Re: command substitution: zsh waits until command exits

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

On Nov 30,  2:39pm, Vincent Lefevre wrote:
}
} but doesn't say when zsh does the substitution and runs the main
} command (it should be clarified).

Look earlier in that man page.  Almost the first thing it says is:

   The following types of expansions are performed in the indicated
   order in five steps:

with "Command Substitution" listed as happening at the third step,

   ... in left-to-right fashion.

(i.e., in the order it is found when scanning the command line).

} For instance, with the following
} command
} 
}   echo $(echo foo; exec >&-; sleep 2)
} 
} zsh waits for 2 seconds before outputting 'foo'. However, since the
} standard output fd of the substituted command has been closed, I assume
} that zsh should be able to replace $(echo foo; exec >&-; sleep 2) by
} 'foo' and run the main command before the command inside $(...) exits.
} Is this a bug?

That really has nothing to do with "when zsh does the substitution" and
everything to do with job control.  It's not a bug.

} Otherwise, is there a way to do what I wish (without temporary files)?

Sure -- background the command yourself, e.g.:

  echo $({ echo foo ; exec >&- ; sleep 2; echo bar >/dev/tty } &)

} More precisely, I want to do something like:
} 
}   some_command $(xterm -e 'tty >&3; exec 3>&-; sleep 999999' 3>&1)
} 
} so that some_command can write data to the xterm (in addition to the
} normal output).

That one is a bit tricky and the problem is that xterm doesn't close
the descriptors that it passes through to the command it's running, so
even when "exec 3>&-" runs *inside* the xterm, the descriptor is still
open.  So you have to run xterm in a way that it's descriptors don't
matter.  Something like:

  echo $(coproc xterm -e 'tty >&3; sleep 999999' 3>&1 ; read -E <&p)