Sample: Gradle custom plugin in build.gradle

build.gradle

class Greeting implements Plugin<Project> {
    void apply(Project target) {
        target.task('greet', type: GreetingTask)
    }
}

class GreetingTask extends DefaultTask {
    def greeting = 'Hello'
    @TaskAction
    def greet() {
        println greeting
    }
}

apply plugin: Greeting

task hi(type:GreetingTask){
    greeting = 'Hi'
}