1. Creating Command Line Applications Using sbt

Creating Command Line Applications Using sbt 

There are several components of sbt that may be used to create a command line application. The launcher and the command system are the two main ones illustrated here.

As described on the launcher page, a launched application implements the xsbti.AppMain interface and defines a brief configuration file that users pass to the launcher to run the application. To use the command system, an application sets up a State instance that provides command implementations and the initial commands to run. A minimal hello world example is given below.

Hello World Example 

There are three files in this example:

  1. build.sbt
  2. Main.scala
  3. hello.build.properties

To try out this example:

  1. Put the first two files in a new directory
  2. In sbt’s shell run publishLocal in that directory
  3. Run sbt @path/to/hello.build.properties to run the application.

Like for sbt itself, you can specify commands from the command line (batch mode) or run them at an prompt (interactive mode).

Build Definition: build.sbt 

The build.sbt file should define the standard settings: name, version, and organization. To use the sbt command system, a dependency on the command module is needed. To use the task system, add a dependency on the task-system module as well.

organization := "org.example"

name := "hello"

version := "0.1-SNAPSHOT"

libraryDependencies += "org.scala-sbt" % "command" % "0.12.0"

Application: Main.scala 

The application itself is defined by implementing xsbti.AppMain. The basic steps are

  1. Provide command definitions. These are the commands that are available for users to run.
  2. Define initial commands. These are the commands that are initially scheduled to run. For example, an application will typically add anything specified on the command line (what sbt calls batch mode) and if no commands are defined, enter interactive mode by running the ‘shell’ command.
  3. Set up logging. The default setup in the example rotates the log file after each user interaction and sends brief logging to the console and verbose logging to the log file.
package org.example

   import sbt._
   import java.io.{File, PrintWriter}

final class Main extends xsbti.AppMain
   /** Defines the entry point for the application.
   * The call to `initialState` sets up the application.
   * The call to runLogged starts command processing. */
   def run(configuration: xsbti.AppConfiguration): xsbti.MainResult =
      MainLoop.runLogged( initialState(configuration) )

   /** Sets up the application by constructing an initial State instance with the supported commands
   * and initial commands to run.  See the State API documentation for details. */
   def initialState(configuration: xsbti.AppConfiguration): State =
      val commandDefinitions = hello +: BasicCommands.allBasicCommands
      val commandsToRun = Hello +: "iflast shell" +: configuration.arguments.map(_.trim)
      State( configuration, commandDefinitions, Set.empty, None, commandsToRun, State.newHistory,
         AttributeMap.empty, initialGlobalLogging, State.Continue )

   // defines an example command.  see the Commands page for details.
   val Hello = "hello"
   val hello = Command.command(Hello) { s =>

   /** Configures logging to log to a temporary backing file as well as to the console. 
   * An application would need to do more here to customize the logging level and
   * provide access to the backing file (like sbt's last command and logLevel setting).*/
   def initialGlobalLogging: GlobalLogging =
      GlobalLogging.initial(MainLogging.globalDefault _, File.createTempFile("hello", "log"))

Launcher configuration file: hello.build.properties 

The launcher needs a configuration file in order to retrieve and run an application. hello.build.properties:

  version: 2.9.1

  org: org.example
  name: hello
  version: 0.1-SNAPSHOT
  class: org.example.Main
  components: xsbti
  cross-versioned: true

  typesafe-ivy-releases: https://repo.typesafe.com/typesafe/ivy-releases/, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]