Reading The Fine Source it appears that variable is just used as the fallback fstab argument used IFF it's defined and the caller hasn't passed that key to the constructor. It doesn't look like it's explicitly populated. That being said it looks like you could be slightly naughty (in breaking encapsulation) and peek inside the object and pull it out with something like $foo->{fstab} after you have an instance.

G'day Rick,

As far as I can tell, Sys::Filesystem does not report what fstab file it uses. It relies on the various Sys::Filesystem::<OS> modules, e.g. Sys::Filesystem::AIX, for the filesystem information. See the "Sys-Filesystem distro page" for a list of those modules.

If you simply want to perform some upfront sanity checks, perhaps code similar to the following might suffice.

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

use Sys::Filesystem ();

if (Sys::Filesystem::->supported()) {
    my $fs = Sys::Filesystem::->new();

    say '$Sys::Filesystem::FSTAB: ',
            $Sys::Filesystem::FSTAB // 'undefined';
    say '$fs->{fstab}: ', $fs->{fstab} // 'undefined';
    say "O/S '$^O' supported.";
    say 'Using: ', ref($fs->{filesystems});
    my @fs_list = $fs->filesystems();

    if (@fs_list) {
        say 'Filesystems found:';
        say "\t$_" for @fs_list;
    }
    else {
        warn "No filesystems found.\n";
    }

    use Data::Dump;
    say '=' x 60;
    say 'Sys::Filesystem Object:';
    say '=' x 60;
    dd $fs;
    say '-' x 60;
}
else {
    warn "O/S '$^O' not supported.\n";
}
[download]

I've included code to produce a lot of additional information which you may find interesting, but probably isn't wanted in production.

I'm strongly against breaking encapsulation and urge you to stick with the published API. I only included $fs->{fstab} and $fs->{filesystems} for demonstration purposes.

I don't have any of the four OSes you list available at home. Here's the output on my Cygwin system.

$Sys::Filesystem::FSTAB: undefined
$fs->{fstab}: undefined
O/S 'cygwin' supported.
Using: Sys::Filesystem::Cygwin
Filesystems found:
        /
        /cygdrive/c
        /cygdrive/d
        /usr/bin
        /usr/lib
============================================================
Sys::Filesystem Object:
============================================================
bless({
  aliases     => {
                   boot_order      => ["fs_mntno"],
                   check_frequency => ["fs_freq"],
                   check_order     => ["fs_passno"],
                   device          => ["fs_spec", "dev"],
                   filesystem      => ["fs_file", "mount_point"],
                   format          => ["fs_vfstype", "vfs", "vfstype"]
+,
                   label           => ["fs_label"],
                   mount_point     => ["fs_file", "filesystem"],
                   options         => ["fs_mntops"],
                   type            => ["fs_vfstype", "vfs"],
                   volume          => ["fs_volume", "fs_vol", "vol"],
                 },
  canondev    => undef,
  filesystems => bless({
                   "/" => {
                     device      => "C:/cygwin64",
                     fs_file     => "/",
                     fs_mntops   => "binary,auto",
                     fs_spec     => "C:/cygwin64",
                     fs_vfstype  => "ntfs",
                     mount_point => "/",
                     mounted     => 1,
                   },
                   "/cygdrive/c" => {
                     device      => "C:",
                     fs_file     => "/cygdrive/c",
                     fs_mntops   => "binary,posix=0,user,noumount,auto
+",
                     fs_spec     => "C:",
                     fs_vfstype  => "ntfs",
                     mount_point => "/cygdrive/c",
                     mounted     => 1,
                   },
                   "/cygdrive/d" => {
                     device      => "D:",
                     fs_file     => "/cygdrive/d",
                     fs_mntops   => "binary,posix=0,user,noumount,auto
+",
                     fs_spec     => "D:",
                     fs_vfstype  => "ntfs",
                     mount_point => "/cygdrive/d",
                     mounted     => 1,
                   },
                   "/usr/bin" => {
                     device      => "C:/cygwin64/bin",
                     fs_file     => "/usr/bin",
                     fs_mntops   => "binary,auto",
                     fs_spec     => "C:/cygwin64/bin",
                     fs_vfstype  => "ntfs",
                     mount_point => "/usr/bin",
                     mounted     => 1,
                   },
                   "/usr/lib" => {
                     device      => "C:/cygwin64/lib",
                     fs_file     => "/usr/lib",
                     fs_mntops   => "binary,auto",
                     fs_spec     => "C:/cygwin64/lib",
                     fs_vfstype  => "ntfs",
                     mount_point => "/usr/lib",
                     mounted     => 1,
                   },
                 }, "Sys::Filesystem::Cygwin"),
  fstab       => undef,
  mtab        => undef,
}, "Sys::Filesystem")
------------------------------------------------------------
[download]

Hi Ken

Thanks very much for the reply.

I just tried your code on RHEL 7. Here's the salient output:

$Sys::Filesystem::FSTAB: undefined
$fs->{fstab}: undefined
O/S 'linux' supported.
Using: Sys::Filesystem::Linux
[download]

An environment issue?

Rick

"An environment issue?"

I don't understand the question. What are you asking about the environment? What issue do think exists?

Other than "[Ll]inux", instead of "[Cc]ygwin", that's identical to my first four lines of output. I assume you got, but didn't post, subsequent lines about "filesystems found", followed by dd $fs output.

— Ken