Override a task setting in an SBT command

build.sbt

lazy val person = settingKey[String]("Person to greet")

lazy val hello = taskKey[Unit]("Greet person")

// https://stackoverflow.com/q/14262798/1535738
def greetEveryone = Command.command("greetEveryone") { state =>
  Seq("John", "Alice", "Bob").foreach { name =>
    val extracted = Project extract state
    val newState = extracted.append(Seq(person := name), state)
    Project.extract(newState).runTask(hello, newState)
  }
  state
}

lazy val root = (project in file("."))
  .settings(
    person := "John",
    hello := {
      println(s"Hello ${person.value}")
    },
    commands ++= Seq(greetEveryone)
  )