Variables in Puppet are marked with a dollar sign ($) character. When using variables within a manifest, it is preferred to enclose the variable within braces "${myvariable}" instead of "$myvariable". All of the facts from facter can be referenced as top scope variables (we will discuss scope in the next section). For example, the fully qualified domain name (FQDN) of the node may be referenced by "${::fqdn}". Variables can only contain alphabetic characters, numerals, and the underscore character (_). As a matter of style, variables should start with an alphabetic character. Never use dashes in variable names.

In the variable example explained in the There's more… section, the fully qualified domain name was referred to as ${::fqdn} rather than ${fqdn}; the double colons are how Puppet differentiates scope. The highest level scope, top scope or global, is referred to by two colons (::) at the beginning of a variable identifier. To reduce namespace collisions, always use fully scoped variable identifiers in your manifests. For a Unix user, think of top scope variables as the / (root) level. You can refer to variables using the double colon syntax similar to how you would refer to a directory by its full path. For the developer, you can think of top scope variables as global variables; however, unlike global variables, you must always refer to them with the double colon notation to guarantee that a local variable isn't obscuring the top scope variable.

Capitalization is important in Puppet. In our previous example, we created a package named httpd. If we wanted to refer to this package later, we would capitalize its type (package) as follows:

Package['httpd']

To refer to a class, for example, the something::somewhere class, which has already been included/defined in your manifest, you can reference it with the full path as follows:

Class['something::somewhere']

When you have a defined type, for example the following defined type:

example::thing {'one':} 

The preceding resource may be referenced later as follows:

Example::Thing['one']

Knowing how to reference previously defined resources is necessary for the next section on metaparameters and ordering.

All the manifests that will be used to define a node are compiled into a catalog. A catalog is the code that will be applied to configure a node. It is important to remember that manifests are not applied to nodes sequentially. There is no inherent order to the application of manifests. With this in mind, in the previous httpd example, what if we wanted to ensure that the httpd process started after the httpd package was installed?

We couldn't rely on the httpd service coming after the httpd package in the manifests. What we have to do is use metaparameters to tell Puppet the order in which we want resources applied to the node. Metaparameters are parameters that can be applied to any resource and are not specific to any one resource type. They are used for catalog compilation and as hints to Puppet but not to define anything about the resource to which they are attached. When dealing with ordering, there are four metaparameters used:

The before and require metaparameters specify a direct ordering; notify implies before and subscribe implies require. The notify metaparameter is only applicable to services; what notify does is tell a service to restart after the notifying resource has been applied to the node (this is most often a package or file resource). In the case of files, once the file is created on the node, a notify parameter will restart any services mentioned. The subscribe metaparameter has the same effect but is defined on the service; the service will subscribe to the file.

The relationship between package and service previously mentioned is an important and powerful paradigm of Puppet. Adding one more resource-type file into the fold, creates what puppeteers refer to as the trifecta. Almost all system administration tasks revolve around these three resource types. As a system administrator, you install a package, configure the package with files, and then start the service.

Diagram of Trifecta (Files require package for directory, service requires files and package)

A key concept of Puppet is that the state of the system when a catalog is applied to a node cannot affect the outcome of Puppet run. In other words, at the end of Puppet run (if the run was successful), the system will be in a known state and any further application of the catalog will result in a system that is in the same state. This property of Puppet is known as idempotency. Idempotency is the property that no matter how many times you do something, it remains in the same state as the first time you did it. For instance, if you had a light switch and you gave the instruction to turn it on, the light would turn on. If you gave the instruction again, the light would remain on.

Indent your manifests using two spaces (not tabs), as follows:

service {'httpd':
  ensure  => running,
}

Always quote your resource names, as follows:

package { 'exim4':

We cannot do this as follows though:

package { exim4:

Use single quotes for all strings, except when:

Consider the following code:

file { '/etc/motd':
  content => "Welcome to ${::fqdn}\n"
}

Puppet doesn't process variable references or escape sequences unless they're inside double quotes.

Always quote parameter values that are not reserved words in Puppet. For example, the following values are not reserved words:

name => 'Nucky Thompson',
mode => '0700',
owner => 'deploy',

However, these values are reserved words and therefore not quoted:

ensure => installed,
enable => true,
ensure => running,

There is only one thing in Puppet that is false, that is, the word false without any quotes. The string "false" evaluates to true and the string "true" also evaluates to true. Actually, everything besides the literal false evaluates to true (when treated as a Boolean):

if "false" {
  notify { 'True': }
}
if 'false' {
  notify { 'Also true': }
}
if false {
  notify { 'Not true': }
}

When this code is run through puppet apply, the first two notifies are triggered. The final notify is not triggered; it is the only one that evaluates to false.

Always include curly braces ({}) around variable names when referring to them in strings, for example, as follows:

source => "puppet:///modules/webserver/${brand}.conf",

Otherwise, Puppet's parser has to guess which characters should be a part of the variable name and which belong to the surrounding string. Curly braces make it explicit.

Always end lines that declare parameters with a comma, even if it is the last parameter:

service { 'memcached':
  ensure => running,
  enable => true,
}

This is allowed by Puppet, and makes it easier if you want to add parameters later, or reorder the existing parameters.

When declaring a resource with a single parameter, make the declaration all on one line and with no trailing comma, as shown in the following snippet:

package { 'puppet': ensure => installed }

Where there is more than one parameter, give each parameter its own line:

package { 'rake':
  ensure   => installed,
  provider => gem,
  require  => Package['rubygems'],
}

To make the code easier to read, line up the parameter arrows in line with the longest parameter, as follows:

file { "/var/www/${app}/shared/config/rvmrc":
  owner   => 'deploy',
  group   => 'deploy',
  content => template('rails/rvmrc.erb'),
  require => File["/var/www/${app}/shared/config"],
}

The arrows should be aligned per resource, but not across the whole file, otherwise it can make it difficult for you to cut and paste code from one file to another.

When declaring file resources which are symlinks, use ensure => link and set the target attribute, as follows:

file { '/etc/php5/cli/php.ini':
  ensure => link,
  target => '/etc/php.ini',
}

If you need to use a template as a part of the module, place it in the module's templates directory and refer to it as follows:

file { '/etc/memcached.conf':
  content => template('memcached/memcached.conf.erb'),
}

Puppet will look for the file in:

MODULEPATH/memcached/templates/memcached.conf.erb

Modules can also contain custom facts, custom functions, custom types, and providers.

For more information about these, refer to Chapter 9, External Tools and the Puppet Ecosystem.

For more details on how to organize your modules, see puppetlabs website:

http://docs.puppetlabs.com/puppet/3/reference/modules_fundamentals.html

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.

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, ':|,')

You can add further tests using the elseif keyword, as follows:

if $::timezone == 'UTC' {
  notify { 'Universal Time Coordinated': }
} elseif $::timezone == 'GMT' {
  notify { 'Greenwich Mean Time': }
} else {
  notify { "$::timezone is not UTC": }
}

You can check whether two values are equal using the == syntax, as in our example:

if $::timezone == 'UTC' {
  
}

Alternatively, you can check whether they are not equal using !=:

if $::timezone != 'UTC' {
  …
}

You can also compare numeric values using < and >:

if $::uptime_days > 365 {
  notify { 'Time to upgrade your kernel!': }
}

To test whether a value is greater (or less) than or equal to another value, use <= or >=:

if $::mtu_eth0 <= 1500 {
  notify {"Not Jumbo Frames": }
}

You can put together the kind of simple expressions described previously into more complex logical expressions, using and, or, and not:

if ($::uptime_days > 365) and ($::kernel == 'Linux') {
  …
}

if ($role == 'webserver') and ( ($datacenter == 'A') or ($datacenter == 'B') ) {
  …
}

You can not only match text using a regular expression, but also capture the matched text and store it in a variable:

$input = 'Puppet is better than manual configuration'
if $input =~ /(.*) is better than (.*)/ {
  notify { "You said '${0}'. Looks like you're comparing ${1}
    to ${2}!": }
}

The preceding code produces this output:

You said 'Puppet is better than manual configuration'. Looks like you're comparing Puppet to manual configuration!

The variable $0 stores the whole matched text (assuming the overall match succeeded). If you put brackets around any part of the regular expression, it creates a group, and any matched groups will also be stored in variables. The first matched group will be $1, the second $2, and so on, as shown in the preceding example.

Puppet's regular expression syntax is the same as Ruby's, so resources that explain Ruby's regular expression syntax will also help you with Puppet. You can find a good introduction to Ruby's regular expression syntax at this website:

http://www.tutorialspoint.com/ruby/ruby_regular_expressions.htm.

In the first example, we used a selector (the ? operator) to choose a value for the $systemtype variable depending on the value of $::operatingsystem. This is similar to the ternary operator in C or Ruby, but instead of choosing between two possible values, you can have as many values as you like.

Puppet will compare the value of $::operatingsystem to each of the possible values we have supplied in Ubuntu, Debian, and so on. These values could be regular expressions (for example, for a partial string match, or to use wildcards), but in our case, we have just used literal strings.

As soon as it finds a match, the selector expression returns whatever value is associated with the matching string. If the value of $::operatingsystem is Fedora, for example, the selector expression will return the redhatlike string and this will be assigned to the variable $systemtype.

Unlike selectors, the case statement does not return a value. case statements come in handy when you want to execute different code depending on the value of some expression. In our second example, we used the case statement to include either the debianlike or redhatlike class, depending on the value of $::operatingsystem.

Again, Puppet compares the value of $::operatingsystem to a list of potential matches. These could be regular expressions or strings, or as in our example, comma-separated lists of strings. When it finds a match, the associated code between curly braces is executed. So, if the value of $::operatingsystem is Ubuntu, then the code including debianlike will be executed.

As with if statements, you can use regular expressions with selectors and case statements, and you can also capture the values of the matched groups and refer to them using $1, $2, and so on:

case $::lsbdistdescription {
  /Ubuntu (.+)/: {
    notify { "You have Ubuntu version ${1}": }
  }
  /CentOS (.+)/: {
    notify { "You have CentOS version ${1}": }
  }
  default: {}
}

Both selectors and case statements let you specify a default value, which is chosen if none of the other options match (the style guide suggests you always have a default clause defined):

$lunch = 'Filet mignon.'
$lunchtype =  $lunch ? {
  /fries/ => 'unhealthy',
  /salad/ => 'healthy',
  default => 'unknown',
}

notify { "Your lunch was ${lunchtype}": }

The output is as follows:

t@mylaptop ~ $ puppet apply lunchtype.pp
Notice: Your lunch was unknown
Notice: /Stage[main]/Main/Notify[Your lunch was unknown]/message: defined 'message' as 'Your lunch was unknown'

When the default action shouldn't normally occur, use the fail() function to halt the Puppet run.

You can concatenate arrays with the + operator or append them with the << operator. In the following example, we use the ternary operator to assign a specific package name to the $apache variable. We then append that value to an array using the << operator:

$apache = $::osfamily ? {
  'Debian' => 'apache2',
  'RedHat' => 'httpd'
} 
$packages = ['memcached'] << $apache
package {$packages: ensure => installed}

If we have two arrays, we can use the + operator to concatenate the two arrays. In this example, we define an array of system administrators ($sysadmins) and another array of application owners ($appowners). We can then concatenate the array and use it as an argument to our allowed users:

$sysadmins = [ 'thomas','john','josko' ]
$appowners = [ 'mike', 'patty', 'erin' ]
$users = $sysadmins + $appowners
notice ($users)

When we apply this manifest, we see that the two arrays have been joined as shown in the following command line output:

t@cookbook:~/.puppet/manifests$ puppet apply --parser=future concat.pp Notice: [thomas, john, josko, mike, patty, erin]
Notice: Compiled catalog for cookbook.example.com in environment production in 0.36 seconds
Notice: Finished catalog run in 0.03 seconds
Merging Hashes

If we have two hashes, we can merge them using the same + operator we used for arrays. Consider our $interfaces hash from a previous example; we can add another interface to the hash:

$iface = {
  'name' => 'eth0',
  'ip'   => '192.168.0.1',
  'mac'  => '52:54:00:4a:60:07' 
}  + {'route' => '192.168.0.254'}
notice ($iface)

When we apply this manifest, we see that the route attribute has been merged into the hash (your results may differ, the order in which the hash prints is unpredictable), as follows:

t@cookbook:~/.puppet/manifests$ puppet apply --parser=future hash2.pp
Notice: {route => 192.168.0.254, name => eth0, ip => 192.168.0.1, mac => 52:54:00:4a:60:07}
Notice: Compiled catalog for cookbook.example.com in environment production in 0.36 seconds
Notice: Finished catalog run in 0.03 seconds

Lambda functions are iterators applied to arrays or hashes. You iterate through the array or hash and apply an iterator function such as each, map, filter, reduce, or slice to each element of the array or key of the hash. Some of the lambda functions return a calculated array or value; others such as each only return the input array or hash.

Lambda functions such as map and reduce use temporary variables that are thrown away after the lambda has finished. Use of lambda functions is something best shown by example. In the next few sections, we will show an example usage of each of the lambda functions.

$count = [1,2,3,4,5]
$sum = reduce($count) | $total, $i | { $total + $i }
notice("Sum is $sum")

t@cookbook:~/.puppet/manifests$ puppet apply --parser future lambda.pp
Notice: Sum is 15
Notice: Compiled catalog for cookbook.example.com in environment production in 0.36 seconds
Notice: Finished catalog run in 0.03 seconds

$filter = filter ($count) | $i | { $i > 3 } notice("Filtered array is $filter")

Notice: Filtered array is [4, 5]

$map = map ($count) | $i | { $i * $i } notice("Square of array is $map")

Notice: Square of array is [1, 4, 9, 16, 25]

$firewall_rules = ['192.168.0.1','80','192.168.0.10','443']
slice ($firewall_rules,2) |$ip, $port| { notice("Allow $ip on $port") }

Notice: Allow 192.168.0.1 on 80
Notice: Allow 192.168.0.10 on 443

slice ($firewall_rules,2) |$ip, $port| { firewall {"$port from $ip": dport  => $port, source => "$ip", action => 'accept', }
}

each ($count) |$c| { notice($c) }

Notice: 1
Notice: 2
Notice: 3
Notice: 4
Notice: 5