On 04/03/2012 07:56, Manfred Lotz wrote:
Hi there,
I've got a strange problem with $_. It seems I do not understand
something which might be basic. Anyway, here is a minimal example.
#! /usr/bin/perl
use strict;
use warnings;
use 5.010;
sub sub1 {
my $cmd = shift;
print "Sub3: Issuing [$cmd]\n";
open my $fh, '-|', "$cmd 2>&1" or die "open: $!";
while (<$fh> ) { print; }
close $fh;
return $?>> 8;
}
sub sub2 {
my $cmd = shift;
say "\nSub1 $cmd";
return 1;
}
my @cmds = ( "uname -a", "date");
foreach ( @cmds ) { say sub1($_); };
foreach ( @cmds ) { say sub2($_); };
Running this gives:
Sub3: Issuing [uname -a]
Linux hogwart 3.0.0-14-generic-pae #23~lucid1-Ubuntu SMP Thu Dec 8
15:03:12 UTC 2011 i686 GNU/Linux 0
Sub3: Issuing [date]
Sun Mar 4 08:54:18 CET 2012
0
Use of uninitialized value $cmd in concatenation (.) or string
at ./foreach1.pl line 20.
Sub1
1
Use of uninitialized value $cmd in concatenation (.) or string
at ./foreach1.pl line 20.
Sub1
1
What am I doing wrong here?
Hi Manfred
You have discovered why many people say it is unsafe to use $_. $_ is a
global variable, and the loop
while (<$fh> ) { print; }
modifies it, leaving it undefined at end of file. Unfortunately the loop
foreach ( @cmds ) { say sub1($_); };
has aliased $_ to each of the elements of @cmds, so the array is getting
corrupted by the subroutine call.
You can get around this either by using a named variable for reading
from the pipe, or by adding
local $_;
at the start of sub1 in anticipation of changing its value, so that the
copy that the main program is using doesn't get corrupted.
Cheers,
Rob
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