Perl anonymous channel without output

Question

Question

Why is nothing printed when using an anonymous channel if I do not print the actual data from the channel?

---

Example

use strict;
use warnings;

my $child_process_id = 0;
my $vmstat_command = 'vmstat 7|';
$child_process_id = open(VMSTAT, $vmstat_command) || die "Error when executing \"$vmstat_command\": $!";


while (<VMSTAT>) {
    print "hi" ;
}
close VMSTAT or die "bad command: $! $?";

Appears to hang

---

use strict;
use warnings;

my $child_process_id = 0;
my $vmstat_command = 'vmstat 7|';
$child_process_id = open(VMSTAT, $vmstat_command) || die "Error when executing \"$vmstat_command\": $!";


while (<VMSTAT>) {
    print "hi" . $_ ;
#                ^^^ Added this
}
close VMSTAT or die "bad command: $! $?";

Print

hiprocs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
hi r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
hi 1  0      0 7264836 144200 307076    0    0     0     1    0   14  0  0 100  0  0

etc...

---

Expected Behavior

It would print hi for each vmstat output line for the first example.

---

Version

perl, v5.10.0
GNU bash, version 3.2.51

---

Miscellaneous

It also seems to hang when using chomp before printing a line (which, as I thought, only removes newline characters).

I feel that I am missing something fundamental for how the channel is read and processed, but cannot find a similar question. If there is one, then fool it, and I will look at it.

---

Any additional information you just need to ask.

Answer 1

print "hi";

to

print "hi\n";

""

, , ,

$|

, . 0 ( , , "$ |" , Perl ). STDOUT, , , - , - . , , Perl rsh . . . getc man- perlfunc. (: , .)