Re: exec | wc isn't very useful...

[Peter Stephenson <pws@xxxxxxx>]

Anthony Heading wrote:
> > A pipeline is fundamentally different from a redirection.
> 
> I don't quite see that - aside from the very real underlying
> implementation issues, what is wrong with a conceptual model
> that views:
>     a | b | c
> as pure syntactic sugar for:
>     a > >(b > >(c))
> ?

Currently, the combination > >(...) is not special; >(...) produces a
file name (either a named pipe or a /dev/fd entry) and > redirects into
it.  In principle that could be changed to use pipes internally.

There are still some major differences, however.

First, a > >(b) creates b as a subprocess and then runs a.  This is
different from a | b as here a is created in a subprocess, then b is
run.  Actually, some shells do it the other way around, but there are
some good uses for having b running in the main shell.

Second, the shell doesn't wait for the processes in the command
substitution to exit.  This is different from a pipeline where the shell
waits for the all the processes before the job is finished.

A more subtle, but possibly the most critical, difference is that in
a > >(b > >(c))
the process c is forked from b, not from the parent shell.  This means
that the parent shell isn't even aware of c.  This makes it impossible
to treat as a pipeline where all the processes are forked from (or run
in) the parent shell.

-- 
Peter Stephenson <pws@xxxxxxx>                  Software Engineer
CSR PLC, Churchill House, Cambridge Business Park, Cowley Road
Cambridge, CB4 0WZ, UK                          Tel: +44 (0)1223 692070


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