Sunday, April 13, 2014

Reading Environment Variables in Puppet

Puppet uses a tool called Facter to discover facts about the system it is going to provision. Some examples of these facts are: $operatingsystem, $hostname, $processorcount, etc. These facts are available as variables in puppet to use in manifests. You can see the full list here: Factor 2.0 Core Facts.

If you want an environment variable, or a bash script parameter to be available to puppet, you need to make it available as a Facter fact. You can do that by simply setting a variable whose name is prefixed with the "FACTER_" string.

So for instance, if you wanted to make a variable called say $my_module_name available within puppet, and wanted its value to be equal to environment variable PRODUCT_MODULE_NAME, you could do the following in a bash script that invokes puppet:

export FACTER_my_module_name=$PRODUCT_MODULE_NAME

Now, $my_module_name variable will be available within your puppet manifests.

Setting a Default if Environment Variable is not set

To take this further, if you wanted your puppet variable to have a default value, if no environment variable was set, you could specify it in puppet manifest (.pp) file like this:

$module_name = $my_module_name ? {
      undef => "Admin_Module",
      default               => $my_module_name 

Note that the above code uses a different variable name called "module_name", whose value will be set to "Admin_Module" if $my_module_name is un-defined, else it will be set to value of "my_module_name" variable.
Now you can use $module_name in all your puppet manifests (instead of using $my_module_name).

To see a sample code where I did this in Bahmni Hospital Management System provisioning scripts, check out this Github commit: Example.

Tip: If you wish to debug what facts are being set in your system, you can use this snippet to print all facts to a file: Printing all puppet facts.

Tip: If your puppet script is running with "sudo", then the environment variables set in current environment won't be passed to the sudo environment. If you want that, then use "-E" switch with sudo to pass environment variables forward.