Getting user inputs

Objective

I want to read user input from the console in a task or a command.

Steps

1. Use the interactionService key to acquire sbt.InteractionService:

lazy val demo1 = taskKey[Int]("demo1")

scalaVersion := "3.8.4"

demo1 := Def.uncached {
  val srv = interactionService.value
  srv.readLine(prompt = "enter a number: ", mask = false) match
    case Some(x) => x.toInt
    case None    => sys.error("error getting an input!")
}

2. For commands, use CommandLineUIService directly:

lazy val demo2 = Command.command("demo2"): s0 =>
  val srv = CommandLineUIService
  srv.readLine(prompt = "enter a number: ", mask = false) match
    case Some(x) => x.toInt
    case None    => sys.error("error getting an input!")
  s0

LocalRootProject / commands += demo2

Test

$ sbt --client
sbt:demo> demo1
enter a number: 1
[success] elapsed time: 1 s
sbt:demo> demo2
enter a number: 1
sbt:demo> shutdown
[info] disconnected

$ sbt --server
sbt:demo> demo1
enter a number: 1
[success] elapsed time: 1 s
sbt:demo> demo2
enter a number: 1
sbt:demo> exit
[info] shutting down sbt server

Alternative

You can also use scala.io.StdIn.readLine function.

lazy val demo3 = taskKey[Int]("demo3")
demo3 := Def.uncached {
  import scala.io.StdIn
  Option(StdIn.readLine("enter a number: ")) match
    case Some(x) => x.toInt
    case None    => sys.error("error getting an input!")
}

Explanation

sbt 2 runs in client-server mode by default, involving sbtn and the sbt server process. This means that when a task prompts the user for input, the prompting happens on the client (sbtn) while the task executes on the server. This client-server separation also existed in sbt 0.13 when Activator provided a web-based frontend to sbt. To abstract over simple user interactions, sbt provides interactionService, initialized to CommandLineUIService. This key is also supported in both sbt 1.x and 2.x, where it enables sbtn terminal virtualization.

Alternatively, you can use scala.io.StdIn.readLine inside a task or command definition. Note that this relies on the sbt server proxying terminal interactions to sbtn over the wire.