Re^2: String searching in file

[cbtshare]

thank you very much, my final code and explanation is below, please let me know if I am not understanding something

use warnings;
use strict;
my $line;
my $awsLists = '/ansible/awsLists'; 
my @command;

@command=`aws ec2 describe-instances --filters "Name=instance-state-na
+me,Values=running" | grep  PrivateDnsName | cut -d ":" -f2| cut -d '"
+' -f2 | sort -u `;


##Initializes an empty hash
my %old;

##Checks if the file is there to be opened and read into the handle $i
+n
if( open my $in, '<', $awsLists ) # OK if file not there
  {
##If file is there then read contents into keys and associating a valu
+e of blank or () means new line? 
##  why wouldn't you do @old{<$in>} = undef; , would it be wrong? 

  @old{ <$in> } = (); # note: keys have \n
  close $in;
  }

###This takes the keys from the the hash previously populate from the 
+file and puts it into scalar  
my $oldsize = keys %old;

## Populates the hash again, but this time with the values of the @com
+mand array
##   Does the old keys get overwritten?(I dont think so, eg, an  array
+ values dont get overwritten everytime you push new values)
@old{ @command } = (); # add new ips


#Does the compare, and if the hash keys from the <$in> and different f
+rom the hash keys of @command
if( keys %old > $oldsize ) # only write if new ips added
  {
  open my $out, '>', $awsLists or die "$! opening $awsLists";
##print directly to the file the new keys difference
  print $out sort keys %old; # sort not needed, but nice touch :)
  close $out;
  }



#    system qq{sed -i '/^ *\$/d' $awsLists};

[poj]

###This takes counts the keys from the the in the hash previously populate from the file and puts it into scalar

my $oldsize = keys %old;

#Does the compare, and if the count of hash keys from the <$in> and different from the hash keys of @command has increased (i.e. new keys added)

if( keys %old > $oldsize ) # only write if new ips added

## If file is there then read contents into keys and associating a value of blank or () means new line?
## why wouldn't you do @old{<$in>} = undef; , would it be wrong?

  @old{ <$in> } = (); # note: keys have \n

In this case no difference but possibly misleading you to think @old{<$in>} = 1; would assign the value 1 to all the keys from <$in> which is not the case. Try this simple example

#!/usr/bin/perl
use strict;
use Data::Dumper;

my @keys = ('a','b','c');
my %hash=();
@hash{@keys} = 1;
print Dumper \%hash;
__END__
$VAR1 = {
          'c' => undef,
          'a' => 1,
          'b' => undef
        };

poj