preprocess files only for set variables

[etiennewan]

I would like to use preprocess as many times as I want to inject variables in my files when they are set, and keep the directives when a variable is undefined.

In my use case (take it as an example), I use preprocess for blog posts.
First, I create a post and I inject the title.
Then, when I finish writing the article, I ask to set the date and to set tags.
And between my final rewiew and the publish, I would like to set the URL for comments from an external service.

At each step, I would like only to preprocess directives when the variables that are echoed are set, and keep for future iterations the remaining directives.