Question

I’m attempting to print out to two different files. For some reason, print statements work fine for one file, but not for the other. When I run this program, filter2.out consists of a single line that reads “Beginning”. filter2.err remains empty.

open(OUTPUT, "> Filter2/filter2.out");
open(ERROR, "> Filter2/filter2.err");
print OUTPUT "Beginning\n";
print ERROR "Beginning\n";

UPDATE: So I was running this at the beginning of a larger program and realized that it only updates the ERROR file in batches or when the file is closed. Any idea why this occurs?

Answer 1

Consider adding

use strict;
use warnings;

to the top of your script. These statements will help catch errors that are otherwise silently ignored by Perl. In addition, consider adding error checking to your open calls: in all likelihood, it’s not actually opening. I’d write it like this:

use strict;
use warnings;

open(OUTPUT, "> Filter2/filter2.out")
    or die "Can't open filter2.out: $!";
open(ERROR, "> Filter2/filter2.err")
    or die "Can't open filter2.err: $!";
print OUTPUT "Beginning\n";
print ERROR "Beginning\n";

for example, by just adding adding strict and warnings I got:

print() on closed filehandle OUTPUT at .\printer.pl line 6.
print() on closed filehandle ERROR at .\printer.pl line 7.

Hmm…!

By adding error checking, I got:

PS C:\dev> perl .\printer.pl
Can't open filter2.out: No such file or directory at .\printer.pl line 4.

Aah! Looking, I didn’t have the folder. After I added the folder, everything ran fine. You’ll probably find something similar.

Finally, you should probably also use the modern, lexical file handles. This helps catch other errors (like re-used handle names.) Thus, the final script would look like:

use strict;
use warnings;

open(my $output, ">", "Filter2/filter2.out")
    or die "Can't open filter2.out: $!";
open(my $error, ">", "Filter2/filter2.err")
    or die "Can't open filter2.err: $!";
print $output "Beginning\n";
print $error "Beginning\n";

Viola! Now you can see exactly where the problem fails, as it fails, and make sure that other libraries or code you write later can’t accidentally interfere with your file handles.