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From: casper@fwi.uva.nl (Casper H.S. Dik)
Newsgroups: comp.lang.perl
Subject: Re: Double-ended pipes
Keywords: pipe
Message-ID: <1991Jun14.111448.25093@fwi.uva.nl>
Date: 14 Jun 91 11:14:48 GMT
References: <1991Jun10.180301.7925@NCoast.ORG> <14054@pasteur.Berkeley.EDU>
Sender: news@fwi.uva.nl
Organization: FWI, University of Amsterdam
Lines: 65
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lars@yosemite.berkeley.edu (Lars E. Thon) writes:

>One feature that I have been missing from perl is having a double-ended
>pipe. This could be useful f.ex. if you want to feed a lot of commands
>to a csh and then analyze output and/or status from the commands. For a
>simple (and contrived) example, consider the following:

>#! /usr/local/gnu/bin/perl

>doublepipe_open(CSH_IN, "| /bin/csh -f |", CSH_OUT); 

>#Silly way of checking whether user ~blah exists
>print CSH_IN "echo ~blah";
>$pathname= <CSH_OUT>;
>print CSH_IN "echo \$status";
>$errcode= <CSH_OUT>;

>if ($errcode) {
>     print "User blah does not exist ...\n";
>}
># END

>I haven't seen a way to do this in perl, except for the obvious (but
>unsatisfactory) method of using a temp file to store the csh output.

>Any ideas about this?
>--

You can easily roll your own:

sub double_pipe {
    local($in,$cmd,$out) = @_;
    # Create an in and an out pipe. Make them unbuffered.
    pipe($out,WTMP);
    pipe(RTMP,$in);
    select($in); $| = 1;
    select($out); $| = 1;
    select(STDOUT);

    unless (fork) {
	# close the unused sides in the child.
	close($in); close($out);

	open(STDIN,"<&RTMP");
	open(STDOUT,">&WTMP");
	# Join stdin and stdout.
	open(STDERR,">&WTMP");
	close(RTMP); close(WTMP);
	exec $cmd;
	die "Can't execute $cmd\n";
    }
    # close the unused sides in the parent.
    close(WTMP); close(RTMP);

}

&double_pipe(SH_IN,"sh",SH_OUT);
print SH_IN "ls -l\n";			# NOTE: trailing \n, and no |'s.
close(SH_IN);
while (<SH_OUT>) { print $_ ; }

#Casper
--
						|	Casper H.S. Dik
	RELIGION KILLS				|	casper@fwi.uva.nl