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

I am trying to pipe the STDOUT from one Perl program into another for counting and other manipulation. I am having all kinds of problems. Help is appreciated. 

[--- piper.pl ---]

open (P, "| from.pl")
while (<P>) {
    print $_;
    $count++;
}
close (P);
print "$count\n";




[--- from.pl ---]

print "hello\n";
print "there,\n";
print "fellow\n";
print "monks\n";

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My intended output from this example would be:

hello
there
fellow
monks
4

Replies are listed 'Best First'.
Re: Pipe Question
by sachmet (Scribe) on Apr 26, 2001 at 00:25 UTC
    Two notes:
    1. Your pipe is on the wrong side. It should be: 
      open(P,'./from.pl|');
while (<P>) {
   print;
   $count++;
}
close(P);
print "$count\n";

      [download]
      which gives you what you expect.
    2. Make sure your scripts are mode +x! If you can't run them from the command line, they won't work.
    In general, though, remember - put the pipe on the side as you would if you ran them on the command line; that is: ./from.pl | ./piper.pl would be what you'd run in this case, so use the pipe after the script name. (If you wanted to write to it, you'd put the pipe before.)
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Re: Pipe Question
by chipmunk (Parson) on Apr 26, 2001 at 00:25 UTC
    You've got the program at the wrong end of the pipe. You opened a pipe for inputting to from.pl. It's also not clear whether from.pl has the proper #! line and execute permissions.

    Try this instead: 

    open(P, "perl from.pl |") or die "Can't pipe from from.pl: $?\n";
while (<P>) {
    print $_;
    $count++;
}
close(P) or die "Can't pipe from from.pl: $?\n";
print "$count\n";

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Got it.
by Bliss (Novice) on Apr 26, 2001 at 00:53 UTC
    (highest thanks)
    two errors uncovered:

    I had tried the pipe on the right earlier, but the count failed. In rechecking the code (it is part of a larger program), I discover that I was resetting the value in my while statement, though I am not sure how. The while statement was:
    while (<R> && (count < 3))
{ ... }

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    Now having removed the count < 3 *and* switching the pipe to the right, we have a winner.

    Thank you all. I am astounded and delighted by the responsiveness by the visitors of this site. Truly, near tears to my eyes.
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Re: Pipe Question
by diarmuid (Beadle) on Apr 26, 2001 at 15:36 UTC
    I think what you need are either sockets perldoc IO::Socket or pr named pipes mkfifo. eg
    From the perl cookbook :
    #Shell :
% mkfifo /path/to/named.pipe


#Reader: 
open(FIFO, "< /path/to/named.pipe")         or die $!;
while (<FIFO>) {
    print "Got: $_";
}
close(FIFO);

#Writer:

open(FIFO, "> /path/to/named.pipe")         or die $1;
print FIFO "Smoke this.\n";
close(FIFO);

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not the semi-colon
by Bliss (Novice) on Apr 26, 2001 at 00:17 UTC
    I know the terminator is missing on the "open", but not in my actual code.

    In this example, nothing prints but the count works.

    If I move the pipe to the right, it prints but the count fails.

    Thanks again.
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Re: Pipe Question
by sutch (Curate) on Apr 26, 2001 at 00:23 UTC
    On my Unix system, scripts are not executable from the current directory without preappending "./" to the filename.

    Your scripts works fine when I replace your open statement with:

    open (P, "| ./from.pl");

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      It shouldn't, unless you're writing to P. See my response below.

      What you're probably seeing is from.pl writing to stdout, and you're not getting a count of 4 underneath the words.
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        You're correct about the placement of the pipe (and not getting a count without its correct placement).

        But on my system this works:

        open (P, "./from.pl |");

        and this does not work:

        open (P, "from.pl |");

        From the shell I must always specify a script's path, unless the path is defined in PATH.

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