    TILDE other_stuff
    SLASH other_stuff
  | other_stuff
  ;

other_stuff: ... ;

Also, the man page of say bash, section invocation, describes in  
detail how commands are interpreted, which can be useful if you do not  
have an example grammar.

  Hans