Arrays are a powerful feature in Puppet; wherever you want to perform the same operation on a list of things, an array may be able to help. You can create an array just by putting its content in square brackets:

$lunch = [ 'franks', 'beans', 'mustard' ]

Here's a common example of how arrays are used:

1. Add the following code to your manifest:

$packages = [
  'ruby1.8-dev', 'ruby1.8',
  'ri1.8', 'rdoc1.8',
  'irb1.8', 'libreadline-ruby1.8',
  'libruby1.8', 'libopenssl-ruby'
]
package { $packages: ensure => installed }

2. Run Puppet and note that each package should now be installed.

Where Puppet encounters an array as the name of a resource, it creates a resource for each element in the array. In the example, a new package resource is created for each of the packages in the $packages array, with the same ensure => installed parameters. This is a very compact way to instantiate many similar resources.

Although arrays will take you a long way with Puppet, it's also useful to know about an even more flexible data structure: the hash.

A hash is like an array, but each of the elements can be stored and looked up by name (referred to as the key); for example, hash.pp:

$interface = {
 'name' => 'eth0',
 'ip'   => '192.168.0.1',
 'mac'  => '52:54:00:4a:60:07'
}
notify { "(${interface['ip']}) at ${interface['mac']} on ${interface['name']}": }

When we run Puppet on this, we see the following notice in the output:

t@cookbook:~/.puppet/manifests$ puppet apply hash.pp
Notice: Compiled catalog for cookbook.example.com in environment production in 0.04 seconds
Notice: (192.168.0.1) at 52:54:00:4a:60:07 on eth0

Hash values can be anything that you can assign to variables, strings, function calls, expressions, and even other hashes or arrays. Hashes are useful to store a bunch of information about a particular thing because by accessing each element of the hash using a key, we can quickly find the information we are looking for.

You can declare literal arrays using square brackets, as follows:

define lunchprint() {
 notify { "Lunch included ${name}":}
}
$lunch = ['egg', 'beans', 'chips']
lunchprint { $lunch: }

Now, when we run Puppet on the preceding code, we see the following notice messages in the output:

t@cookbook:~$ puppet apply lunchprint.pp
Notice: Compiled catalog for cookbook.strangled.net in environment production in 0.02 seconds
Notice: Lunch included egg
Notice: Lunch included beans
Notice: Lunch included chips
Notice: Applied catalog in 0.04 seconds

However, Puppet can also create arrays for you from strings, using the split function, as follows:

$menu = 'egg beans chips'
$items = split($menu, ' ')
lunchprint { $items: }

Running puppet apply against this new manifest, we see the same messages in the output:

t@cookbook:~$ puppet apply lunchprint2.pp
Notice: Compiled catalog for cookbook.strangled.net in environment production in 0.02 seconds
Notice: Lunch included egg
Notice: Lunch included beans
Notice: Lunch included chips
Notice: Applied catalog in 0.21 seconds

The character to split on can be any character or string:

$menu = 'egg and beans and chips' $items = split($menu, ' and ')

The character can also be a regular expression, for example, a set of alternatives separated by a | (pipe) character:

$lunch = 'egg:beans,chips'
$items = split($lunch, ':|,')