In Java you can pass system properties from the command line like this:

java -D MyProperty=foo MyClass

And you can then get them in your code like so:

public class MyClass {
  public String getMyProperty() {
    return System.getProperty("MyProperty");
  }
}

Pretty easy, no?

You can pass system arguments the same way with Gradle:

gradle -D MyProperty=foo run

But what if you want to manipulate or use those properties in Gradle first?

Instead of -D you can use -P to pass properties to the Gradle object, and then you can do whatever you want with it.

gradle -P MyProperty=foo MyClass

And then you can use your properties in Gradle and then pass them to the System properties thusly:

task setProperty << {

    if (project.hasProperty("ENV")) {
        println "project has property ENV"
        System.properties["ENV"] = "$ENV"
    } else {
        println "project does not have property ENV"
        System.properties["ENV"] = "dev"
    }

    println "ENV: " + System.properties["ENV"]
}

Some references:

https://docs.gradle.org/current/userguide/writing_build_scripts.html#N10FDD

https://docs.gradle.org/current/userguide/build_environment.html

http://mrhaki.blogspot.com/2010/10/gradle-goodness-pass-command-line.html

http://mrhaki.blogspot.com/2010/09/gradle-goodness-different-ways-to-set.html