Can't capture STDERR to Variable

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

Maybe it's something very obvious to anyone but me? ;)

I want to analyze an external program's STDOUT and STDERR and don't like to redirect STDERR to a file which I later have to read, and delete.

So I thought I'd redirect STDERR to a variable like this:

close STDERR;
open STDERR,">",\$x or die;
`ls /some/nonexisting/path`;
print $x;
[download]

I expected $x to read "/some/nonexisting/path: No such file or directory" but it's simply empty.

Does anyone of you see what's wrong and can give me a hint?

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

Comment on Can't capture STDERR to Variable Select or Download Code

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Re: Can't capture STDERR to Variable by moritz (Cardinal) on Jul 21, 2011 at 10:18 UTC
With your call to open you only capture Perl's STDERR, not that of processes that Perl starts. IPC::Run or IPC::Open3 allow you to capture STDERR of external commands. I hope that your use of ls is only an example - directory listings are easily obtained in Perl with opendir, readdir and closedir, which make errors readily available in $!. (Or File::Find if you want recursive directory listings). Perl 6 - second systems done right	[reply]
Re: Can't capture STDERR to Variable by salva (Canon) on Jul 21, 2011 at 10:21 UTC
Use Capture::Tiny or some other similar module. File handles opened to scalar references can not be used outside the Perl process where they were created (and even then, most modules wrapping external C libraries will also fail to recognize them as valid file handles).	[reply]
Re: Can't capture STDERR to Variable by davido (Cardinal) on Jul 21, 2011 at 10:22 UTC
qx// and the backticks operator don't capture STDERR. There's a discussion of this in perlop (the preceding link is anchored to the relevant section). The docs do provide some examples of how to finesse STDERR to do what you want. Dave	[reply]
Re: Can't capture STDERR to Variable by zentara (Cardinal) on Jul 21, 2011 at 11:41 UTC
Here is a general purpose example using IPC::Open3. You can run your exact command, instead of bash in the example. #!/usr/bin/perl use warnings; use strict; use IPC::Open3; use IO::Select; my $pid = open3(\WRITE, \READ,\ERROR,"/bin/bash"); my $sel = new IO::Select(); $sel->add(\READ); $sel->add(\ERROR); my($error,$answer)=('',''); while(1){ print "Enter command\n"; chomp(my $query = <STDIN>); #send query to bash print WRITE "$query\n"; foreach my $h ($sel->can_read) { my $buf = ''; if ($h eq \ERROR) { sysread(ERROR,$buf,4096); if($buf){print "ERROR-> $buf\n"} } else { sysread(READ,$buf,4096); if($buf){print "$query = $buf\n"} } } } waitpid($pid, 1); # It is important to waitpid on your child process, # otherwise zombies could be created. [download] I'm not really a human, but I play one on earth. Old Perl Programmer Haiku ................... flash japh	[reply] [d/l]
Re^2: Can't capture STDERR to Variable by ikegami (Patriarch) on Jul 21, 2011 at 19:31 UTC
`open3` is a pretty low-level tool. It does some things quite well, but this isn't one of them. See instead: IPC::Run3, IPC::Run PS — Your code has a race condition that could deadlock both processes since you only passed two of the three handles to `select`.	[reply] [d/l] [select]
Re: Can't capture STDERR to Variable by Skeeve (Parson) on Jul 22, 2011 at 07:11 UTC
Thanks to all of you! Capture::Tiny looks promising and I will consider it for a rewrite of my script. For my task at hand, merging STDOUT and STDERR with "2>&1" is sufficient, but I don't like it as a "solution". I think it is just a workaround. And to answer moritz' Question: Yes! ls is just an example. The real program I call is openssl. `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]