Understanding Hashes

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November 19th, 2012, 12:24 PM

gw1500se

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Understanding Hashes

I am trying to reverse engineer a hash but I am don't have enough experience with them to understand what I am seeing. The following is the output from Dumper for my hash called @changes:

Code:

$VAR1 = [
          {
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/local',
            'type' => 'stat',
            'change_time' => {
                             'new' => '1353344714',
                             'old' => '1352733300'
                           },
            'modification_time' => {
                                   'new' => '1353344714',
                                   'old' => '1312223768'
                                 }
          },
          {
            'uid' => '0',
            'permissions' => '644',
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/local/addedfile2.txt',
            'type' => [
                      'newfile',
                      '100000'
                    ],
            'modification_time' => '1353344714',
            'size' => '0',
            'gid' => '0'
          },
          {
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/bin',
            'type' => 'stat',
            'change_time' => {
                             'new' => '1353344778',
                             'old' => '1352733300'
                           },
            'modification_time' => {
                                   'new' => '1353344778',
                                   'old' => '1334360360'
                                 }
          },
          {
            'uid' => '0',
            'permissions' => '755',
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/bin/pinfo.tmp',
            'type' => [
                      'newfile',
                      '100000'
                    ],
            'modification_time' => '1096294431',
            'size' => '100272',
            'gid' => '0'
          },
          {
            'uid' => '0',
            'permissions' => '755',
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/bin/pinky.tmp',
            'type' => [
                      'newfile',
                      '100000'
                    ],
            'modification_time' => '1236179771',
            'size' => '26248',
            'gid' => '0'
          },
          {
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/games',
            'type' => 'stat',
            'change_time' => {
                             'new' => '1353344685',
                             'old' => '1352733269'
                           },
            'modification_time' => {
                                   'new' => '1353344685',
                                   'old' => '1092330129'
                                 }
          },
          {
            'uid' => '0',
            'permissions' => '644',
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/games/addedfile.txt',
            'type' => [
                      'newfile',
                      '100000'
                    ],
            'modification_time' => '1353344685',
            'size' => '0',
            'gid' => '0'
          },
          {
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/bin/pgawk',
            'type' => 'stat',
            'change_time' => {
                             'new' => '1353345281',
                             'old' => '1352733298'
                           },
            'modification_time' => {
                                   'new' => '1353345281',
                                   'old' => '1099590961'
                                 }
          },
          {
            'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/bin/dmesg',
            'type' => 'stat',
            'change_time' => {
                             'new' => '1353345307',
                             'old' => '1352733298'
                           },
            'modification_time' => {
                                   'new' => '1353345307',
                                   'old' => '1236338436'
                                 }
          }
        ];

I understand how to get the individual elements (e.g. $ref->{type}) but when there is only a single one. It is not clear what I am seeing when there are multiples. I tried treating it like an array in a foreach but that gives me bad index in hash errors.

Code:

foreach $change (@changes) {
   print("change type is ".$change->{type}."\n");
}

Is my syntax messed up because $change is itself a hash not a scalar or am I not understanding the structure of the hash Dumper is giving me? TIA.

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November 19th, 2012, 01:11 PM

Laurent_R

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You are using a relatively complex data structure, not a simple hash.

Try to use Dumper to look at:
$VAR1->[0], $VAR1->[1], ..., $VAR1->[9].

For example, $VAR1->[0] will print something like:

Perl Code:

Original - Perl Code
0 HASH(0x200bc5f0) 'change_time' => HASH(0x200bc6b0) 'new' => 1353344714 'old' => 1352733300 'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/local' 'modification_time' => HASH(0x200bc728) 'new' => 1353344714 'old' => 1312223768 'type' => 'stat'

0  HASH(0x200bc5f0)
   'change_time' => HASH(0x200bc6b0)
      'new' => 1353344714
      'old' => 1352733300
   'file' => '/nobackup/fbe/cel5.03-x86_64-1.0/usr/local'
   'modification_time' => HASH(0x200bc728)
      'new' => 1353344714
      'old' => 1312223768
   'type' => 'stat'

So you have an array of hashes.

But each hash in this array contains some straight forward hash values ('type' and 'file') and also references to nested hashes ('change_time' and 'modification_time' are themselves hashes with a 'new' and an 'old' element).

You can continue to explore. For example try to print:

Code:

$VAR1->[0]{file}

you should get this:
/nobackup/fbe/cel5.03-x86_64-1.0/usr/local

If you print:

Code:

$VAR1->[0]{change_time}

You'll get a reference to a hash, something like this:

Code:

HASH(0x200bc6b0)

Try further to print:

Code:

$VAR1->[0]{change_time}{new}

I hope I have given you an idea on how to explore your data structure, just continue.

Last edited by Laurent_R : November 19th, 2012 at 01:40 PM.

November 19th, 2012, 01:31 PM

gw1500se

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Thanks. I see my problem, it was the way I referenced elements of the array. The [ ] goes after the -> not before.

That leaves me with another problem. How do I get the number of elements of the array in that case ($#VAR1 will not work)?

November 19th, 2012, 02:13 PM

gw1500se

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One additional question, although not really related to the hash issue. Note the time values in the Dumper output (e.g. change_time, modification_time). Can you recognize what they are? If I pass those values to 'localtime' they are completely wrong. They must be some other kind of time stamp but I don't recognize it. TIA.

November 19th, 2012, 02:36 PM

keath

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Quote:

Originally Posted by gw1500se

That leaves me with another problem. How do I get the number of elements of the array in that case ($#VAR1 will not work)?

The scalar keyword is handy.

Code:

my $number = scalar @$VAR1;

print "There are $number elements in the array\n";

If the integer doesn't correspond to localtime, I have no idea. Looks like it would correspond to the filesystem, but it could be UTC.

November 19th, 2012, 03:38 PM

Laurent_R

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The timestamps look OK to me:

Code:

$ perl -e '$c = gmtime 1353344714; print "new = $c"'
new = Mon Nov 19 17:05:14 2012
$
$ perl -e '$c = gmtime 1352733300; print "old = $c"'
old = Mon Nov 12 15:15:00 2012