Capturing STDERR from a piped open

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

Updated
Hi,

After consulting the Cookbook (16.7), I've written the following sub.

sub TestRadiusAuth
{
    my $cmd = "$radclient" .
        " -t $radius_config->{auth_timeout}" .
        " -r $radius_config->{auth_retries}" .
        " -i 1 $radius_config->{ip}:$radius_config->{auth_port}" .
        " auth $radius_config->{secret} <<EOF\n" .
        "User-Name = \"$test_user\"\n" .
        "Password = \"$test_pass\"\n" .
        "NAS-IP-Address = \"$eth0\"\n" .
        "EOF\n";
    my $response;
    open(CMD, "$cmd 2>&1 |") or die "Could not execute $cmd:$!\n";
    while (<CMD>)
    {
        chomp();
        return (1,$_) if /Session-Timeout/;
        $response .= $_;
    }
    return (0,$response);
}
[download]

My problem is that the redirection (2>&1) within the piped open does not work as expected. If I deliberately force $cmd to return an error, it still comes to _my_ STDERR, rather than the STDOUT of the CMD filehandle.

I figure that I'm doing something fundamentally stupid, but I can't see it for the life of me.
Would appreciate any advice.

Comment on Capturing STDERR from a piped open Download Code

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Re: Capturing STDERR from a piped open by McDarren (Abbot) on Aug 13, 2005 at 16:03 UTC
Just went for a walk and it suddenly hit me.... I needed to change the following line: " auth $radius_config->{secret} <<EOF\n" . to: " auth $radius_config->{secret} 2>&1 <<EOF\n" . All is good now, thanks for the replies :)	[reply]
Re: Capturing STDERR from a piped open by tlm (Prior) on Aug 13, 2005 at 15:30 UTC
FWIW, it works fine for my (on Linux): `use strict; use warnings; my $cmd = 'ls nonexistent'; open( CMD, "$cmd 2>&1 \|" ) or die "Could not execute $cmd: $!\n"; while ( <CMD> ) { chomp; print "<< $_ >>\n"; } __END__ % perl 483558.pl << ls: nonexistent: No such file or directory >>` [download] the lowliest monk	[reply] [d/l]
Re^2: Capturing STDERR from a piped open by McDarren (Abbot) on Aug 13, 2005 at 15:45 UTC
Thanks, Your example works fine for me too, so I've updated my original post to show the full subroutine. $cmd calls radclient to do a test auth against a remote radius server. If I deliberately cause an error, say for example using an incorrect port (causing "radclient: no response from server"), it still comes to _my_ STDERR when I run it. Any clues?	[reply]
Re^3: Capturing STDERR from a piped open by tlm (Prior) on Aug 13, 2005 at 15:55 UTC
AFAICT, you're trying to feed the external program's stdin through a here-doc, ~~which looks quite hopeless to me~~. Read what the Good Cookbook has to say about accessing stdin, stdout, and stderr of another process (recipe 16.9, pp. 639-641). Update: I guess it was not so hopeless after all. :-/ the lowliest monk	[reply]
Re: Capturing STDERR from a piped open by graff (Chancellor) on Aug 13, 2005 at 15:33 UTC
That's odd -- it works for me. In a path where "foo.bar" does not exist, running this gives me "I got the error", and nothing else: `perl -e 'open(C,"ls foo.bar 2>&1 \|") or die $!; while(<C>){ print "I got the error\n" if (/No such/) }'` [download] Maybe you need to be sure about the particular shell that is being invoked when you do the pipeline open? (Mine is /bin/bash; I think C-shells are brain-damaged when it comes to redirecting stderr.)	[reply] [d/l]
Re^2: Capturing STDERR from a piped open by tlm (Prior) on Aug 13, 2005 at 15:38 UTC
On Unix it's always `/bin/sh -c`. the lowliest monk	[reply]