perl - STDOUT redirected to variable not catching pipe output

Question

I want to temporarily redirect stdout to an in memory variable. Prints are correctly redirected to my variable but not the output of a pipe (bc in my example). What is going on?

#!/usr/bin/perl

my $stdout_sink;
open(my $orig_stdout, ">&STDOUT") || die $!;
close STDOUT;
open(STDOUT, ">", $stdout_sink) || die $!;

# produce some output in different ways
print "before bc
"; # ok
open my $fh, "| bc";
print $fh "2+2
";   # not ok
close $fh;

close STDOUT;  
open(STDOUT, ">&", $orig_stdout) || die $!;
print "$stdout_sink";

Actual ouput will be:

before bc

Expected output:

before bc
4

Answer 1

This is ... not possible.

Standard output of piped opens and system calls are written to file descriptor 1. Normally, Perl's STDOUT file handle is associated with file descriptor 1, but that can be manipulated.

In this example, the system calls writes to STDOUT filehandle, which writes to the file foo.

close STDOUT;             # closes file descriptor 1
open STDOUT, '>', 'foo';  # reopens STDOUT as file descriptor 1
system("echo bar");
close STDOUT;
print STDERR "foo: ",`cat foo`;
# result:  "foo: bar"

But in this example, the system calls writes to the BAZ filehandle.

close STDOUT;             # closes file descriptor 1
open BAZ, '>', 'baz';     # attaches fd 1 to handle BAZ
open STDOUT, '>', 'foo';  # opens new file descriptor, maybe fd 3
system("echo bar");
close STDOUT;
print STDERR "foo: ",`cat foo`;
print STDERR "baz: ",`cat baz`;
# result:  "foo: baz: bar"

An in-memory filehandle is not a real filehandle. If you call fileno on it, you will (generally, may be OS dependent) get a negative number.

open STDOUT, '>', $scalar;
print STDERR fileno(STDOUT);     #   -1

Piped opens and system calls will not be able to write to this filehandle.

You will need a more complicated workaround, like writing the piped open output to a file, and then copying that file into the in-memory variable.