On Unix (but not on Windows), lstat and stat on the directories can help.

(Actually, this works for every file, but df only looks at mountpoints, i.e. directories.)

#!/usr/bin/perl

use strict;
use warnings;

for my $d (qw( / /bin /sbin /lib /usr /usr/bin /usr/sbin /usr/lib /pro
+c /sys /run /tmp /var/run /var/tmp )) {
    print "$d: on device ",(lstat($d))[0];
    if (-l $d) {
        print " (symlink to ",readlink($d),")";
    }
    print "\n";
}

[download]

Running that on a recent debian in a VM shows:

$ perl lstat.pl
/: on device 1795
/bin: on device 1795 (symlink to usr/bin)
/sbin: on device 1795 (symlink to usr/sbin)
/lib: on device 1795 (symlink to usr/lib)
/usr: on device 1795
/usr/bin: on device 1795
/usr/sbin: on device 1795
/usr/lib: on device 1795
/proc: on device 99
/sys: on device 100
/run: on device 105
/tmp: on device 1795
/var/run: on device 1795 (symlink to /run)
/var/tmp: on device 1795
$

[download]

Most stuff is on device 1795, the root filesystem of this VM. /proc is a completely different device (that of the virtual proc filesystem), and so are /run (105) and /sys (100). /var/run is a symlink stored on device 1795 (root filesystem), it points to /run, which is on device 105.

Due to the way symlinks work, everything that appears to be in /var/run/ is really in /run.

Changing lstat() to stat() allows the system to follow symlinks instead of working on them:

#!/usr/bin/perl

use strict;
use warnings;

for my $d (qw( / /bin /sbin /lib /usr /usr/bin /usr/sbin /usr/lib /pro
+c /sys /run /tmp /var/run /var/tmp )) {
    print "$d: on device ",(stat($d))[0];
    if (-l $d) {
        print " (followed symlink to ",readlink($d),")";
    }
    print "\n";
}

[download]
$ perl stat.pl
/: on device 1795
/bin: on device 1795 (followed symlink to usr/bin)
/sbin: on device 1795 (followed symlink to usr/sbin)
/lib: on device 1795 (followed symlink to usr/lib)
/usr: on device 1795
/usr/bin: on device 1795
/usr/sbin: on device 1795
/usr/lib: on device 1795
/proc: on device 99
/sys: on device 100
/run: on device 105
/tmp: on device 1795
/var/run: on device 105 (followed symlink to /run)
/var/tmp: on device 1795
$

[download]

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

In reply to Re^2: An anomaly with Filesys::DfPortable, I need your eyes by afoken
in thread An anomaly with Filesys::DfPortable, I need your eyes by Intrepid

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