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Writing powerful conditional statements

Puppet's if statement allows you to change the manifest based on the value of a variable or an expression. With it, you can apply different resources or parameter values depending on certain facts about the node, for example, the operating system, or the memory size. You can also set variables within the manifest which can change the behavior of included classes. For example, nodes in data center A might need to use different DNS servers than nodes in data center B, or you might need to include one set of classes for an Ubuntu system, and a different set for other systems.

How to do it…

Here's an example of a useful conditional statement:

Add the following code to your manifest:

if $::operatingsystem == 'Ubuntu' {
  notify { 'Running on Ubuntu': }
} else {
  notify { 'Non-Ubuntu system detected. Please upgrade
    to Ubuntu immediately.': }
}

How it works…

Puppet treats whatever follows an if keyword as an expression and evaluates it. If the expression evaluates to true, Puppet will execute the code within the curly braces.

Optionally, you can add an else branch, which will be executed if the expression evaluates to false.

There's more…

Here are some more tips on using if statements.

Elsif branches

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

if $::operatingsystem == 'Ubuntu' {
  notify { 'Running on Ubuntu': }
} elsif $::operatingsystem == 'Debian' {
  notify { 'Close enough...': }
} else {
  notify { 'Non-Ubuntu system detected. Please upgrade to Ubuntu
    immediately.': }
}

Comparisons

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

if $::operatingsystem == 'Ubuntu' {
  …

}

Or, you can check if they are not equal using !=:

if $::operatingsystem != 'CentOS' {
     
}

You can also compare numeric values using < and >:

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

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

if $::lsbmajdistrelease <= 12 {
     
}

Combining expressions

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

if ($::uptime_days > 365) and ($::operatingsystem == 'Ubuntu') {
     
}

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