Difference between $0 and __FILE__

[solegaonkar]

Hello Monks, Often wondered what is the difference between the two special values $0 and __FILE__ in a Perl script. But always brushed it off saying "Perl has many ways to do anything". But, today, I saw some code containing:

unless ($0 eq __FILE__) .....

That sets me thinking, what exactly is the difference between the two. Can they be different? When?

[BrowserUk]

Within a module (say a.pm) used from a program (say b.pl), __FILE__ will be a.pm but $0 will be b.pl.

The test you posted checks whether the file that contains it is being run as a 'main' program, or is being used or required, or done or eval'd.

[pobocks]

__FILE__ is the file name that the __FILE__ token is in, and $0 is a variable containing the name of the program, as given to the shell.

They can most certainly be different - one case that comes to mind is a __FILE__ token in a library called by your program, but there are many others, particularly since you can assign to $0 (with varying effects on different OS), but you can't meaningfully assign to __FILE__.

Example code:

File 1 - firstfile

#!/usr/bin/perl -w
use strict;

print $0 . "\n";
print __FILE__ . "\n";

do 'filetwo';

File 2 - filetwo

#!/usr/bin/perl -w
use strict;

print $0 . "\n";
print __FILE__ . "\n";

It's simplistic, but should show the basic concept.

[solegaonkar]

That explains it! Thank you!!

[BitDreamer]

This is an old question, but there's more of an answer. If a module is found in the current directory by matching '.' in @INC, then the value of __FILE__ is just the filename of the module with no path information. This is important if something does a chdir, then the module is no longer in "the current directory".

[Your Mother]

Good point; part of why I eschew chdir style code. A little more on your call out–

package AB; # File: /Users/moo/AB.pm
use strict;
use Path::Tiny;
sub ohai { __FILE__ }
sub ohai_der { path(__FILE__)->absolute }
1;

moo@cow[54]~>perl -MAB -le 'print AB->ohai; print AB->ohai_der'
AB.pm
/Users/moo/AB.pm

moo@cow[55]~>perl -MAB -le 'chdir "/tmp"; print AB->ohai; print AB->oh
+ai_der;'
AB.pm
/private/tmp/AB.pm