How do I extend gradle's clean task to delete a file?

So far I've added the following to my build.gradle

apply plugin: 'base' 
clean << {
    delete '${rootDir}/api-library/auto-generated-classes/'
    println '${rootDir}/api-library/auto-generated-classes/'
}

However, not only is my file not deleted, but the print statement shows that ${rootDir} is not being converted to the root directory of my project.

Why won't this work, what concepts am I missing?

You just need to use double quotes. Also, drop the << and use doFirst instead if you are planning to do the deletion during execution. Something like this:

clean.doFirst {
    delete "${rootDir}/api-library/auto-generated-classes/"
    println "${rootDir}/api-library/auto-generated-classes/"
}

Gradle build scripts are written in Groovy DSL. In Groovy you need to use double quotes for string interpolation (when you are using ${} as placeholders). Take a look at here.

<< is equivalent for clean.doLast. doFirst and doLast are ordering the operations at the execution phase, which is seldom relevant for delete operations.

In this case you don't need any of them. The clean task from base is of type [Delete][1], so you simply need to pass it a closure to tell it at configuration time what to delete when it executes:

clean {
    delete 'someFile'
}

AS [mushfek0001][2] correctly points it out in his answer, you should use double quotes for variable interpolation to work:

clean {
    delete "${buildDir}/someFile"
}

You need to have at least the base plugin applied for this to work, most other plugins, like the Java plugin either apply base or declare their own clean task of type delete [Delete][1] task. The error you would get if you don't have this is a missing clean method one.

apply plugin: 'base'

[1]: https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/Delete.html "Gradle Delete Task" [2]: https://stackoverflow.com/a/29813360/740641

In order to extend the clean task, you can use

clean.doFirst {}

or

clean.doLast {}

These will allow you to inject your own actions into the clean process. In order to delete files and directories you can use the "file" API which doesn't require any additional plugins.

Here is an example that will delete both a file and a directory as the last step in the clean task:

clean.doLast {
    file('src/main/someFile.txt').delete()
    file('src/main/libs').deleteDir()
}

Below one works for me (I prefer to use dependsOn),

task customCleanUp(type:Delete) {
   delete "your_folder", "your_file"
}

tasks.clean.dependsOn(tasks.customCleanUp)

Gradle Kotlin Script analogue:

tasks {
    getByName<Delete>("clean") {
        delete.add("logs") // add accepts argument with Any type
    }
}

There are options in gradle where you can extend a particular task ("clean" in your case) with doFirst (before the actual task), doLast (after the actual task).

You just need to replace single quotes with double quotes.

clean.doFirst {
    delete "${rootDir}/api-library/auto-generated-classes"
    println "${rootDir}/api-library/auto-generated-classes"
}

Another solution for Kotlin DSL (build.gradle.kts):

tasks.clean {
    delete += listOf(
        "${rootDir}/logs.txt",
        projectDir.list()?.first { it/* name */ .contains("temp") }
    )
}

This deletes a file called logs.txt at the project root and deletes any directory/file that has temp in its name in this subproject (the folder containing this build file).