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 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.

Getting ready

Install the puppetlabs-stdlib module using puppet:

t@mylaptop ~ $ puppet module install puppetlabs-stdlib
Notice: Preparing to install into /home/thomas/.puppet/modules ...
Notice: Downloading from https://forgeapi.puppetlabs.com ...
Notice: Installing -- do not interrupt ...
/home/thomas/.puppet/modules
└── puppetlabs-stdlib (v4.5.1)

This installs the module from the forge into my user's puppet directory; to install into the system directory, run the command as root or use sudo. For the purpose of this example, we'll continue working as our own user.

How to do it...

Using the file_line resource type, we can ensure that a line exists or is absent in a config file. Using file_line we can quickly make edits to files without controlling the entire file.

1. Create a manifest named oneline.pp that will use file_line on a file in /tmp:

     file {'/tmp/cookbook':
       ensure => 'file',
     }
     file_line {'cookbook-hello':
       path    => '/tmp/cookbook',
       line    => 'Hello World!',
       require => File['/tmp/cookbook'],
     }

2. Run puppet apply on the oneline.pp manifest:

   t@mylaptop ~/.puppet/manifests $ puppet apply oneline.pp 
   Notice: Compiled catalog for mylaptop in environment production in 0.39 seconds
   Notice: /Stage[main]/Main/File[/tmp/cookbook]/ensure: created
   Notice: /Stage[main]/Main/File_line[cookbook-hello]/ensure: created
   Notice: Finished catalog run in 0.02 seconds
   

3. Now verify that /tmp/cookbook contains the line we defined:

   t@mylaptop ~/.puppet/manifests $ cat /tmp/cookbook
   Hello World!
   

How it works…

We installed the puppetlabs-stdlib module into the default module path for Puppet, so when we ran puppet apply, Puppet knew where to find the file_line type definition. Puppet then created the /tmp/cookbook file if it didn't exist. The line Hello World! was not found in the file, so Puppet added the line to the file.

There's more…

We can define more instances of file_line and add more lines to the file; we can have multiple resources modifying a single file.

Modify the oneline.pp file and add another file_line resource:

  file {'/tmp/cookbook':
    ensure => 'file',
  }
  file_line {'cookbook-hello':
    path    => '/tmp/cookbook',
    line    => 'Hello World!',
    require => File['/tmp/cookbook'],
  }
  file_line {'cookbook-goodbye':
    path    => '/tmp/cookbook',
    line    => 'So long, and thanks for all the fish.',
    require => File['/tmp/cookbook'],
  }

Now apply the manifest again and verify whether the new line is appended to the file:

t@mylaptop ~/.puppet/manifests $ puppet apply oneline.pp 
Notice: Compiled catalog for mylaptop in environment production in 0.36 seconds
Notice: /Stage[main]/Main/File_line[cookbook-goodbye]/ensure: created
Notice: Finished catalog run in 0.02 seconds
t@mylaptop ~/.puppet/manifests $ cat /tmp/cookbook 
Hello World!
So long, and thanks for all the fish.

The file_line type also supports pattern matching and line removal as we'll show you in the following example:

  file {'/tmp/cookbook':
    ensure => 'file',
  }
  file_line {'cookbook-remove':
    ensure  => 'absent',
    path    => '/tmp/cookbook',
    line    => 'Hello World!',
    require => File['/tmp/cookbook'],
  }
  file_line {'cookbook-match':
    path    => '/tmp/cookbook',
    line    => 'Oh freddled gruntbuggly, thanks for all the fish.',
    match   => 'fish.$',
    require => File['/tmp/cookbook'],
  }

Verify the contents of /tmp/cookbook before your Puppet run:

t@mylaptop ~/.puppet/manifests $ cat /tmp/cookbook 
Hello World!
So long, and thanks for all the fish.

Apply the updated manifest:

t@mylaptop ~/.puppet/manifests $ puppet apply oneline.pp 
Notice: Compiled catalog for mylaptop in environment production in 0.30 seconds
Notice: /Stage[main]/Main/File_line[cookbook-match]/ensure: created
Notice: /Stage[main]/Main/File_line[cookbook-remove]/ensure: removed
Notice: Finished catalog run in 0.02 seconds

Verify that the line has been removed and the goodbye line has been replaced:

t@mylaptop ~/.puppet/manifests $ cat /tmp/cookbook 
Oh freddled gruntbuggly, thanks for all the fish.

Editing files with file_line works well if the file is unstructured. Structured files may have similar lines in different sections that have different meanings. In the next section, we'll show you how to deal with one particular type of structured file, a file using INI syntax.