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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?

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!!
Re: Difference between $0 and __FILE__
by BitDreamer (Sexton) on Jul 20, 2017 at 19:04 UTC
    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".

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

      package AB; # File: /Users/moo/ 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' /Users/moo/
      moo@cow[55]~>perl -MAB -le 'chdir "/tmp"; print AB->ohai; print AB->oh +ai_der;' /private/tmp/

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