1. Commands


What is a “command”? 

A “command” looks similar to a task: it’s a named operation that can be executed from the sbt console.

However, a command’s implementation takes as its parameter the entire state of the build (represented by State) and computes a new State. This means that a command can look at or modify other sbt settings, for example. Typically, you would resort to a command when you need to do something that’s impossible in a regular task.


There are three main aspects to commands:

  1. The syntax used by the user to invoke the command, including:

    • Tab completion for the syntax
    • The parser to turn input into an appropriate data structure
  2. The action to perform using the parsed data structure. This action transforms the build State.
  3. Help provided to the user

In sbt, the syntax part, including tab completion, is specified with parser combinators. If you are familiar with the parser combinators in Scala’s standard library, these are very similar. The action part is a function (State, T) => State, where T is the data structure produced by the parser. See the Parsing Input page for how to use the parser combinators.

State provides access to the build state, such as all registered Commands, the remaining commands to execute, and all project-related information. See States and Actions for details on State.

Finally, basic help information may be provided that is used by the help command to display command help.

Defining a Command 

A command combines a function State => Parser[T] with an action (State, T) => State. The reason for State => Parser[T] and not simply Parser[T] is that often the current State is used to build the parser. For example, the currently loaded projects (provided by State) determine valid completions for the project command. Examples for the general and specific cases are shown in the following sections.

See Command.scala for the source API details for constructing commands.

General commands 

General command construction looks like:

val action: (State, T) => State = ...
val parser: State => Parser[T] = ...
val command: Command = Command("name")(parser)(action)

No-argument commands 

There is a convenience method for constructing commands that do not accept any arguments.

val action: State => State = ...
val command: Command = Command.command("name")(action)

Single-argument command 

There is a convenience method for constructing commands that accept a single argument with arbitrary content.

// accepts the state and the single argument
val action: (State, String) => State = ...
val command: Command = Command.single("name")(action)

Multi-argument command 

There is a convenience method for constructing commands that accept multiple arguments separated by spaces.

val action: (State, Seq[String]) => State = ...

// <arg> is the suggestion printed for tab completion on an argument
val command: Command = Command.args("name", "<arg>")(action)

Full Example 

The following example is a sample build that adds commands to a project. To try it out:

  1. Create build.sbt and project/CommandExample.scala.
  2. Run sbt on the project.
  3. Try out the hello, helloAll, failIfTrue, color, and printState commands.
  4. Use tab-completion and the code below as guidance.

Here’s build.sbt:

import CommandExample._

ThisBuild / organization := "com.example"
ThisBuild / scalaVersion := "2.12.18"
ThisBuild / version      := "0.1.0-SNAPSHOT"

lazy val root = (project in file("."))
    commands ++= Seq(hello, helloAll, failIfTrue, changeColor, printState)

Here’s project/CommandExample.scala:

import sbt._
import Keys._

// imports standard command parsing functionality
import complete.DefaultParsers._

object CommandExample {
  // A simple, no-argument command that prints "Hi",
  //  leaving the current state unchanged.
  def hello = Command.command("hello") { state =>

  // A simple, multiple-argument command that prints "Hi" followed by the arguments.
  //   Again, it leaves the current state unchanged.
  def helloAll = Command.args("helloAll", "<name>") { (state, args) =>
    println("Hi " + args.mkString(" "))

  // A command that demonstrates failing or succeeding based on the input
  def failIfTrue = Command.single("failIfTrue") {
    case (state, "true") => state.fail
    case (state, _) => state

  // Demonstration of a custom parser.
  // The command changes the foreground or background terminal color
  //  according to the input.
  lazy val change = Space ~> (reset | setColor)
  lazy val reset = token("reset" ^^^ "\033[0m")
  lazy val color = token( Space ~> ("blue" ^^^ "4" | "green" ^^^ "2") )
  lazy val select = token( "fg" ^^^ "3" | "bg" ^^^ "4" )
  lazy val setColor = (select ~ color) map { case (g, c) => "\033[" + g + c + "m" }

  def changeColor = Command("color")(_ => change) { (state, ansicode) =>

  // A command that demonstrates getting information out of State.
  def printState = Command.command("printState") { state =>
    import state._
    println(definedCommands.size + " registered commands")
    println("commands to run: " + show(remainingCommands))

    println("original arguments: " + show(configuration.arguments))
    println("base directory: " + configuration.baseDirectory)

    println("sbt version: " + configuration.provider.id.version)
    println("Scala version (for sbt): " + configuration.provider.scalaProvider.version)

    val extracted = Project.extract(state)
    import extracted._
    println("Current build: " + currentRef.build)
    println("Current project: " + currentRef.project)
    println("Original setting count: " + session.original.size)
    println("Session setting count: " + session.append.size)


  def show[T](s: Seq[T]) =
    s.map("'" + _ + "'").mkString("[", ", ", "]")