pschoonveld has asked for the wisdom of the Perl Monks concerning the following question:

I need to execute a program, write some data to its stdin and then read its stdout. A little research has yieled that open FILE, "| $prog |"; does not work for some less than obvious reasons.

Gladly, perl comes with IPC::Open2, which allows you to direct the STDIN and STDOUT of an executed program and let your script control them. But, for some reason my code isn't working. Here is is: 

#!/usr/bin/perl
use IPC::Open2;

print open2(\*BOIN, \*BOOUT, "cat"); 

print BOIN "proclist\n";
print BOIN "quit\n";
close BOIN;

while (<BOOUT>) {
    print;
}
close BOOUT;

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Prints nothing. Any thoughts?

Replies are listed 'Best First'.
Re: IPC::Open2
by btrott (Parson) on May 19, 2000 at 21:48 UTC
    Because you've switched BOIN and BOOUT.

    You need to write to BOOUT and read from BOIN--you were doing the opposite. It should read: 

        #!/usr/bin/perl -w
    use strict;

    use IPC::Open2;

    print open2(\*BOIN, \*BOOUT, "cat"); 

    print BOOUT "proclist\n";
    print BOOUT "quit\n";
    close BOOUT;

    while (<BOIN>) {
        print;
    }
    close BOIN;

    [download]
    If you'd had warnings on in your original, you would have received this warning: 
        Filehandle main::BOIN opened only for input at foo.pl line 8.
    Filehandle main::BOIN opened only for input at foo.pl line 9.

    [download]
    By the way, as perlipc warns, watch out for Unix buffering. In this case (using cat) you might want to use the -u flag for unbuffered output. In your case, it doesn't matter much, since you close the output filehandle. But in general, it's something to think about.
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Re: IPC::Open2
by pschoonveld (Pilgrim) on May 19, 2000 at 21:53 UTC
    Damn, I'm stupid. RTFM for me. Thanks.
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