gw1500se has asked for the wisdom of the Perl Monks concerning the following question:

I would like to run a command into which I will feed data via a pipe (sysopen). However, at the same time I would like to read the output from that command. I cannot seem to find an example or a good explanation of how to sysopen a command to write to its STDIN and read from its STDOUT and STDERR. Can someone tell me how to do this? Thanks.

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Re: Setting up 2 way pipe for a command
by shmem (Chancellor) on Jun 29, 2008 at 15:01 UTC

    See IPC::Open2, or IPC::Open3 for reading STDERR as well.

    --shmem 

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
[reply]
      Thanks. I was looking for a built-in perl method. I forgot to check for modules.
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Re: Setting up 2 way pipe for a command
by gloryhack (Deacon) on Jun 29, 2008 at 21:03 UTC
    When it's up to me to implement this, I almost always reach for IPC::Run. The documentation is a bit unwieldy, but once you work through it the implementation is about as straightforward as it can be.
[reply]
      Thanks for the suggestion. I may have to switch to IPC::Run as I cannot get open2 to work. It seems to block when I try to read the output. I tried setting autoflush for the output but it doesn't seem to help. I should make it clear that I am trying to make this interactive so I need the output for each input (if there is one) while the command is running. I believe there is a 1:1 correspondence between a line into the command and a line out. The line in does what it is supposed to but at that point the script hangs waiting for a line on the output side so the next input line never goes. However, it is possible that there is not a 1:1 correspondence but until I can manage the blocking I won't really know. It appears like pump might do what I need but I have to read up on it more and see some examples.
[reply]
      I switched to IPC::Run but has not faired any better. I keep getting a premature end error even though there is input data. Here is the relevent code segment I have now after many various iterations: 
      my $h=start(\@cmd,\$in,\$out,\$err);
                while (!eof(FILELIST)) {
                        $thisline=readline(FILELIST);
                        ($thisfile,$thisfilesize)=split(/\t/,$thisline
+);
                        $in="$thisfile\n";
                        $h->pump;
                        while ($h->pumpable) {
                                $h->pump;
                        }
                        if (length($out)>0) {
                                putmsg($out);
                        }
                        $total+=$thisfilesize;
                        $complete=sprintf("%.2f",$total/$dumpsize);
                        putmsg("$complete%\n");
                }
                finish $h;

      [download]
      In one of my iterations I avoided the premature end message however, the script simply quits on the first call of 'pump'. No error message, nothing. Does anyone one see anything obvious? Thanks.
[reply]
[d/l]
        Uh... "premature end error"? As in HTTP server error response? If this is indeed the case, try running the thing from the command line, or at least printing a text/plain header somewhere ahead of the block above.

        Maybe on that iteration that didn't puke, the application being run via IPC::Run didn't output anything?

[reply]

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