How can I join a nested Perl hash? - perl

Question

I have a Perl hash, where I store information about LUNs. It has the following structure:
my %luns = (
360000 => {
Devices => [
{ Major_Minor => "8:144",
SCSI_Address => "1:0:0:8",
SCSI_Device => "sdj",
SCSI_Host => "host1",
},
{ Major_Minor => "129:48",
SCSI_Address => "3:0:0:8",
SCSI_Device => "sder",
SCSI_Host => "host3",
},
],
DM_Device => "dm-13",
Size => "45G",
WWID => 360000,
},
360001 => {
Devices => [
{ Major_Minor => "70:144",
SCSI_Address => "1:0:1:39",
SCSI_Device => "sddb",
SCSI_Host => "host1",
},
{ Major_Minor => "135:48",
SCSI_Address => "3:0:1:39",
SCSI_Device => "sdij",
SCSI_Host => "host3",
},
],
DM_Device => "dm-53",
Size => "200G",
WWID => 360000,
},
);
How can I use join to get a comma-separated list of all SCSI_Devices, for example, of 360000?

Answer 1

You're working with a Hash of Hash of Array of Hash. To learn how to work with such structures, I recommend reading perldsc - Perl Data Structures Cookbook.
In this instance, the following loop will print out each of your device lists:
for my $id ( sort { $a <=> $b } keys %luns ) {
my #devices = map { $_->{SCSI_Device} } #{ $luns{$id}{Devices} };
print "$id - #devices\n";
}
Outputs:
360000 - sdj sder
360001 - sddb sdij
Live Demo

Answer 2

You say you want a list of values for LUN 360000, so for a start you need
$luns->{36000}
which is another hash with a Devices element, which has an array reference as a value, and DM_Device, Size, and WWID elements, whose values are simple scalars.
So presumably you want the list that is
$luns->{36000}{Devices}
which is an array of references to hashes, each of which has Major_Minor, SCSI_Address, SCSI_Device, and SCSI_Host elements.
It sounds like you want the SCSI_Device element, and map is the ideal tool to help you with this
my #scsi_devices = map { $_->{SCSI_Device} } #{ $luns->{360000}{Devices} };
That last step is a big leap, and it may help to separate it in your code. For instance, you can copy the reference to the list of devices for 360000, like this
my $devices = $luns->{360000}{Devices};
and extract the SCSI_Device from each of the hashes in that array with
my #scsi_devices = map { $_->{SCSI_Device} } #$devices;
Either way, the array reference must be dereferenced and the required element from each hash in that array must be extracted.
To get a CSV record, unless the data may contain commas of double-quotes, you simply need to join the result of that map
print join(',', #scsi_devices), "\n";
output
sdj,sder
Although I think this falls short of what you actually need. If this isn't clear then please ask.