Apply Octopus Deploy variables to config files during deployment

ConfigFileVariables.ps1

<#
#    Any variables in $OctopusParameters that use the following format will be used.
#
#    {config file name w/o extension (aka BaseName)}:{xpath to node value to change}
#
#    Examples:
#
#    Given the configFilename = log4net.config
#    And an Octopus variable named = log4net://configuration/log4net/appender[@name='FileAppender']/file/@value
#    The value of the Octopus variable will be placed into the value of that node.
#
#>
function Set-ConfigurationVariables
{
  param($configFilename)

    Write-Host "Checking for Octopus Deploy deployment parameters..."
	if (!$OctopusParameters)
	{
        Write-Host "No deployment parameters were provided from Octopus Deploy. Verify that Octopus Deploy is running this script or that you have specified $OctopusParameters from the executing environment."
        Return
    }
    
	$configFile = Get-ChildItem $configFilename
	if (!$configFile)
	{
		Write-Host "No configuration file was specified or found."
		Return
	}
    
	Write-Host "Applying variables to $configFile.Fullname"
	$configuration = [xml](Get-Content $configFile)
	$variablesForThisConfig = $OctopusParameters.Keys | Where-Object {$_.StartsWith($configFile.BaseName + ":", "CurrentCultureIgnoreCase")}

	if (!$variablesForThisConfig)
	{
		Write-Host "No variables for this config file were found."
		Return
	}

	foreach ($variable in $variablesForThisConfig)
	{
		# Skip past the config file base name to get the xpath from the variable name.
        $query = $variable.SubString(($configFile.BaseName + ":").Length)
		Write-Host "Attempting to apply variable to $query"
		
		$node = $configuration.SelectSingleNode($query)    
		if ($node)
		{
			Write-Host "Node located, applying value."
			$node.Set_InnerText($OctopusParameters[$variable])
		}
		else
		{
			Write-Host "Specified node could not be found."
		}
	}

	$configuration.Save($configFile.Fullname)
	Write-Host "Done applying variables to $configFile.Fullname"
}

function Invoke-ApplyConfigurationFilesVariables
{
    dir *.config | ForEach-Object {
      Set-ConfigurationVariables($_)
    }
}

Write-Host Running
Invoke-ApplyConfigurationFilesVariables
Write-Host Done

Sample Octopus Variables for sample.config

$OctopusParameters = @{
  "sample://configuration/someSection/theSubNode" = "Value from script"
}

sample.config

<configuration>
  <someSection>
    <theSubNode>OriginalValue</theSubNode>
  </someSection>
</configuration>