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Re: Shell Command Substitution and fork()

    Date:        Tue, 21 Jun 2016 20:46:11 +0200
    From:        Edgar =?iso-8859-1?B?RnXf?= <>
    Message-ID:  <>

  | Does anyone know how those ksh's achieve that? Are there any drawbacks?

I haven't look at their sources, but I would assume they don't fork.

  | Could ash be taught the same thing?

It already has code to attempt to do that, but it doesn't currently
work properly in all cases, so has been disabled (long ago).   Of course
there is only a big speedup when the command is a shell builtin, but printf
qualifies (as does echo of course) for that.   But even for other cases,
when this eventually gets fixed other (simple) commands would be able to
just vfork() - as it is now a full fork() is required.

[As I understand it, the issue is with correct cleanup, especially in the
case of errors - nothing n the sub-shell environ is allowed to affect the
parent shell, so anything in the cmd-sub that changes anything at all has
to be undone - fork() makes that simple to get right, exit() cleans up

  | Or can someone think of a POSIX-compliant way to put the output of a
  | command (think printf) into a variable without Command Substitution?

In general, no, that's what command substitution is for after all
(to get the output of a command into the command line, so it can then
be assigned, or whatevr other use you need it for.)

But in some cases, depending upon exactly what the printf is doing,
there can be other ways.


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