Oh, the fun things we can write!

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So now we’ve used Puppet to manage a file on our computer. The /etc/motd file is now owned by root and has a fun little sentence in it. We can write all we want out to that file. But sooner or later, we’re going to want to put something a little more interesting. Perhaps we’ll want the hostname or operating system installed?

We’ll take a little side trip first, though, and learn about $variables.

A variable is a symbolic name that contains some value, such as a string, like “Hi Mom!,” or a number, like 42. It is called a variable not because it changes, but because we can choose any value we want to store in the variable. As a matter of fact, in the Puppet language, once a variable has been set, it is not allowed to be changed during the rest of that run. They can more accurately be called constants–another computer sciencey term which simply means a variable that doesn’t change!

Let’s try that out.

$message = "Hi Mom! I am learning about Puppet and it's fun."

notify { $message: }

A notify resource is typically how we output messages during the Puppet run. It will simply display the message used as its title to the console.

[509:ben@ganymede] code $ puppet apply message.pp
Notice: Compiled catalog for ganymede.local in environment production in 0.02 seconds
Notice: Hi Mom! I am learning about Puppet and it's fun.
Notice: /Stage[main]/Main/Notify[Hi Mom! I am learning about Puppet and it's fun.]/message: defined 'message' as 'Hi Mom! I am learning about Puppet and it's fun.'
Notice: Finished catalog run in 0.10 seconds

Here, we stored a value (the string “Hi Mom! I am learning about Puppet and it’s fun.”) into a variable named $message, and then referred to that variable later on in the code. Let’s try that with our motd.pp file from the earlier post.

$content = "Whee, Puppet is fun!

I am learning all kinds of very cool things now.

file { '/etc/motd':
    ensure  => file,
    owner   => 'root',
    group   => 'root',
    mode    => '0644',
    content => $content,

This time we stored a multiple line value into our variable. Notice that on line #11, we passed the variable directly to the content parameter. As the Puppet run proceeds, the symbolic variable is replaced with its value.

That’s handy, but writing out the full file into a variable is not really much of an improvement. How do we accommodate for slight variations? We certainly don’t want to cut & paste and make tiny modifications. Let’s learn how to embed a variable into a string. That will be handy.

Edit the top of the source file, or manifest, to look like this:

$name    = " Your name goes here"
$content = "Whee, Puppet is fun!

I am learning all kinds of very cool things now.
  Administered by: ${name}

Now when you apply your manifest, the /etc/motd file will have your name embedded in it. Notice the curly {} braces on the highlighted line? That’s how we tell Puppet unambiguously which part of the string is your variable and which is not.

It’s worth pointing out that string interpolation, such as $message = "This contains another ${variable} embedded within itself." should use the curly {} braces around the variable name, but this is a syntax error when used outside a string!

Now are you ready for something really cool? I thought so.

Puppet makes a whole list of magic variables containing all sorts of information about the machine you’re running on–things like the hostname or other network information, or what operating system is running. They’re available for you to use in any manifest any time you like. You can simply trust that they’ll be set and Puppet will determine their values automatically. These are called facts and you can inspect them by running facter on your Puppet VM.

[510:ben@ganymede] code $ facter
augeas => {
  version => "1.4.0"
disks => {
  sda => {
    model => "VMware Virtual S",
    size => "20.00 GiB",
    size_bytes => 21474836480,
    vendor => "VMware,"
dmi => {
  bios => {
    release_date => "05/20/2014",
    vendor => "Phoenix Technologies LTD",
    version => "6.00"

Well, that was quite the data dump! Lets narrow it down a bit:

[511:ben@ganymede] code $ facter osfamily
[512:ben@ganymede] code $ facter kernel
[513:ben@ganymede] code $ facter uptime
22:35 hours

Well, that’s pretty useful. Let’s try including some of these facts into our /etc/motd file. Update your manifest to look like this:

$content = "Welcome to ${fqdn}

  This machine has been running ${osfamily}
on a ${kernel}-${kernelrelease} platform
        for ${uptime}.

file { '/etc/motd':
    ensure  => file,
    owner   => 'root',
    group   => 'root',
    mode    => '0644',
    content => $content,

Now we’ll run it and see what our results are.

[514:ben@ganymede] code $ sudo puppet apply test.pp
Notice: Compiled catalog for ganymede.puppetlabs.vm in environment production in 0.28 seconds
Notice: /Stage[main]/Main/File[/etc/motd]/ensure: defined content as '{md5}958237052eb198ed69008e6266c84601'
Notice: Applied catalog in 0.17 seconds
[515:ben@ganymede] code $ cat /etc/motd
Welcome to ganymede.puppetlabs.vm

  This machine has been running RedHat
on a Linux-2.6.32-504.el6.x86_64 platform
        for 22:37 hours.

Now that’s getting darn right useful.