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03.27.2008 at 06:38AM PDT, ID: 23273934

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9.0

IPC::Open2 not returning data on READER

Zone: Perl Programming Language

Tags: Perl, 5.8.0, OSF, Perl, 5.8.0

I am using IPC::Open2 to create a shell that I can communicate with in Perl on a *nix machine. I create the shell using the following code:

open2(\*READER, \*WRITER, "ksh");

and it creates the shell and returns the PID #.

Then, when I want to execute a command in the shell, I use the following code:

      my @results = ();
      print WRITER "$cmd\n";
      print WRITER "echo Done\n";
      while(<READER>)
      {
if/Done/{last;}
push(@results, $_);
}

For the sake of this example, lets say that $cmd = "ls". This code works very well on most of my machines. I have 1 machines that is running OSF1 that I do not get any data back from the READER. I can change the command to $cmd = "ls > /tmp/t.txt" and the t.txt file is created with the data, so I know that the commands are executing in the shell.

Does anyone know what would prevent data from being returned on the READER handle?

Thanks...

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Question Stats

Zone: Programming

Question Asked By: nbtzlp3

Solution Provided By: ahoffmann

Participating Experts: 1

Solution Grade: A

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03.28.2008 at 12:10AM PDT, ID: 21228462

Rank: Guru

ahoffmann:

depends on the implementation of the shell and on the used echo command (shell built.in or something like /usr/bin/echo) which most likely write to tty instead of STDOUT

03.28.2008 at 05:26AM PDT, ID: 21229697

nbtzlp3:

I'm not sure I understand "implementation of the shell". I am using the ksh shell. Is there something I need to check about the shell to determine if it will work properly?

When you are commenting on the echo command. Is that due to my using "echo Done\n", so that I can determine when the output is complete?

If so, the issue is that the original command doesn't produce any output on the READER handle at all. The while<READER> loop is never entered.

If I have misunderstood, please let me know.

Thanks...

03.30.2008 at 12:52PM PDT, ID: 21241719

Rank: Guru

ahoffmann:

> .. using the ksh shell
ksh is one of those shells with infinite variations of implementation, So please tell us if you use the buil-in echo or one of the binary echo programs

> commenting on the echo
sometimes you can identify it with:
which echo
(but that's unreliable in some cases), just try what happens if you do in a shell:

echo -e 'e\te'
/bin/echo -e 'e\te'
/usr/bin/echo -e 'e\te'

> .. original command
what is the "original command"?

03.31.2008 at 08:51AM PDT, ID: 21246471

nbtzlp3:

I believe that I am using the built in echo. When I run the echo commands that you listed, I get the same output for all: -e e e

The original command that I am referencing is "ls". If you look at the code that I am executing:

my @results = ();
print WRITER "$cmd\n";
print WRITER "echo Done\n";
while(<READER>)
{
if/Done/{last;}
push(@results, $_);
}

I stated that for the sake of this exercise, I am using "ls" as $cmd. I'm not sure what the echo command has to do with the issue. Shouldn't the ls command return data on the <READER> handle?

Thanks....

03.31.2008 at 12:34PM PDT, ID: 21248370

Rank: Guru

ahoffmann:

> I'm not sure what the echo command has to do with the issue.
The example you posted in the question uses echo.
Does it work if you replace echo by ksh's print or by /usr/bin/echo?

03.31.2008 at 12:39PM PDT, ID: 21248413

nbtzlp3:

Echo is only used so that when I am reading the data from the <READER> handle, I know when to exit the loop. I should FIRST received the data output by the ls command and then after I loop through all of that, then I should see the word DONE (as output by the echo command). I an NOT receiving ANY data on the <READER> handle, therefore, I'm not even getting to the echo output.

I will try the print statement instead and see if it gives me any different results.

Thanks....

03.31.2008 at 11:51PM PDT, ID: 21251716

Rank: Guru

ahoffmann:

> Echo is only used so that when I am reading the data from the ..
hope you're aware that this breaks your reader if you have a file or whatever named "Done".
Anyway, please replace
open2(\*READER, \*WRITER, "ksh");
by
open2(\*READER, \*WRITER, "/bin/ksh") || die "ERROR: $!"
or
my $pid = open2(\*READER, \*WRITER, "/bin/ksh"); print "PID: $pid\n";

If this doesnt help or give more hints, then please replace ksh by sh (I mean Bourne shell, not bash!).
Also open2 and open3 are known to have problems with STDIN and buffering, see man IPC::Open3 (which also relates to my very first comment).

04.01.2008 at 10:11AM PDT, ID: 21256002

nbtzlp3:

Yes, I understand about the string "Done" breaking the reader. The commands that I am actually using won't generate that string.

I have tried /bin/ksh and /bin/sh with no change in behavior.

04.01.2008 at 01:34PM PDT, ID: 21257845

Rank: Guru

ahoffmann:

hmm, running out of ideas ...
set your $cmd to produse a huge output (> 8kb)

04.01.2008 at 02:41PM PDT, ID: 21258383

nbtzlp3:

We might be getting somewhere. I used a "cat /fullpathto/largetxtfile.txt" as the command. The file exists and is > 8kb. I got the following error when I printed the command to <WRITER>:

cat: Cannot stat stdout

Does this mean anything to you?

04.01.2008 at 11:30PM PDT, ID: 21260562

Rank: Guru

ahoffmann:

> cat: Cannot stat stdout
well, that seems to prove what I said in my very first comment: sounds like your shell (ksh) is writing to the tty, not STDOUT
Another reason may be a corrupted IPC module. Try to use IPC::open3 or code that yourself using fork(), dup() and open() (examples are in perl's man page).

04.02.2008 at 05:00AM PDT, ID: 21262062

nbtzlp3:

I had previously tried open3 instead of open 2 with no results. So, if the shell is writing to the tty instead of STDOUT, what can I do to redirect the output? Would it be something like print "$cmd >&2"?

Thanks...

04.02.2008 at 05:14AM PDT, ID: 21262161

nbtzlp3:

Whoops, maybe that should be "$cmd >&1" instead.

04.02.2008 at 09:51AM PDT, ID: 21264962

Rank: Guru

ahoffmann:

> .. writing to the tty instead of STDOUT, what can I do to redirect the output?
i.g. you cannot.
The only possibilty I know is to run your shell in a screen session.

04.02.2008 at 10:00AM PDT, ID: 21265032

nbtzlp3:

Then I have a silly question. This code works on many other machines. Why doesn't this shell write it to STDOUT like the others do and will the other shells on this machine have the same issue?

04.02.2008 at 11:27PM PDT, ID: 21270083

Rank: Guru

ahoffmann:

> .. works on many other machines.
that's a vague description.
Are these machines the same UNIX kernel, the same shell, the same perl, the same perl modules? (means that shell, perl and modules are compiled for the system but are the same version)

Accepted Solution

20080236-EE-VQP-29 / EE_QW_2_20070628