Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl-Sensitive Sunglasses

Difference between $0 and __FILE__

by solegaonkar (Beadle)
on Sep 26, 2012 at 04:05 UTC ( #995692=perlquestion: print w/ replies, xml ) Need Help??
solegaonkar has asked for the wisdom of the Perl Monks concerning the following question:

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?

Comment on Difference between $0 and __FILE__
Download Code
Replies are listed 'Best First'.
Re: Difference between $0 and __FILE__
by BrowserUk (Pope) on Sep 26, 2012 at 04:28 UTC

    Within a module (say used from a program (say, __FILE__ will be but $0 will be

    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.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    RIP Neil Armstrong

Re: Difference between $0 and __FILE__
by pobocks (Chaplain) on Sep 26, 2012 at 04:45 UTC

    __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.

    for(split(" ","tsuJ rehtonA lreP rekcaH")){print reverse . " "}print "\b.\n";
      That explains it! Thank you!!

Log In?

What's my password?
Create A New User
Node Status?
node history
Node Type: perlquestion [id://995692]
Approved by lidden
Front-paged by lidden
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others meditating upon the Monastery: (2)
As of 2015-11-27 00:12 GMT
Find Nodes?
    Voting Booth?

    What would be the most significant thing to happen if a rope (or wire) tied the Earth and the Moon together?

    Results (710 votes), past polls