linux - Perl anonymous pipe no output -

question

why nothing printed when using anonymous pipe, unless print actual data pipe ?

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example

use strict; use warnings;  $child_process_id = 0; $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 hang

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use strict; use warnings;  $child_process_id = 0; $vmstat_command = 'vmstat 7|'; $child_process_id = open(vmstat, $vmstat_command) || die "error when executing \"$vmstat_command\": $!";   while (<vmstat>) {     print "hi" . $_ ; #                ^^^ added } close vmstat or die "bad command: $! $?"; 

prints

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

etc...

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expected behaviour

would print hi every line of output of vmstat first example.

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versions

perl, v5.10.0 gnu bash, version 3.2.51 

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misc

it appears hang when using chomp before printing line (which thought removes newlines).

i feel i'm missing fundamental how pipe read , processed not find similar question. if there 1 dupe , i'll have @ it.

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any further information needed ask.

alter

print "hi"; 

to

print "hi\n"; 

and "works"

the reason fails output line buffered default

setting $| flush buffer straight away

if set nonzero, forces flush right away , after every write or print on selected output channel. default 0 (regardless of whether channel buffered system or not; "$|" tells whether you've asked perl explicitly flush after each write). stdout typically line buffered if output terminal , block buffered otherwise. setting variable useful when outputting pipe or socket, such when running perl program under rsh , want see output it's happening. has no effect on input buffering. see getc entry in perlfunc manpage that. (mnemonic: when want pipes piping hot.)