How can close and reopen STDOUT in Perl? - perl

Question

I'd like to close STDOUT to prevent my code from outputing a particular image that I need for further computation but do not want on my web page.
So i want to close STDOUT, do what I have to do with my code, then reopen STDOUT to output stuff to a web page. (Not to a file)
What I tried is:
close STDOUT;
# my code here
open STDOUT;
This doesn't work...
Thanks

Answer 1

There are several ways to approach your problem, and many of them do not require you to close STDOUT and risk fubaring your program's standard I/O channels.
For example, you can use the (1-arg) select command to direct the output of print commands somewhere else temporarily.
print $stuff_you_want_to_send_to_STDOUT;
select(NOT_STDOUT);
# now default print sends things to NOT_STDOUT.
# This doesn't need to be a real filehandle, though you may get warning
# messages if it is not.
...;
print $the_image_you_dont_want_to_go_to_STDOUT;
...;
select(STDOUT);
# now print sends things to STDOUT agin
print $more_stuff_you_do_want_to_go_to_STDOUT;
You can also reassign the *STDOUT glob at run-time without closing any handles.
*OLD_STDOUT = *STDOUT;
print $for_STDOUT;
*STDOUT = *NOT_STDOUT; # again, doesn't need to be a real filehandle
print $stuff_to_suppress;
*STDOUT = *OLD_STDOUT; # restore original STDOUT
print $more_stuff_for_STDOUT;

Answer 2

It's bad to close STDOUT since much assumes it's always open. It's better to redirect it to /dev/null (unix) or nul (Windows).
If you want to redirect the file descriptor,
use Sub::ScopeFinalizer qw( scope_finalizer );
{
open(my $backup_fh, '>&', \*STDOUT) or die $!;
my $guard = scope_finalizer { open(STDOUT, '>&', $backup_fh) or die $!; };
open(STDOUT, '>', '/dev/null') or die $!;
...
}
If you just want to redirect STDOUT,
{
local *STDOUT;
open(STDOUT, '>', '/dev/null') or die $!;
...
}
If you just want to redirect the default output handle,
use Sub::ScopeFinalizer qw( scope_finalizer );
{
open(my $null_fh, '>', '/dev/null') or die $!;
my $backup_fh = select($null_fh);
my $guard = scope_finalizer { select($backup_fh); };
...
}

Answer 3

You can implement something to catch STDOUT like so:
sub stdout_of (&) {
my $code = shift;
local *STDOUT;
open STDOUT, '>', \(my $stdout_string = '')
or die "reopen STDOUT: $!";
$code->();
return $stdout_string;
}
And then use it like so:
my $stdout = stdout_of { print "hello world" };
Localizing the filehandle inside stdout_of() allows you to avoid the tricks of closing and re-opening STDOUT.

Answer 4

Read the documentation for open.
Search for "Here is a script that saves, redirects, and restores STDOUT and STDERR using various methods".
What you want to do is not close STDOUT, but rather redirect it to /dev/null temporarily.

Answer 5

To (re)open STDOUT or STDERR as an in-memory file, close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
From the perl doc: http://perldoc.perl.org/functions/open.html
You have a : after your close, don't do that. The open above should also work with jus
open STDOUT;
This thread in perl monks might help you too: http://www.perlmonks.org/?node_id=635010

Answer 6

I checked 2 ways:
via select
via *OLD_STDOUT = * STDOUT, and see they are not usable in common case.
The reason is these 2 approachs redirect STDOUT only if "print" or something else is used in a Perl Script. But if you use "system()" call or call subscript, their output got to standard STDOUT anyway =((.
My point of view, the indeed solution is to be:
#!/usr/bin/perl -w
my $file1 = "/tmp/out.txt";
my $file2 = "/tmp/err.txt";
open my $oldSTDOUT, ">&STDOUT";
open OLDERR, ">&",\*STDERR;
open(STDOUT, ">$file1") or print("Can't redirect stdout: to $file1 ");
open(STDERR, ">$file2") or print("Can't redirect stderr: to $file2 ");
print "THIS OUTPUT ISN'T GOT TO STANDARD OUTPUT\n";
system("pwd"); # this output isn;t got to standard output too, that is right!
close(STDOUT);
close(STDERR);
open STDOUT, ">>&", $oldSTDOUT;
open STDERR, ">>&OLDERR";
print "BUT THIS OUTPUT IS SEEN IN A STANDARD OUTPUT\n";
I checked this solution and it worked for me.