Getting Gradle 'rootProject' object to honor env vars - gradle

Question

I ran across the following configuration in a Gradle project's buildfile (build.gradle):
codenarcMain {
configFile = rootProject.file("gradle/codenarc/CodeNarcMain.groovy")
}
When I Google "Gradle rootProject" I find this link which makes it look like rootProject is a ProjectDescriptor instance.
But looking at ProjectDescriptor, I don't see any property called file. I see a buildFile and projectDir, but no file property.
Ultimately, I am trying to get Gradle to load the CodeNarc config file from outside the build directory. On my system I have an env var called $CODENARC_HOME with the following directory structure:
CODENARC_HOME/ (say this is /home/myuser/tools/codenarc/)
CodeNarcMain.groovy
CodeNarcTest.groovy
README.md
Now I would like to change the CodeNarc config in Gradle to look something like this:
codenarcMain {
configFile = rootProject.file("CODENARC_HOME/CodeNarcMain.groovy")
}
And then, no matter where CODENARC_HOME is defined, the Gradle build will still be able to locate the config file.
So my questions:
What is the file property on rootProject, and why don't I see it in the API docs (linked above)?; and
How to get rootProject.file(...) to honor system/env vars inside its file path string argument?

Answer 1

rootProject in settings.gradle is-a ProjectDescriptor. rootProject in build.gradle is-a Project. Environment variables and system properties can be accessed in the standard Java way:
codenarcMain {
configFile = rootProject.file("${System.getenv("CODENARC_HOME")}/CodeNarcMain.groovy")
// or: System.getProperty("codenarc.home")
}