Script hangs when executing command over ssh

eg2014 has asked for the wisdom of the Perl Monks concerning the following question:

Hi PerlMonks, I have two text files on the "remote" sever, one is 29864 bytes (about 30 K) and the other is 35498 bytes (about 35 K). And the following code

use strict;
use Net::SSH::Perl;

my $ssh = Net::SSH::Perl->new('localhost',debug => 1, protocol => '2,1
+');
$ssh->login('username','password');
my ($stdout, $stderr, $exit) = $ssh->cmd('cat /tmp/a 1>&2');
print $stdout . "\n";
print $stderr . "\n";
print $exit . "\n";
[download]

Although everything works when the remote program outputs to stdout, when it outputs to stderr, and the output is large, my script hangs. /tmp/a is the smaller file and the script runs to completion. However /tmp/b is the larger file and causes the above to hang. Any information would be helpful. Thank you kind monks.

Comment on Script hangs when executing command over ssh Download Code

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Re: Script hangs when executing command over ssh by marinersk (Priest) on Jun 18, 2015 at 16:47 UTC
One oddity I ran into, too long ago to remember the details, was an issue with the way `STDERR`was buffered. You may be bumping into that; I wish I remembered more about the problem. Something about `STDERR`being held until the end of the run and then released? Sorry, it's been too long. One thing I'm noticing is that your command line is redirecting `STDOUT`to `STDERR`using the `1>&2`technique. However, in your invocation, you seem to be wishing to capture `$stdout`and `$stderr`separately. Perhaps you'd have more luck getting rid of the command redirect and simply capturing the data raw with your call to `$ssh->cmd`? `my ($stdout, $stderr, $exit) = $ssh->cmd('cat /tmp/a');` [download]	[reply] [d/l] [select]
Re^2: Script hangs when executing command over ssh by eg2014 (Initiate) on Jun 18, 2015 at 17:00 UTC
Thank you for your reply. Yes perhaps it does have to do with the STDERR buffering, and it seems to be at about 32K. Do I need to increase that buffer somehow? The actual program I am running is outputting to STDERR (and STDOUT) and I have no control over that output. I just gave the above code as an example of the more complicated one.	[reply]
Re^3: Script hangs when executing command over ssh by marinersk (Priest) on Jun 18, 2015 at 17:24 UTC
If there's a way to work with the buffer, hopefully a more Unix/Linux-oriented Monk will come along with a suggestion. However, I don't think you understood my point about getting rid of the piping. Try removing the `1>&2`from your command, and see if the problem goes away.	[reply] [d/l]
Re^4: Script hangs when executing command over ssh by eg2014 (Initiate) on Jun 18, 2015 at 18:28 UTC
Re^5: Script hangs when executing command over ssh by marinersk (Priest) on Jun 18, 2015 at 18:56 UTC
Some notes below your chosen depth have not been shown here
Re: Script hangs when executing command over ssh by salva (Canon) on Jun 19, 2015 at 06:53 UTC
That looks like a Net::SSH::Perl Bug. 30k is quite suspecting, the max size of a SSH packet. You should try using Net::OpenSSH instead.	[reply]
Re: Script hangs when executing command over ssh by eg2014 (Initiate) on Jun 18, 2015 at 18:38 UTC
Regarding the last comments, I would like to add that I need to distinguish the STDOUT from the STDERR and redirecting STDERR to STDOUT is not an option.	[reply]
Re^2: Script hangs when executing command over ssh by marinersk (Priest) on Jun 18, 2015 at 19:01 UTC
redirecting STDERR to STDOUT is not an option Then it is might be doubly imperative that you stop using `1>&2`, because `1>&2`causes ~~`STDERR`~~ `STDOUT`to be redirected to ~~`STDOUT`~~ `STDERR`.	[reply] [d/l] [select]