1. Tasks


Tasks and settings are introduced in the getting started guide, which you may wish to read first. This page has additional details and background and is intended more as a reference.


Both settings and tasks produce values, but there are two major differences between them:

  1. Settings are evaluated at project load time. Tasks are executed on demand, often in response to a command from the user.
  2. At the beginning of project loading, settings and their dependencies are fixed. Tasks can introduce new tasks during execution, however.


There are several features of the task system:

  1. By integrating with the settings system, tasks can be added, removed, and modified as easily and flexibly as settings.
  2. Input Tasks use parser combinators to define the syntax for their arguments. This allows flexible syntax and tab-completions in the same way as Commands.
  3. Tasks produce values. Other tasks can access a task’s value by calling value on it within a task definition.
  4. Dynamically changing the structure of the task graph is possible. Tasks can be injected into the execution graph based on the result of another task.
  5. There are ways to handle task failure, similar to try/catch/finally.
  6. Each task has access to its own Logger that by default persists the logging for that task at a more verbose level than is initially printed to the screen.

These features are discussed in detail in the following sections.

Defining a Task 

Hello World example (sbt) 


lazy val hello = taskKey[Unit]("Prints 'Hello World'")

hello := println("hello world!")

Run “sbt hello” from command line to invoke the task. Run “sbt tasks” to see this task listed.

Define the key 

To declare a new task, define a lazy val of type TaskKey:

lazy val sampleTask = taskKey[Int]("A sample task.")

The name of the val is used when referring to the task in Scala code and at the command line. The string passed to the taskKey method is a description of the task. The type parameter passed to taskKey (here, Int) is the type of value produced by the task.

We’ll define a couple of other keys for the examples:

lazy val intTask = taskKey[Int]("An int task")
lazy val stringTask = taskKey[String]("A string task")

The examples themselves are valid entries in a build.sbt or can be provided as part of a sequence to Project.settings (see .scala build definition).

Implement the task 

There are three main parts to implementing a task once its key is defined:

  1. Determine the settings and other tasks needed by the task. They are the task’s inputs.
  2. Define the code that implements the task in terms of these inputs.
  3. Determine the scope the task will go in.

These parts are then combined just like the parts of a setting are combined.

Defining a basic task 

A task is defined using :=

intTask := 1 + 2

stringTask := System.getProperty("user.name")

sampleTask := {
   val sum = 1 + 2
   println("sum: " + sum)

As mentioned in the introduction, a task is evaluated on demand. Each time sampleTask is invoked, for example, it will print the sum. If the username changes between runs, stringTask will take different values in those separate runs. (Within a run, each task is evaluated at most once.) In contrast, settings are evaluated once on project load and are fixed until the next reload.

Tasks with inputs 

Tasks with other tasks or settings as inputs are also defined using :=. The values of the inputs are referenced by the value method. This method is special syntax and can only be called when defining a task, such as in the argument to :=. The following defines a task that adds one to the value produced by intTask and returns the result.

sampleTask := intTask.value + 1

Multiple settings are handled similarly:

stringTask := "Sample: " + sampleTask.value + ", int: " + intTask.value
Task Scope 

As with settings, tasks can be defined in a specific scope. For example, there are separate compile tasks for the compile and test scopes. The scope of a task is defined the same as for a setting. In the following example, Test/sampleTask uses the result of Compile/intTask.

Test / sampleTask := (Compile / intTask).value * 3
On precedence 

As a reminder, infix method precedence is by the name of the method and postfix methods have lower precedence than infix methods.

  1. Assignment methods have the lowest precedence. These are methods with names ending in =, except for !=, <=, >=, and names that start with =.
  2. Methods starting with a letter have the next highest precedence.
  3. Methods with names that start with a symbol and aren’t included in

    1. have the highest precedence. (This category is divided further according to the specific character it starts with. See the Scala specification for details.)

Therefore, the previous example is equivalent to the following:

(Test / sampleTask).:=( (Compile / intTask).value * 3 )

Additionally, the braces in the following are necessary:

helloTask := { "echo Hello" ! }

Without them, Scala interprets the line as ( helloTask.:=("echo Hello") ).! instead of the desired helloTask.:=( "echo Hello".! ).

Separating implementations 

The implementation of a task can be separated from the binding. For example, a basic separate definition looks like:

// Define a new, standalone task implemention
lazy val intTaskImpl: Initialize[Task[Int]] =
   Def.task { sampleTask.value - 3 }

// Bind the implementation to a specific key
intTask := intTaskImpl.value

Note that whenever .value is used, it must be within a task definition, such as within Def.task above or as an argument to :=.

Modifying an Existing Task 

In the general case, modify a task by declaring the previous task as an input.

// initial definition
intTask := 3

// overriding definition that references the previous definition
intTask := intTask.value + 1

Completely override a task by not declaring the previous task as an input. Each of the definitions in the following example completely overrides the previous one. That is, when intTask is run, it will only print #3.

intTask := {

intTask := {

intTask :=  {
    sampleTask.value - 3

Getting values from multiple scopes 


The general form of an expression that gets values from multiple scopes is:


NOTE! Make sure to assign the ScopeFilter as a val! This is an implementation detail requirement of the .all macro.

The all method is implicitly added to tasks and settings. It accepts a ScopeFilter that will select the Scopes. The result has type Seq[T], where T is the key’s underlying type.


A common scenario is getting the sources for all subprojects for processing all at once, such as passing them to scaladoc. The task that we want to obtain values for is sources and we want to get the values in all non-root projects and in the Compile configuration. This looks like:

lazy val core = project

lazy val util = project

val filter = ScopeFilter( inProjects(core, util), inConfigurations(Compile) )

lazy val root = project.settings(
   sources := {
      // each sources definition is of type Seq[File],
      //   giving us a Seq[Seq[File]] that we then flatten to Seq[File]
      val allSources: Seq[Seq[File]] = sources.all(filter).value

The next section describes various ways to construct a ScopeFilter.


A basic ScopeFilter is constructed by the ScopeFilter.apply method. This method makes a ScopeFilter from filters on the parts of a Scope: a ProjectFilter, ConfigurationFilter, and TaskFilter. The simplest case is explicitly specifying the values for the parts:

val filter: ScopeFilter =
      inProjects( core, util ),
      inConfigurations( Compile, Test )
Unspecified filters 

If the task filter is not specified, as in the example above, the default is to select scopes without a specific task (global). Similarly, an unspecified configuration filter will select scopes in the global configuration. The project filter should usually be explicit, but if left unspecified, the current project context will be used.

More on filter construction 

The example showed the basic methods inProjects and inConfigurations. This section describes all methods for constructing a ProjectFilter, ConfigurationFilter, or TaskFilter. These methods can be organized into four groups:

  • Explicit member list (inProjects, inConfigurations, inTasks)
  • Global value (inGlobalProject, inGlobalConfiguration, inGlobalTask)
  • Default filter (inAnyProject, inAnyConfiguration, inAnyTask)
  • Project relationships (inAggregates, inDependencies)

See the API documentation for details.

Combining ScopeFilters 

ScopeFilters may be combined with the &&, ||, --, and - methods:

  • a && b Selects scopes that match both a and b
  • a || b Selects scopes that match either a or b
  • a -- b Selects scopes that match a but not b
  • -b Selects scopes that do not match b

For example, the following selects the scope for the Compile and Test configurations of the core project and the global configuration of the util project:

val filter: ScopeFilter =
   ScopeFilter( inProjects(core), inConfigurations(Compile, Test)) ||
   ScopeFilter( inProjects(util), inGlobalConfiguration )

More operations 

The all method applies to both settings (values of type Initialize[T]) and tasks (values of type Initialize[Task[T]]). It returns a setting or task that provides a Seq[T], as shown in this table:

Target Result
Initialize[T] Initialize[Seq[T]]
Initialize[Task[T]] Initialize[Task[Seq[T]]]

This means that the all method can be combined with methods that construct tasks and settings.

Missing values 

Some scopes might not define a setting or task. The ? and ?? methods can help in this case. They are both defined on settings and tasks and indicate what to do when a key is undefined.

? On a setting or task with underlying type T, this accepts no arguments and returns a setting or task (respectively) of type Option[T]. The result is None if the setting/task is undefined and Some[T] with the value if it is.
?? On a setting or task with underlying type T, this accepts an argument of type T and uses this argument if the setting/task is undefined.

The following contrived example sets the maximum errors to be the maximum of all aggregates of the current project.

// select the transitive aggregates for this project, but not the project itself
val filter: ScopeFilter =
   ScopeFilter( inAggregates(ThisProject, includeRoot=false) )

maxErrors := {
   // get the configured maximum errors in each selected scope,
   // using 0 if not defined in a scope
   val allVersions: Seq[Int] =
      (maxErrors ?? 0).all(filter).value
Multiple values from multiple scopes 

The target of all is any task or setting, including anonymous ones. This means it is possible to get multiple values at once without defining a new task or setting in each scope. A common use case is to pair each value obtained with the project, configuration, or full scope it came from.

  • resolvedScoped: Provides the full enclosing ScopedKey (which is a Scope + AttributeKey[_])
  • thisProject: Provides the Project associated with this scope (undefined at the global and build levels)
  • thisProjectRef: Provides the ProjectRef for the context (undefined at the global and build levels)
  • configuration: Provides the Configuration for the context (undefined for the global configuration)

For example, the following defines a task that prints non-Compile configurations that define sbt plugins. This might be used to identify an incorrectly configured build (or not, since this is a fairly contrived example):

// Select all configurations in the current project except for Compile
lazy val filter: ScopeFilter = ScopeFilter(
   inAnyConfiguration -- inConfigurations(Compile)

// Define a task that provides the name of the current configuration
//   and the set of sbt plugins defined in the configuration
lazy val pluginsWithConfig: Initialize[Task[ (String, Set[String]) ]] =
   Def.task {
      ( configuration.value.name, definedSbtPlugins.value )

checkPluginsTask := {
   val oddPlugins: Seq[(String, Set[String])] =
   // Print each configuration that defines sbt plugins
   for( (config, plugins) <- oddPlugins if plugins.nonEmpty )
      println(s"$config defines sbt plugins: ${plugins.mkString(", ")}")

Advanced Task Operations 

The examples in this section use the task keys defined in the previous section.

Streams: Per-task logging 

Per-task loggers are part of a more general system for task-specific data called Streams. This allows controlling the verbosity of stack traces and logging individually for tasks as well as recalling the last logging for a task. Tasks also have access to their own persisted binary or text data.

To use Streams, get the value of the streams task. This is a special task that provides an instance of TaskStreams for the defining task. This type provides access to named binary and text streams, named loggers, and a default logger. The default Logger, which is the most commonly used aspect, is obtained by the log method:

myTask := {
  val s: TaskStreams = streams.value
  s.log.debug("Saying hi...")

You can scope logging settings by the specific task’s scope:

myTask / logLevel := Level.Debug

myTask / traceLevel := 5

To obtain the last logging output from a task, use the last command:

$ last myTask
[debug] Saying hi...
[info] Hello!

The verbosity with which logging is persisted is controlled using the persistLogLevel and persistTraceLevel settings. The last command displays what was logged according to these levels. The levels do not affect already logged information.

Conditional task 

(Requires sbt 1.4.0+)

When Def.task { ... } consists of an if-expression at the top-level, a conditional task (or Selective task) is automatically created:

bar := {
  if (number.value < 0) negAction.value
  else if (number.value == 0) zeroAction.value
  else posAction.value

Unlike the regular (Applicative) task composition, conditional tasks delays the evaluation of then-clause and else-clause as naturally expected of an if-expression. This is already possible with Def.taskDyn { ... }, but unlike dynamic tasks, conditional task works with inspect command.

Dynamic Computations with Def.taskDyn 

It can be useful to use the result of a task to determine the next tasks to evaluate. This is done using Def.taskDyn. The result of taskDyn is called a dynamic task because it introduces dependencies at runtime. The taskDyn method supports the same syntax as Def.task and := except that you return a task instead of a plain value.

For example,

val dynamic = Def.taskDyn {
  // decide what to evaluate based on the value of `stringTask`
  if(stringTask.value == "dev")
    // create the dev-mode task: this is only evaluated if the
    //   value of stringTask is "dev"
    Def.task {
    // create the production task: only evaluated if the value
    //    of the stringTask is not "dev"
    Def.task {
      intTask.value + 5

myTask := {
  val num = dynamic.value
  println(s"Number selected was $num")

The only static dependency of myTask is stringTask. The dependency on intTask is only introduced in non-dev mode.

Note: A dynamic task cannot refer to itself or a circular dependency will result. In the example above, there would be a circular dependency if the code passed to taskDyn referenced myTask.

Using Def.sequential 

sbt 0.13.8 added Def.sequential function to run tasks under semi-sequential semantics. This is similar to the dynamic task, but easier to define. To demonstrate the sequential task, let’s create a custom task called compilecheck that runs Compile / compile and then Compile / scalastyle task added by scalastyle-sbt-plugin.

lazy val compilecheck = taskKey[Unit]("compile and then scalastyle")

lazy val root = (project in file("."))
    Compile / compilecheck := Def.sequential(
      Compile / compile,
      (Compile / scalastyle).toTask("")

To call this task type in compilecheck from the shell. If the compilation fails, compilecheck would stop the execution.

root> compilecheck
[info] Compiling 1 Scala source to /Users/x/proj/target/scala-2.10/classes...
[error] /Users/x/proj/src/main/scala/Foo.scala:3: Unmatched closing brace '}' ignored here
[error] }
[error] ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed

Handling Failure 

This section discusses the failure, result, and andFinally methods, which are used to handle failure of other tasks.


The failure method creates a new task that returns the Incomplete value when the original task fails to complete normally. If the original task succeeds, the new task fails. Incomplete is an exception with information about any tasks that caused the failure and any underlying exceptions thrown during task execution.

For example:

intTask := sys.error("Failed.")

intTask := {
   println("Ignoring failure: " + intTask.failure.value)

This overrides the intTask so that the original exception is printed and the constant 3 is returned.

failure does not prevent other tasks that depend on the target from failing. Consider the following example:

intTask := if(shouldSucceed) 5 else sys.error("Failed.")

// Return 3 if intTask fails. If intTask succeeds, this task will fail.
aTask := intTask.failure.value - 2

// A new task that increments the result of intTask.
bTask := intTask.value + 1

cTask := aTask.value + bTask.value

The following table lists the results of each task depending on the initially invoked task:

invoked task intTask result aTask result bTask result cTask result overall result
intTask failure not run not run not run failure
aTask failure success not run not run success
bTask failure not run failure not run failure
cTask failure success failure failure failure
intTask success not run not run not run success
aTask success failure not run not run failure
bTask success not run success not run success
cTask success failure success failure failure

The overall result is always the same as the root task (the directly invoked task). A failure turns a success into a failure, and a failure into an Incomplete. A normal task definition fails when any of its inputs fail and computes its value otherwise.


The result method creates a new task that returns the full Result[T] value for the original task. Result has the same structure as Either[Incomplete, T] for a task result of type T. That is, it has two subtypes:

  • Inc, which wraps Incomplete in case of failure
  • Value, which wraps a task’s result in case of success.

Thus, the task created by result executes whether or not the original task succeeds or fails.

For example:

intTask := sys.error("Failed.")

intTask := {
   intTask.result.value match {
      case Inc(inc: Incomplete) =>
         println("Ignoring failure: " + inc)
      case Value(v) =>
         println("Using successful result: " + v)

This overrides the original intTask definition so that if the original task fails, the exception is printed and the constant 3 is returned. If it succeeds, the value is printed and returned.


The andFinally method defines a new task that runs the original task and evaluates a side effect regardless of whether the original task succeeded. The result of the task is the result of the original task. For example:

intTask := sys.error("I didn't succeed.")

lazy val intTaskImpl = intTask andFinally { println("andFinally") }

intTask := intTaskImpl.value

This modifies the original intTask to always print “andFinally” even if the task fails.

Note that andFinally constructs a new task. This means that the new task has to be invoked in order for the extra block to run. This is important when calling andFinally on another task instead of overriding a task like in the previous example. For example, consider this code:

intTask := sys.error("I didn't succeed.")

lazy val intTaskImpl = intTask andFinally { println("andFinally") }

otherIntTask := intTaskImpl.value

If intTask is run directly, otherIntTask is never involved in execution. This case is similar to the following plain Scala code:

def intTask(): Int =
  sys.error("I didn't succeed.")

def otherIntTask(): Int =
  try { intTask() }
  finally { println("finally") }


It is obvious here that calling intTask() will never result in “finally” being printed.