How do I get the size of a file in megabytes using Perl?

Question

I want to get the size of a file on disk in megabytes. Using the -s operator gives me the size in bytes, but I'm going to assume that then diving this by a magic number is a bad idea:

my $size_in_mb = (-s $fh) / (1024 * 1024);

Should I just use a read-only variable to define 1024 or is there a programmatic way to obtain the size of a byte?

EDIT: Updated the incorrect calculation.

Answer 1

Well, there's not 1024 bytes in a meg, there's 1024 bytes in a K, and 1024 K in a meg...

That said, 1024 is a safe "magic" number that will never change in any system you can expect your program to work in.

Answer 2

1) You don't want 1024. That gives you kilobytes. You want 1024*1024, or 1048576.

2) Why would dividing by a magic number be a bad idea? It's not like the number of bytes in a megabyte will ever change. Don't overthink things too much.

Answer 3

Don't get me wrong, but: I think that declaring 1024 as a Magic Variable goes a bit too far, that's a bit like "$ONE = 1; $TWO = 2;" etc.

A Kilobyte has been falsely declared as 1024 Bytes since more than 20 years, and I seriously doubt that the operating system manufacturers will ever correct that bug and change it to 1000.

What could make sense though is to declare non-obvious stuff, like "$megabyte = 1024 * 1024" since that is more readable than 1048576.

Answer 4

Since the -s operator returns the file size in bytes you should probably be doing something like

my $size_in_mb = (-s $fh) / (1024 * 1024);

and use int() if you need a round figure. It's not like the dimensions of KB or MB is going to change anytime in the near future :)

Answer 5

I would read this into a variable rather than use a magic number. Even if magic numbers are not going to change, like the number of bytes in a megabyte, using a well named constant is a good practice because it makes your code more readable. It makes it immediately apparent to everybody else what your intention is.

Answer 6

If you'd like to avoid magic numbers, try the CPAN module Number::Bytes::Human.

use Number::Bytes::Human qw(format_bytes);
my $size = format_bytes(-s $file); # 4.5M

Answer 7

You could of course create a function for calculating this. That is a better solution than creating constants in this instance.

sub size_in_mb {
    my $size_in_bytes = shift;
    return $size_in_bytes / (1024 * 1024);
}

No need for constants. Changing the 1024 to some kind of variable/constant won't make this code more readable.