Re: Is & a command?

In addition to Michael's comment, I think Roger is making the mistake of
treating a specific for a universal.

"*The [ is actually the name of the command*" is not a universal statement
that all [ are commands, or that all opening delimiters are commands, but a
specific statement that in the particular shell they are
implemented/treated/equivalent as commands. The scope of the statement is
that particular language's implementation or design (and perhaps its
"denotational semantics".)

It is as wrongheaded as would be "English has prepositions, so is the XML &
delimiter a preposition?"

On the other hand, if Roger is asking "Is it possible to write a parser for
XML in which each delimiter is used as the 'name' of a 'command' used to
parse the next  section after it?"  then the answer is yes: there are
several classes of parsers that can be implemented like that (e.g. some of
bottom up, shift-reduce, PEG, etc.)

Loosely, a top-down parser says "I am looking now for z, y or z" while a
bottom-up parser say "I am presented now with a z: what are the
ramifications?"  So in the bottom-up case, you could regard of the z as
(the name of) a command, if it helped. (The threaded interpreted languages
like Forth work like that, IIRC:  there is no distinct scanner, so a new
command could be added just using any unique character.)

Parsers can be implemented accepting grammars, or by hardcoding, and by
everything in between.  (The fundamental implementation issue is not
whether a lexical token is treated as a string, a message, a method name, a
command, or whatever, but whether your implementation needs a stack or
not:  a stack is not required to parse XML text, but it is needed one to
judge well-formedness.)

Regards
Rick



On Mon, Nov 14, 2022 at 8:53 PM Roger L Costello <[email protected]> wrote:

> Hi Folks,
>
>
>
> In UNIX there is a command named “test” which is used like this:
>
>
>
>              test expression
>
>
>
> Upon encountering that, a UNIX-aware application (e.g., a UNIX shell) will
> call the test command to evaluate expression.
>
>
>
> Interestingly, these two forms are equivalent:
>
>
>
>              test expression
>
>              [ expression ]
>
>
>
> The book [1] that I am reading says this about the second form:
>
>
>
> The [ is actually the name of the command
>              (who said anything about command names
>              having to be alphanumeric characters?). It
>              still initiates execution of the same test
>              command, only in this format, test expects
>              to see a closing ] at the end of the expression.
>
>
>
> Okay, now to XML ….
>
>
>
> An XML document may contain this:
>
>
>
>              &lt;
>
>
>
> Upon encountering that, an XML-aware application (e.g., an XML parser)
> will call a routine to convert the entity. One might say this:
>
>
>
> The & is actually the name of the command.
>              It initiates execution of the & command and
>              expects to see a closing ; at the end.
>
>
>
> So, …………….
>
>
>
> Is & a command?
>
>
>
> Likewise, ……………..
>
>
>
> Is < a command?
>
>
>
> Is " a command?
>
>
>
> If the answers are no, then what are they? What is the difference between
> a command and whatever those things are?
>
>
>
> /Roger
>
>
>
> [1] UNIX Shell Programming by Kochan and Wood, p. 155.
>