Making quick edits to config files

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 which 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. You can use an exec resource to do jobs like this.

How to do it…

This example shows how to use exec to append a line to a text file:

1. Add the following code to your manifests/site.pp file:

   define append_if_no_such_line($file,$line) {
     exec { "/bin/echo '${line}' >> '${file}'":
       unless => "/bin/grep -Fx '${line}' '${file}'",
     }
   }

2. Modify your manifests/nodes.pp file as follows:

   node 'cookbook' {
     append_if_no_such_line { 'enable-ip-conntrack':
       file => '/etc/modules',
       line => 'ip_conntrack...