TransformOctopusConfig.ps1

function transform($xml, $xdt, $output)
{
    Add-Type -LiteralPath "Microsoft.Web.XmlTransform.dll"

    $xmldoc = New-Object Microsoft.Web.XmlTransform.XmlTransformableDocument;
    $xmldoc.PreserveWhitespace = $true
    $xmldoc.Load($xml);

    $transform = New-Object Microsoft.Web.XmlTransform.XmlTransformation($xdt);
    if ($transform.Apply($xmldoc) -eq $false)
    {
        throw "Transformation failed."
    }
    
    $xmldoc.Save($output)
}

function substitute($line, $octopusValues)
{
    $regex = [regex] "(#\{\b[a-zA-Z0-9-_.]+\})"
    $groups = $regex.Matches($line)
    if ($groups.Count -eq 0)
    {
        return $line
    }
    
    foreach($group in $groups)
    {
        $octVariable = $group.Value.Trim("#{").Trim("}")
        write-host "[DEBUG] group.Value:" $octVariable
        Try
        {
            $token = $octopusValues | select -ExpandProperty "$octVariable" -ErrorAction Stop
            $token = substitute $token $octopusValues
            $line = $line.Replace($group.Value, $token)
        }
        Catch
        {
            Write-Host "[WARNING] Could not find value of $octVariable"
            Break            
        }
    }
    
    return $line
}

function Transform-OctopusConfig($json, $xml, $xdt, $output)
{
    if (!$json -or !(Test-Path -path $json -PathType Leaf)) {
        throw "File not found. $json";
    }
    if (!$xml -or !(Test-Path -path $xml -PathType Leaf)) {
        throw "File not found. $xml";
    }
    if (!$xdt -or !(Test-Path -path $xdt -PathType Leaf)) {
        throw "File not found. $xdt";
    }
    
    $outputTemp = $output + ".tmp"
    transform $xml $xdt $outputTemp
    
    $octopusValues = (Get-Content $json | ConvertFrom-Json)
    
    $lines = Get-Content $outputTemp

    foreach($line in $lines)
    {
        $line = substitute $line $octopusValues
        $line | Out-File -filePath $output -Append
    }
}