When you need to have Puppet change a particular setting in a config file, it's common to simply deploy the whole file with Puppet. This isn't always possible, though; especially if it's a file that several different parts of your Puppet manifest may need to modify.
What would be useful is a simple recipe to add a line to a config file if it's not already present, for example, adding a module name to /etc/modules to tell the kernel to load that module at boot. There are several ways to do this, the simplest is to use the file_line type provided by the puppetlabs-stdlib module. In this example, we install the stdlib module and use this type to append a line to a text file.