perl function to parse the command output

Question

I have written a perl function that executes a command and parses the command output. The command will provide the output mentioned below. In the function call i pass a number between 1 to 5 as arguments . '1' correspond to lan, 2 corresponds to wan, '3' corresponds to name and so on. ( see the output below) . for example if 1 is passed as an argument in the function call the expected output is '0' ( 1 = lan and value of lan = 0 ) . When i execute the script im not getting the expected output. null is returned.Any suggestions?

command output 1 :

[
    {
       "lan" : 0,           #1
       "wan" : 0,           #2
       "name" : "XYZ",      #3 
       "packets" : 0,       #4
       "bytes" : 0          #5
    }
]

Function call

$self->{'stats_obj'} = Statistics->new( ip => "ip addr")

    my $result = $self->{'stats_obj'}->statistics_get('1');
       INFO('statistics:' . $result );

Function:

sub statistics_get{
    my ($self, $option)= @_;

    my $result = $self->_get_hyd_tc();

    return $result->{$option};

}

sub _get_hyd_tc {

    my ($self)  = @_;

    my $opt;
    my %result;
    my $line;

    my $cmd     = 'cmd goes here';

    $self->execute($cmd);

    my $count =0;

    foreach my $line (  $self->output() ) {
    chomp $line;

    if ( $line =~ /(Requested table doesn't.*)/i ){

        ERROR('table doesnt exist'  . $line)

    }

    if ($line =~ /(.*)/) {

        $opt = $1;
        $count = 0;

   }
     elsif ( $line =~ /(.*)/) {

        my $key = $1;
        my $value = $2;
        $result{$opt}{++$count} = $value;
       }
    }

    return \%result;

}

Your elsif in _get_hyd_tc will never be entered. Why are you using the same condition in both the if and the elsif? And why are you using a condition that will match any string? .* means match zero or more characters. — ThisSuitIsBlackNot
– ThisSuitIsBlackNot, Commented May 19, 2015 at 21:22
Are the comments in the input in the real input or did you put them in? If you put them in, that's a JSON-format and your problem can be solved without any parsing on your part as the JSON module comes with perl. — Patrick J. S.
– Patrick J. S., Commented May 19, 2015 at 21:59

Sobrique · Accepted Answer · 2015-07-04 17:49:13Z

You are approaching this the wrong way.

Your snippet you give is JSON. So really - your best bet by far is to process it as JSON and not try and parse it yourself.

Something like this:

use strict;
use warnings;

use JSON;
use Data::Dumper;

my $command_output = '[
    {
       "lan" : 0,           
       "wan" : 0,           
       "name" : "XYZ",       
       "packets" : 0,       
       "bytes" : 0          
    }
]';


my $json_ob = decode_json( $command_output );
print Dumper \$json_ob;

print $json_ob -> [0] -> {'name'};

Those [] in your text denotes an array. usually that means multiple elements. You could iterate those elements, but as you've just got one, accessing it via [0] does the trick.

Now, you could if you really want map the attributes from 'words' to numeric, but ... there isn't really any need.

But to answer your question - why is null returned - it's because:

if ($line =~ /(.*)/) {

Will always evaluate as true - zero or more of anything.

And therefore you will never run the second elsif loop and so this never happens:

    $result{$opt}{++$count} = $value;

And so you never have anything other than an empty array to return.

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