>perl out.pl d.d

initial: fileno(*STDOUT) = 1
initial: fileno(*STDERR) = 2

redirected: fileno(*STDOUT) = 1
redirected: fileno(*STDERR) = 2


hello - this is contents of d.d

testing stdout capture 1
testing stderr capture
testing stdout capture 2


>perl out.pl 

initial: fileno(*STDOUT) = 1
initial: fileno(*STDERR) = 2

redirected: fileno(*STDOUT) = -1
redirected: fileno(*STDERR) = -1
hello - this is $captured_output

testing stdout capture 1
testing stdout capture 2


##</code><code>##

#!/usr/bin/perl
use strict;                                               
use FileHandle;                                            
                                                                    
my $use_file = shift;

local *SAVED_STDOUT;
local *SAVED_STDERR;

open SAVED_STDOUT, ">&STDOUT" 
      or die "couldn't save STDOUT: $!\n"; 
open SAVED_STDERR, ">&STDERR" 
      or die "couldn't save STDERR: $!\n"; 
 
printf("\ninitial: fileno(*STDOUT) = %d\n", 
                        fileno(*STDOUT));  # 1
printf("initial: fileno(*STDERR) = %d\n", 
                        fileno(*STDERR));  # 2

my $captured_output = ""; 
 
# redirect standard out to file or string, as requested
close STDOUT; 
if ($use_file) {
    open STDOUT, ">", "$use_file" 
        or die "couldn't re-open STDOUT: $!\n"; 
}
else {
    open STDOUT, ">", \$captured_output 
         or die "couldn't re-open STDOUT: $!\n"; 
}
 
close STDERR; 
open STDERR, ">&STDOUT" 
      or die "couldn't re-open STDERR: $!\n"; 
 
printf SAVED_STDOUT "\nredirected: fileno(*STDOUT) = %d\n", 
                    fileno(*STDOUT);  # 1
printf SAVED_STDOUT "redirected: fileno(*STDERR) = %d\n", 
                     fileno(*STDERR);  # 2

select STDOUT; $|++;
select STDERR; $|++;
select STDOUT;

print STDOUT "testing stdout capture 1\n"; 
print STDERR "testing stderr capture\n"; 
print STDOUT "testing stdout capture 2\n"; 
 
close STDOUT; open STDOUT, ">&SAVED_STDOUT"; 
close STDERR; open STDERR, ">&SAVED_STDERR"; 
 
if ($use_file) {
    printf SAVED_STDOUT 
          "\n\nhello - this is contents of $use_file\n\n";
    system("cat $use_file");
}
else {
    printf "hello - this is \$captured_output\n\n";
    printf STDERR $captured_output, "\n"; 
}