Facter 2.4: Custom Facts Walkthrough

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Extend Facter by writing your own custom facts to provide information to Puppet.

Adding Custom Facts to Facter

Sometimes you need to be able to write conditional expressions based on site-specific data that just isn’t available via Facter, or perhaps you’d like to include it in a template.

Since you can’t include arbitrary Ruby code in your manifests, the best solution is to add a new fact to Facter. These additional facts can then be distributed to Puppet clients and are available for use in manifests and templates, just like any other fact would be.

The Concept

You can add new facts by writing snippets of Ruby code on the Puppet master. Puppet then uses Plugins in Modules to distribute the facts to the client.

Loading Custom Facts

Facter offers a few methods of loading facts:

  • $LOAD_PATH, or the Ruby library load path
  • The environment variable FACTERLIB
  • Facts distributed using pluginsync

You can use these methods of loading facts to do things like test files locally before distributing them, or you can arrange to have a specific set of facts available on certain machines.

Facter searches all directories in the Ruby $LOAD_PATH variable for subdirectories named ‘facter’, and loads all Ruby files in those directories. If you had some directory in your $LOAD_PATH like ~/lib/ruby, set up like this:

└── facter
    ├── rackspace.rb
    ├── system_load.rb
    └── users.rb

Facter tries to load ‘facter/system_load.rb’, ‘facter/users.rb’, and ‘facter/rackspace.rb’.

Facter also checks the environment variable FACTERLIB for a colon-delimited set of absolute paths to directories, then tries to load all Ruby files in those directories. This allows you to do something like this:

$ pwd
$ ls my_facts
$ ls my_other_facts
$ export FACTERLIB="/home/user/my_facts:/home/user/my_other_facts"
$ facter system_load users
system_load => 0.25
users => thomas,pat

Facter can also easily load fact files distributed using pluginsync. Running facter -p loads all the facts that have been distributed via pluginsync, so if you’re using a lot of custom facts inside Puppet, you can easily use these facts with standalone Facter.

Custom facts can be distributed to clients using the Plugins in Modules method.

Two Parts of Every Fact

Setting aside external facts for now, every fact has at least two elements:

  1. A call to Facter.add('fact_name'), which determines the name of the fact.
  2. A setcode statement, which Facter evaluates to determine the fact’s value.

Facts can get a lot more complicated than that, but every fact contains at least those two parts.

Executing Shell Commands in Facts

Puppet gets information about a system from Facter, and the most common way for Facter to get that information is by executing shell commands. You can then parse and manipulate the output from those commands using standard Ruby code. The Facter API gives you a few ways to execute shell commands:

  • If all you want to do is run the command and use the verbatim output as your fact’s value, you can pass the command into setcode directly. For example: setcode 'uname --hardware-platform'.
  • If your fact is any more complicated than that, you can call Facter::Core::Execution.exec('uname --hardware-platform') from within the setcode doend block. As always, Facter uses whatever the setcode statement returns as the fact’s value.
  • In any case, remember that your shell command is also a Ruby string, so you’ll need to escape special characters if you want to pass them through.

It’s important to note that not everything that works in the terminal will work in a fact. You can use the pipe (|) and similar operators just as you normally would, but Bash-specific syntax like if statements do not work. The best way to handle this limitation is to write your conditional logic in Ruby.

An Example

Let’s say you need to get the output of uname --hardware-platform to single out a specific type of workstation. To do this, create a new custom fact. Start by giving the fact a name, in this case hardware_platform, and create your new fact in a file, hardware_platform.rb, on the Puppet master server:

# hardware_platform.rb

Facter.add('hardware_platform') do
  setcode do
    Facter::Core::Execution.exec('/bin/uname --hardware-platform')

You can then use the instructions in the Plugins in Modules guide to copy the new fact to a module and distribute it. The value of the new fact is then available to use in your manifests and templates during your next Puppet run.

The best place to find ideas on writing your own custom facts is to look at the code for Facter’s core facts, which contains a wealth of examples of how to retrieve different types of system data and return useful facts.

Using Other Facts

You can write a fact that uses other facts by accessing Facter.value(:somefact). If the named fact is unresolved, Facter.value returns nil; but if the fact can’t be found at all, Facter throws an error.

For example:

Facter.add(:osfamily) do
  setcode do
    distid = Facter.value(:lsbdistid)
    case distid
    when /RedHatEnterprise|CentOS|Fedora/
    when 'ubuntu'

Configuring Facts

Facts have a few properties that you can use to customize how facts are evaluated.

Confining Facts

One of the more commonly used properties is the confine statement, which restricts the fact to only run on systems that matches another given fact.

An example of the confine statement would be something like the following:

Facter.add(:powerstates) do
  confine :kernel => 'Linux'
  setcode do
    Facter::Core::Execution.exec('cat /sys/power/states')

This fact uses sysfs on Linux to get a list of the power states that are available on the given system. Since this is only available on Linux systems, we use the confine statement to ensure that this fact isn’t needlessly run on systems that don’t support this type of enumeration.

Fact Precedence

A single fact can have multiple resolutions, each of which is a different way of ascertaining what the value of the fact should be. It’s very common to have different resolutions for different operating systems, for example. It’s easy to confuse facts and resolutions because they are superficially identical — to add a new resolution to a fact, you simply add the fact again, only with a different setcode statement.

When a fact has more than one resolution, make sure that Facter executes only one of them. Otherwise, each subsequent resolution overrides the one before it, and you might not get the value that you want.

Facter decides the issue of precedence using the weight property. Once Facter rules out any resolutions that are excluded because of confine statements, it executes the resolution with the highest weight. If that resolution doesn’t return a value, Facter moves on to the next resolution (by descending weight) until it gets a suitable value for the fact.

By default, the weight of a fact is the number of confines for that resolution, so more specific resolutions take priority over less specific resolutions.

# Check to see if this server has been marked as a postgres server.
Facter.add(:role) do
  has_weight 100
  setcode do
    if File.exist? '/etc/postgres_server'

# Guess if this is a server by the presence of the pg_create binary.
Facter.add(:role) do
  has_weight 50
  setcode do
    if File.exist? '/usr/sbin/pg_create'

# If this server doesn't look like a server, it must be a desktop.
Facter.add(:role) do
  setcode do

Timing Out

If you have facts that are unreliable and may not finish running, you can use the timeout property. If a fact is defined with a timeout and the evaluation of the setcode block exceeds the timeout, Facter will halt the resolution of that fact and move on.

# Sleep
Facter.add(:sleep, :timeout => 10) do
  setcode do
      sleep 999999

Structured Facts

While the norm is for a fact to return a single string, Facter 2.0 introduced structured facts, which take the form of either a hash or an array. To create a structured fact, simply return a hash or an array from the setcode statement. You can see some relevant examples in the writing structured facts section of the Fact Overview.

Aggregate Resolutions

If your fact combines the output of multiple commands, it might make sense to use aggregate resolutions. An aggregate resolution is split into “chunks”, each one responsible for resolving one piece of the fact. After all of the chunks are resolved separately, they’re combined into a single flat or structured fact and returned.

Aggregate resolutions have several key differences compared to simple resolutions, beginning with the fact declaration. To introduce an aggregate resolution, you’ll need to add the :type => :aggregate parameter:

Facter.add(:fact_name, :type => :aggregate) do
    # Chunks go here.
    # Aggregate block goes here.

Each step in the resolution then gets its own chunk statement with an arbitrary name:

chunk(:one) do
    'Chunk one returns this.'

chunk(:two) do
    'Chunk two returns this.'

In a simple resolution, the code always includes a setcode statement that determines the fact’s value. Aggregate resolutions never have a setcode statement. Instead, they have an optional aggregate block that combines the chunks. Whatever value the aggregate block returns will be the fact’s value. Here’s an example that just combines the strings from the two chunks above:

aggregate do |chunks|
  result = ''

  chunks.each_value do |str|
    result += str

# Returns "Chunk one returns this. Chunk two returns this."

If the chunk blocks either all return arrays or all return hashes, you can omit the aggregate block. If you do, Facter automatically merges all of your data into one array or hash and uses that as the fact’s value.

For more examples of aggregate resolutions, see the aggregate resolutions section of the Fact Overview page.

Viewing Fact Values

If your Puppet masters are configured to use PuppetDB or the inventory service, you can view and search all of the facts for any node, including custom facts. See the PuppetDB or inventory service documentation for more information.

External Facts

What Are External Facts?

External facts provide a way to use arbitrary executables or scripts as facts, or set facts statically with structured data. If you’ve ever wanted to write a custom fact in Perl, C, or a one-line text file, this is how.

Fact Locations

The best way to distribute external facts is with pluginsync, which added support for them in Puppet 3.4/Facter 2.0.1. To add external facts to your Puppet modules, place them in <MODULEPATH>/<MODULE>/facts.d/.

If you’re not using pluginsync, then external facts must go in a standard directory. The location of this directory varies depending on your operating system, whether your deployment uses Puppet Enterprise or open source Puppet, and whether you are running as root/Administrator. When calling Facter from the command line, you can specify the external facts directory with the --external-dir option.

Note: These directories do not necessarily exist by default; you might need to create them. If you create the directory, restrict access so that only Administrators can write to it.

In a module (recommended):


On Unix/Linux/Mac OS X:

/etc/facter/facts.d/ # Puppet Open Source
/etc/puppetlabs/facter/facts.d/ # Puppet Enterprise

On Windows 2003:

C:\Documents and Settings\All Users\Application Data\PuppetLabs\facter\facts.d\

On other supported Windows Operating Systems (Windows Vista, 7, 8, 2008, 2012):


When running as a non-root or non-Administrator user:

<HOME DIRECTORY>/.facter/facts.d/

Executable Facts — *nix

Executable facts on *nix work by dropping an executable file into the standard external fact path above.

An example external fact written in Python:

#!/usr/bin/env python
data = {"key1" : "value1", "key2" : "value2" }

for k in data:
    print "%s=%s" % (k,data[k])

You must ensure that the script has its execute bit set:

chmod +x /etc/facter/facts.d/my_fact_script.py

For Facter to parse the output, the script must return key-value pairs on STDOUT in this format:


Using this format, a single script can return multiple facts.

Executable Facts — Windows

Executable facts on Windows work by dropping an executable file into the external fact path for your version of Windows. Unlike *nix, the external facts interface expects Windows scripts to end with a known extension. Line endings can be either LF or CRLF. Facter 2.4 supports the following extensions:

  • .com and .exe: binary executables
  • .bat and .cmd: batch scripts
  • .ps1: PowerShell scripts

As with *nix facts, each script must return key-value pairs on STDOUT in this format:


Using this format, a single script can return multiple facts in one return.

Batch Scripts

For consistently usable output, the file encoding for .bat/.cmd files must be ANSI or UTF8 without BOM (Byte Order Mark).

Here is a sample batch script that outputs facts using the required format:

@echo off
echo key1=val1
echo key2=val2
echo key3=val3
REM Invalid - echo 'key4=val4'
REM Invalid - echo "key5=val5"

PowerShell Scripts

The encoding options for .ps1 files are relatively open. PowerShell determines the encoding of the file at runtime.

Here is a sample PowerShell script that outputs facts using the required format:

Write-Host "key1=val1"
Write-Host 'key2=val2'
Write-Host key3=val3

You should be able to save and execute this PowerShell script on the command line.

Structured Data Facts

Facter can parse structured data files stored in the external facts directory and set facts based on their contents.

Structured data files must use one of the supported data types and must have the correct file extension. At the moment, Facter supports the following extensions and data types:

  • .yaml: YAML data, in the following format:
key1: val1
key2: val2
key3: val3
  • .json: JSON data, in the following format:
    "key1": "val1",
    "key2": "val2",
    "key3": "val3"
  • .txt: Key value pairs, in the following format:

As with executable facts, structured data files can set multiple facts at once.

Structured Data Facts on Windows

Facter supports all of the above types on Windows with the following caveats:

  • The line endings can be either LF or CRLF.
  • The file encoding must be either ANSI or UTF8 without BOM (Byte Order Mark).


If your external fact doesn’t appear in Facter’s output, running Facter in debug mode should give you a meaningful reason and tell you which file is causing the problem:

sudo facter --debug

For instance, let’s say you used an invalid hyphen instead of an equals sign in your script test.sh:


echo "key1-value1"

Running facter --debug should yield a useful error message:

Fact file /etc/facter/facts.d/sample.txt was parsed but returned an empty data set

To find performance bottlenecks, you can run Facter in timing mode, which outputs how long it takes to parse your external facts:

facter --timing

The output should look similar to the timing for Ruby facts, but names external facts with their full paths. For example:

$ facter --timing
kernel: 14.81ms
/usr/lib/facter/ext/abc.sh: 48.72ms
/usr/lib/facter/ext/foo.sh: 32.69ms
/usr/lib/facter/ext/full.json: 104.71ms
/usr/lib/facter/ext/sample.txt: 0.65ms

External Facts and stdlib

If you find that an external fact does not match what you have configured in your facts.d directory, make sure you have not defined the same fact using the external facts capabilities found in the stdlib module.


While external facts provide a mostly-equal way to create variables for Puppet, they have a few drawbacks:

  • An external fact cannot internally reference another fact. However, due to parse order, you can reference an external fact from a Ruby fact.
  • External executable facts are forked instead of executed within the same process.
  • Distributing executable facts through pluginsync requires Puppet 3.4.0 or greater.
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