Harvey Quamen wrote:
>
> Hello all:
Hello,
> I'm working my way through "Learning Perl" (3rd ed) and got stuck in
> ch 14 (Process Management, pages 201-202 actually, to be specific)
> where the "date" command is is opened through a filehandle with a
> piped open. Just toying around to see how it worked, I put it in a
> loop and was surprised to see that it worked perfectly for the first
> iteration but then I got an error on every subsequent iteration:
>
> Use of uninitialized value in concatenation (.) at line 11, <DATE> line 1.
>
> Line 11 is the print line, but if I swap the filehandle with $now =
> `date` everything works great.
>
> #!/usr/bin/perl -w
> use strict;
>
> my $now;
>
> open DATE, "date|"
> or die "Darth Vader prevents me from piping from date: $!";
>
> foreach (1..4) {
> $now = <DATE>;
This will read from "date|" until the end of file is reached and
subsequent reads will return undef. Since date only outputs one line
then that is all you can read from this file handle. $now = `date`
executes the date command each time the expression is evaluated. See
the I/O Operators section of the perlop document.
> print "It's now $now\n";
> sleep (1 + rand 10);
> }
>
> close DATE;
You should test the success of close() when using pipes.
close DATE
or die "Darth Vader prevents me from closing pipe from date: $!";
John
--
use Perl;
program
fulfillment
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