Pipe Question

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

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Re: Pipe Question by sachmet (Scribe) on Apr 26, 2001 at 00:25 UTC
Two notes: 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. 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.)	[reply] [d/l] [select]
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";` [download]	[reply] [d/l]
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)) { ... }` [download] 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.	[reply] [d/l] [select]
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.	[reply]
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);` [download]	[reply] [d/l]
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");` [download]	[reply] [d/l]
Re: Re: Pipe Question by sachmet (Scribe) on Apr 26, 2001 at 00:26 UTC
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.	[reply]
Re: Re: Re: Pipe Question by sutch (Curate) on Apr 26, 2001 at 00:39 UTC
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.	[reply] [d/l] [select]


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