How to define a custom task in project/Build.scala in sbt?

Question

With the following task declaration in project/Build.scala, the print task is not recognised when I type in print at an SBT console.

lazy val print = task { println("print") }

What's wrong?

Answer 1

You need a TaskKey for this to work that can be instantiated by using the taskKey macro:

lazy val printTask = taskKey[Unit]("print")

I recommend having a look at the corresponding documentation about tasks. The documentation says:

The name of the val is used when referring to the task in Scala code. The string passed to the TaskKey method is used at runtime, such as at the command line

Answer 2

taskKey[Unit]("print") := println("print")

Then in your SBT console,

> print
print

---

In more complex code, you'll usually see keys separate from the settings.

val printTask = taskKey[Unit]("print")

printTask := println("print")