Iterating over multiple items

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' ]

How to do it…

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.

How it works…

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 parameters (ensure => installed). This is a very compact way to instantiate many similar resources.

There's more…

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.

Using hashes

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 notify in the output:

t@cookbook:~/.puppet/manifests$ puppet apply hash.pp
Notice: (192.168.0.1) at 52:54:00:4a:60:07 on etho

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 for which we are looking.

Creating arrays with the split function

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@mylaptop ~ $ puppet apply lunchprint.pp 
...
Notice: Lunch included chips
Notice: Lunch included beans
Notice: Lunch included egg

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@mylaptop ~ $ puppet apply lunchprint2.pp 
...
Notice: Lunch included chips
Notice: Lunch included beans
Notice: Lunch included egg.

Note that split takes two arguments: the first argument is the string to be split. The second argument is the character to split on; in this example, a single space. As Puppet works its way through the string, when it encounters a space, it will interpret it as the end of one item and the beginning of the next. So, given the string 'egg beans chips', this will be split into three items.

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, ':|,')