In SDL Web 8.5 we saw the introduction of replacement tokens in the content delivery configuration files. Whereas previously we'd simply had XML files with attributes and elements that we filled in with the relevant values, the replacement tokens allowed for the values to be provided externally when the configuration file is used. The commonest way to do this is probably by using environment variables, but you can also pass them as arguments to the java runtime. (A while ago, I wasted a bunch of time writing a script to pass environment variables in via java arguments. You don't need to do this.)

So anyway - taking the deployer config as my example, we started to see this kind of thing:

<Queues>
 <Queue Id="ContentQueue" Adapter="FileSystem" Verbs="Content" Default="true">
  <Property Value="${queuePath}" Name="Destination"/>
 </Queue>
 <Queue Id="CommitQueue" Adapter="FileSystem" Verbs="Commit,Rollback">
  <Property Value="${queuePath}/FinalTX" Name="Destination"/>
 </Queue>
 <Queue Id="PrepareQueue" Adapter="FileSystem" Verbs="Prepare">
  <Property Value="${queuePath}/Prepare" Name="Destination"/>
 </Queue>

Or from the storage conf of the disco service:

<Role Name="TokenServiceCapability" Url="${tokenurl:-http://localhost:8082/token.svc}">

So if you have an environment variable called queuePath, it will be used instead of ${queuePath}. In the second example, you can see that there's also a syntax for providing a default value, so if there's a tokenurl environment variable, that will be used, and if not, you'll get http://localhost:8082/token.svc.

This kind of replacement token is very common in the *nix world, where it's taken to even further extremes. Most of this is based on the venerable Shell Parameter Expansion syntax.

All this is great for automated deployments and I'm sure the team running SDL's cloud services makes full use of this technique. For now, I'm still using my own scripts to replace values in the config files, so a recent addition turned out to be a bit inconvenient. In Tridion Sites 9, the queue Ids in the deployer config have also been tokenised. So now we have this kind of thing:

<Queue Default="true" Verbs="Content" Adapter="FileSystem" Id="${contentqueuename:-ContentQueue}">
  <Property Name="Destination" Value="${queuePath}"/>
</Queue>

Seeing as I had an XPath that locates the Queue elements by ID, this wasn't too helpful. (Yes, yes, in the general case it's great, but I'm thinking purely selfishly!) Shooting from the hip I quickly updated my script with an awesome regex :-) , so instead of

$config = [xml](gc $deployerConfig)

I had

$config = [xml]((gc $deployerConfig) -replace '\$\{(?:.*?):-(.*?)\}','$1')

About ten seconds after finishing this, I realised that what I should be doing instead is fixing my XPath to glom onto the Verbs attribute instead, but you can't just throw away a good regex. So - I present to you, this beautiful regex for converting shell parameter expansions (or whatever they are called) into their default values while using the PowerShell. In other words, ${contentqueuename:-ContentQueue} becomes ContentQueue.

How does it work? Here it comes, one piece at a time:

'            single quote. Otherwise Powershell will interpret characters like $ and {, which you don't want
\            a slash to escape the dollar from the regex
$            the opening dollar of the expansion espression
\{           match the {, also escaped from the regex
(?:.*?)      match zero or more of anything, non-greedily, and without capturing
:-           match the :-
(.*?)        match zero or more of anything non-greedily. No ?: this time so it's captured for use later as $1
\}           match the }
'            single quote 

 The second parameter of -replace is '$1', which translates to "the first capture". Note the single quotes, for the same reason as before

So there you have it. Now if ever you need to rip through a bunch of config files and blindly accept the defaults for everything, you know how. But meh... you could also just not provide any values in the environment. I refuse to accept that this hack is useless. A reason will emerge. The universe abhors a scripting hack with no purpose.