If you send a NULL ("\0") to STDOUT, nothing happens with a standalone script, but it will be recognised as STDIN by a script to which it is piped.

I believe the following script (no_woes.pl) resolves all of the various issues that I've seen in this thread. 

#!/usr/bin/env perl

use strict;
use warnings;
use constant READ_TIMEOUT => 0.5;

use Text::ASCIITable;
use IO::Select;

if (-t \*STDIN) {
    warn "[$$] STDIN:  TTY\n";
}
else {
    warn "[$$] STDIN:  PIPE\n";
}

if (-t \*STDOUT) {
    warn "[$$] STDOUT: TTY\n";
}
else {
    warn "[$$] STDOUT: PIPE\n";
}

$| = 1;
print "\0" if -t \*STDIN;

my $io_select = IO::Select->new(\*STDIN);

if ($io_select->can_read(READ_TIMEOUT)) {
    print for <STDIN>;
} else {
    print get_table();
}

sub get_table {
    my $t = Text::ASCIITable::->new({
        headingText => 'Basket'
    });

    $t->setCols('Id', 'Name', 'Price');
    $t->addRow(1, 'Dummy product 1', 24.4);
    $t->addRow(2, 'Dummy product 2', 21.2);
    $t->addRow(3, 'Dummy product 3', 12.3);
    $t->addRowLine();
    $t->addRow('', 'Total', 57.9);

    return $t;
}

[download]

Standalone output:

$ ./no_woes.pl
[1564] STDIN:  TTY
[1564] STDOUT: TTY
.------------------------------.
|            Basket            |
+----+-----------------+-------+
| Id | Name            | Price |
+----+-----------------+-------+
|  1 | Dummy product 1 |  24.4 |
|  2 | Dummy product 2 |  21.2 |
|  3 | Dummy product 3 |  12.3 |
+----+-----------------+-------+
|    | Total           |  57.9 |
'----+-----------------+-------'

[download]

Piped output:

$ ./no_woes.pl | ./no_woes.pl
[1566] STDIN:  PIPE
[1566] STDOUT: TTY
[1565] STDIN:  TTY
[1565] STDOUT: PIPE
.------------------------------.
|            Basket            |
+----+-----------------+-------+
| Id | Name            | Price |
+----+-----------------+-------+
|  1 | Dummy product 1 |  24.4 |
|  2 | Dummy product 2 |  21.2 |
|  3 | Dummy product 3 |  12.3 |
+----+-----------------+-------+
|    | Total           |  57.9 |
'----+-----------------+-------'

[download]

Multiple pipes:

$ ./no_woes.pl | ./no_woes.pl | ./no_woes.pl
[1568] STDIN:  PIPE
[1568] STDOUT: PIPE
[1567] STDIN:  TTY
[1567] STDOUT: PIPE
[1569] STDIN:  PIPE
[1569] STDOUT: TTY
.------------------------------.
|            Basket            |
+----+-----------------+-------+
| Id | Name            | Price |
+----+-----------------+-------+
|  1 | Dummy product 1 |  24.4 |
|  2 | Dummy product 2 |  21.2 |
|  3 | Dummy product 3 |  12.3 |
+----+-----------------+-------+
|    | Total           |  57.9 |
'----+-----------------+-------'

$ ./no_woes.pl | ./no_woes.pl | ./no_woes.pl | ./no_woes.pl
[1571] STDIN:  PIPE
[1571] STDOUT: PIPE
[1572] STDIN:  PIPE
[1572] STDOUT: PIPE
[1573] STDIN:  PIPE
[1573] STDOUT: TTY
[1570] STDIN:  TTY
[1570] STDOUT: PIPE
.------------------------------.
|            Basket            |
+----+-----------------+-------+
| Id | Name            | Price |
+----+-----------------+-------+
|  1 | Dummy product 1 |  24.4 |
|  2 | Dummy product 2 |  21.2 |
|  3 | Dummy product 3 |  12.3 |
+----+-----------------+-------+
|    | Total           |  57.9 |
'----+-----------------+-------'

[download]

That was run on Cygwin: "perl 5, version 36, subversion 0 (v5.36.0) built for cygwin-thread-multi".

AM comment:

"Note: If you are on windows this will never work as select is only implemented for sockets, and not STDIN."

That same code works on Win10 (Strawberry Perl): "perl 5, version 26, subversion 3 (v5.26.3) built for MSWin32-x64-multi-thread". The output is very similar, so I've put it in a spoiler to avoid cluttering the thread.

<Reveal this spoiler or all in this thread>

— Ken

In reply to Re: IO::Select woes [Prime with NULL] by kcott
in thread IO::Select woes by Anonymous Monk

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