in reply to What script is this, and where is it? (Re: who am I?)
in thread who am I?

All of those mechanisms break if the script is being called by a symlink, and particularly so if nested symlinks are involved. You need to readlink($0) if (-l $0) first, and then check, and even then, it won't give you the same output as FindBin::RealBin.

Create the following directory structure in /var/tmp. Set up directories as follows (you'll need root for /usr/local/bin, so feel free to make the last step anywhere else in your path that you'd like): 

cd /var/tmp
mkdir mydir
mkdir datadir
mkdir otherdir
cd otherdir
ln -s ../mydir symdir

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Now take the following code and put it in /var/tmp/mydir/myname.pl: 
#!/usr/bin/perl

use strict;
use warnings;

use FindBin;
use File::Basename qw( dirname );
use File::Spec::Functions qw( rel2abs );

my $name = $0;
$name = readlink($name) if(-l $name);

print "Finding a data path:\n\n";
print 'dirname($name):',dirname($name),"/../datadir\n";
print 'dirname(rel2abs($name)):',dirname(rel2abs($name)),"/../datadir\
+n";
print 'dirname($0):',dirname($0),"/../datadir\n";
print 'dirname(rel2abs($0)):',dirname(rel2abs($0)),"/../datadir\n";
print '$FindBin::RealBin:',$FindBin::RealBin,"/../datadir\n";

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Symlink /var/tmp/symdir/myname.pl to /usr/local/bin/myname (or somewhere else on your path, if you can't or don't want to tamper with a system directory). Run 'myname'. You get: 

Finding a data path:

dirname($name):/var/tmp/otherdir/symdir/../datadir
dirname(rel2abs($name)):/var/tmp/otherdir/symdir/../datadir
dirname($0):/usr/local/bin/../datadir
dirname(rel2abs($0)):/usr/local/bin/../datadir
$FindBin::RealBin:/var/tmp/mydir/../datadir

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Only FindBin correctly points you to the correct data directory location.

Frankly, even without symlink forests, FindBin is more readable, though.

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Re^2: What script is this, and where is it? (Re: who am I?)
by ikegami (Patriarch) on Sep 24, 2008 at 22:45 UTC

    All of those mechanisms break if the script is being called by a symlink, and particularly so if nested symlinks are involved. You need to readlink($0) if (-l $0) first,

    Agreed.

    Only FindBin correctly points you to the correct data directory location.

    That's wrong. 

    /var/tmp/otherdir/symdir/../datadir

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    is the same directory as 
    /var/tmp/mydir/../datadir

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    so you demonstrated that working with readlinked $0 works.

    even then, it won't give you the same output as FindBin::RealBin.

    Doesn't matter.

    FindBin is more readable, though.

    What good is readable if it doesn't work as well. And I don't see how dirname($script_qn) is less readable than $FindBin::RealBin.

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      Edit: claims withdrawn. See below. Original message preserved below the jump.

      Read more... (2 kB)
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        *cough* ... look again.

        /var/tmp/otherdir/symdir/../datadir' => '/var/tmp/otherdir/datadir'

        I read it fine the first time. 

        /var/tmp/otherdir/symdir/../datadir

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        is equivalent to 

        /var/tmp/datadir

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        and not equivalent to 

        /var/tmp/otherdir/datadir

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        In "/var/tmp/otherdir/symdir/../datadir", the ".." is that of "mydir" which is a hard link to "/var/tmp".

        Do like I did and test before contradicting someone. 

        $ cd /var/tmp
$ mkdir mydir
$ mkdir datadir
$ mkdir otherdir
$ touch mydir/in_mydir
$ touch datadir/in_datadir
$ touch otherdir/in_otherdir
$ cd otherdir
$ ln -s ../mydir symdir
$ ls /var/tmp/otherdir/symdir/../datadir
in_datadir
$ perl -le'opendir $dh, $ARGV[0] or die; print for grep /in/, readdir 
+$dh' /var/tmp/otherdir/symdir/../datadir
in_datadir

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