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Getting Started with sbt

Getting Started with sbt 

sbt uses a small number of concepts to support flexible and powerful build definitions. There are not that many concepts, but sbt is not exactly like other build systems and there are details you will stumble on if you haven’t read the documentation.

The Getting Started Guide covers the concepts you need to know to create and maintain an sbt build definition.

It is highly recommended to read the Getting Started Guide!

If you are in a huge hurry, the most important conceptual background can be found in .sbt build definition, scopes, and more kinds of setting. But we don’t promise that it’s a good idea to skip the other pages in the guide.

It’s best to read in order, as later pages in the Getting Started Guide build on concepts introduced earlier.

Thanks for trying out sbt and have fun!

Installing sbt 

To create an sbt project, you’ll need to take these steps:

Ultimately, the installation of sbt boils down to a launcher JAR and a shell script, but depending on your platform, we provide several ways to make the process less tedious. Head over to the installation steps for Mac, Windows, Linux, or manual installation.

Tips and Notes 

If you have any trouble running sbt, see Setup Notes on terminal encodings, HTTP proxies, and JVM options.

Installing sbt on Mac 

Installing from a third-party package 

Note: Third-party packages may not provide the latest version. Please make sure to report any issues with these packages to the relevant maintainers.

Macports 

$ port install sbt

Homebrew 

$ brew install sbt

Installing from a universal package 

Download ZIP or TGZ package, and expand it.

Installing manually 

See instruction to install manually.

Installing sbt on Windows 

Windows installer 

Download msi installer and install it.

Installing from a universal package 

Download ZIP or TGZ package and expand it.

Installing manually 

See instruction to install manually.

Installing sbt on Linux 

Installing from a universal package 

Download ZIP or TGZ package and expand it.

RPM and DEB 

The following packages are also officially supported:

Note: Please report any issues with these to the sbt-launcher-package project.

Gentoo 

In the official tree there is no ebuild for sbt. But there are ebuilds to merge sbt from binaries. To merge sbt from this ebuilds you can do: 

$ mkdir -p /usr/local/portage && cd /usr/local/portage
$ git clone git://github.com/whiter4bbit/overlays.git
$ echo "PORTDIR_OVERLAY=$PORTDIR_OVERLAY /usr/local/portage/overlays" >> /etc/make.conf
$ emerge sbt-bin

Note: Please report any issues with the ebuild here.

Installing manually 

See instruction to install manually.

Installing sbt manually 

Manual installation requires downloading sbt-launch.jar and creating a script to start it.

Unix 

Put sbt-launch.jar in ~/bin.

Create a script to run the jar, by creating ~/bin/sbt with these contents: 

SBT_OPTS="-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M"
java $SBT_OPTS -jar `dirname $0`/sbt-launch.jar "$@"

Make the script executable: 

$ chmod u+x ~/bin/sbt

Windows 

Manual installation for Windows varies by terminal type and whether Cygwin is used. In all cases, put the batch file or script on the path so that you can launch sbt in any directory by typing sbt at the command prompt. Also, adjust JVM settings according to your machine if necessary.

Non-Cygwin 

For non-Cygwin users using the standard Windows terminal, create a batch file sbt.bat: 

set SCRIPT_DIR=%~dp0
java -Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M -jar "%SCRIPT_DIR%sbt-launch.jar" %*

and put the downloaded sbt-launch.jar in the same directory as the batch file.

Cygwin with the standard Windows termnial 

If using Cygwin with the standard Windows terminal, create a bash script ~/bin/sbt: 

SBT_OPTS="-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M"
java $SBT_OPTS -jar sbt-launch.jar "$@"

Replace sbt-launch.jar with the path to your downloaded sbt-launch.jar and remember to use cygpath if necessary. Make the script executable: 

$ chmod u+x ~/bin/sbt

Cygwin with an Ansi terminal 

Cygwin with an Ansi terminal (supports Ansi escape sequences and is configurable via stty), create a bash script ~/bin/sbt: 

SBT_OPTS="-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M"
stty -icanon min 1 -echo > /dev/null 2>&1
java -Djline.terminal=jline.UnixTerminal -Dsbt.cygwin=true $SBT_OPTS -jar sbt-launch.jar "$@"
stty icanon echo > /dev/null 2>&1

Replace sbt-launch.jar with the path to your downloaded sbt-launch.jar and remember to use cygpath if necessary. Then, make the script executable: 

$ chmod u+x ~/bin/sbt

In order for backspace to work correctly in the scala console, you need to make sure your backspace key is sending the erase character as configured by stty. For the default cygwin terminal (mintty) you can find a setting under Options -> Keys “Backspace sends ^H” which will need to be checked if your erase key is the cygwin default of ^H.

Note: Other configurations are currently unsupported. Please submit a pull request implementing or describing that support.

Hello, World 

This page assumes you’ve installed sbt.

Create a project directory with source code 

A valid sbt project can be a directory containing a single source file. Try creating a directory hello with a file hw.scala, containing the following: 

object Hi {
  def main(args: Array[String]) = println("Hi!")
}

Now from inside the hello directory, start sbt and type run at the sbt interactive console. On Linux or OS X the commands might look like this: 

$ mkdir hello
$ cd hello
$ echo 'object Hi { def main(args: Array[String]) = println("Hi!") }' > hw.scala
$ sbt
...
> run
...
Hi!

In this case, sbt works purely by convention. sbt will find the following automatically:

By default, sbt will build projects with the same version of Scala used to run sbt itself.

You can run the project with sbt run or enter the Scala REPL with sbt console. sbt console sets up your project’s classpath so you can try out live Scala examples based on your project’s code.

Build definition 

Most projects will need some manual setup. Basic build settings go in a file called build.sbt, located in the project’s base directory.

For example, if your project is in the directory hello, in hello/build.sbt you might write: 

name := "hello"

version := "1.0"

scalaVersion := "2.10.3"

Notice the blank line between every item. This isn’t just for show; they’re actually required in order to separate each item. In .sbt build definition you’ll learn more about how to write a build.sbt file.

If you plan to package your project in a jar, you will want to set at least the name and version in a build.sbt.

Setting the sbt version 

You can force a particular version of sbt by creating a file hello/project/build.properties. In this file, write: 

sbt.version=0.13.6

to force the use of sbt 0.13.6. sbt is 99% source compatible from release to release. Still, setting the sbt version in project/build.properties avoids any potential confusion.

Directory structure 

This page assumes you’ve installed sbt and seen the Hello, World example.

Base directory 

In sbt’s terminology, the “base directory” is the directory containing the project. So if you created a project hello containing hello/build.sbt and hello/hw.scala as in the Hello, World example, hello is your base directory.

Source code 

Source code can be placed in the project’s base directory as with hello/hw.scala. However, most people don’t do this for real projects; too much clutter.

sbt uses the same directory structure as Maven for source files by default (all paths are relative to the base directory): 

src/
  main/
    resources/
       <files to include in main jar here>
    scala/
       <main Scala sources>
    java/
       <main Java sources>
  test/
    resources
       <files to include in test jar here>
    scala/
       <test Scala sources>
    java/
       <test Java sources>

Other directories in src/ will be ignored. Additionally, all hidden directories will be ignored.

sbt build definition files 

You’ve already seen build.sbt in the project’s base directory. Other sbt files appear in a project subdirectory.

project can contain .scala files, which are combined with .sbt files to form the complete build definition. See .scala build definition for more. 

build.sbt
project/
  Build.scala

You may see .sbt files inside project/ but they are not equivalent to .sbt files in the project’s base directory. Explaining this will come later, since you’ll need some background information first.

Build products 

Generated files (compiled classes, packaged jars, managed files, caches, and documentation) will be written to the target directory by default.

Configuring version control 

Your .gitignore (or equivalent for other version control systems) should contain: 

target/

Note that this deliberately has a trailing / (to match only directories) and it deliberately has no leading / (to match project/target/ in addition to plain target/).

Running 

This page describes how to use sbt once you have set up your project. It assumes you’ve installed sbt and created a Hello, World or other project.

Interactive mode 

Run sbt in your project directory with no arguments: 

$ sbt

Running sbt with no command line arguments starts it in interactive mode. Interactive mode has a command prompt (with tab completion and history!).

For example, you could type compile at the sbt prompt: 

> compile

To compile again, press up arrow and then enter.

To run your program, type run.

To leave interactive mode, type exit or use Ctrl+D (Unix) or Ctrl+Z (Windows).

Batch mode 

You can also run sbt in batch mode, specifying a space-separated list of sbt commands as arguments. For sbt commands that take arguments, pass the command and arguments as one argument to sbt by enclosing them in quotes. For example, 

$ sbt clean compile "testOnly TestA TestB"

In this example, testOnly has arguments, TestA and TestB. The commands will be run in sequence (clean, compile, then testOnly).

Continuous build and test 

To speed up your edit-compile-test cycle, you can ask sbt to automatically recompile or run tests whenever you save a source file.

Make a command run when one or more source files change by prefixing the command with ~. For example, in interactive mode try: 

> ~ compile

Press enter to stop watching for changes.

You can use the ~ prefix with either interactive mode or batch mode.

See Triggered Execution for more details.

Common commands 

Here are some of the most common sbt commands. For a more complete list, see Command Line Reference.

clean Deletes all generated files (in the target directory).
compile Compiles the main sources (in src/main/scala and src/main/java directories).
test Compiles and runs all tests.
console Starts the Scala interpreter with a classpath including the compiled sources and all dependencies. To return to sbt, type :quit, Ctrl+D (Unix), or Ctrl+Z (Windows).
run <argument>* Runs the main class for the project in the same virtual machine as sbt.
package Creates a jar file containing the files in src/main/resources and the classes compiled from src/main/scala and src/main/java.
help <command> Displays detailed help for the specified command. If no command is provided, displays brief descriptions of all commands.
reload Reloads the build definition (build.sbt, project/*.scala, project/*.sbt files). Needed if you change the build definition.

Tab completion 

Interactive mode has tab completion, including at an empty prompt. A special sbt convention is that pressing tab once may show only a subset of most likely completions, while pressing it more times shows more verbose choices.

History Commands 

Interactive mode remembers history, even if you exit sbt and restart it. The simplest way to access history is with the up arrow key. The following commands are also supported:

! Show history command help.
!! Execute the previous command again.
!: Show all previous commands.
!:n Show the last n commands.
!n Execute the command with index n, as shown by the !: command.
!-n Execute the nth command before this one.
!string Execute the most recent command starting with 'string.'
!?string Execute the most recent command containing 'string.'

.sbt build definition 

This page describes sbt build definitions, including some “theory” and the syntax of build.sbt. It assumes you know how to use sbt and have read the previous pages in the Getting Started Guide.

.sbt vs .scala Build Definition 

An sbt build definition can contain files ending in .sbt, located in the base directory of a project, and files ending in .scala, located in the project/ subdirectory of the base directory.

This page discusses .sbt files, which are suitable for most cases. The .scala files are typically used for sharing code across .sbt files and for more complex build definitions. See .scala build definition (later in Getting Started) for more on .scala files.

What is a Build Definition? 

After examining a project and processing build definition files, sbt ends up with an immutable map (set of key-value pairs) describing the build.

For example, one key is name and it maps to a string value, the name of your project.

Build definition files do not affect sbt’s map directly.

Instead, the build definition creates a huge list of objects with type Setting[T] where T is the type of the value in the map. A Setting describes a transformation to the map, such as adding a new key-value pair or appending to an existing value. (In the spirit of functional programming with immutable data structures and values, a transformation returns a new map — it does not update the old map in-place.)

In build.sbt, you might create a Setting[String] for the name of your project like this: 

name := "hello"

This Setting[String] transforms the map by adding (or replacing) the name key, giving it the value "hello". The transformed map becomes sbt’s new map.

To create the map, sbt first sorts the list of settings so that all changes to the same key are made together, and values that depend on other keys are processed after the keys they depend on. Then sbt walks over the sorted list of Settings and applies each one to the map in turn.

Summary: A build definition defines a list of Setting[T], where a Setting[T] is a transformation affecting sbt’s map of key-value pairs and T is the type of each value.

How build.sbt defines settings 

build.sbt defines a Seq[Setting[_]]; it’s a list of Scala expressions, separated by blank lines, where each one becomes one element in the sequence. If you put Seq( in front of the .sbt file and ) at the end and replace the blank lines with commas, you’d be looking at the equivalent .scala code.

Here’s an example: 

name := "hello"

version := "1.0"

scalaVersion := "2.10.3"

Each Setting is defined with a Scala expression. The expressions in build.sbt are independent of one another, and they are expressions, rather than complete Scala statements. These expressions may be interspersed with vals, lazy vals, and defs. Top-level objects and classes are not allowed in build.sbt. Those should go in the project/ directory as full Scala source files.

On the left, name, version, and scalaVersion are keys. A key is an instance of SettingKey[T], TaskKey[T], or InputKey[T] where T is the expected value type. The kinds of key are explained below.

Keys have a method called :=, which returns a Setting[T]. You could use a Java-like syntax to call the method: 

name.:=("hello")

But Scala allows name := "hello" instead (in Scala, a single-parameter method can use either syntax).

The := method on key name returns a Setting, specifically a Setting[String]. String also appears in the type of name itself, which is SettingKey[String]. In this case, the returned Setting[String] is a transformation to add or replace the name key in sbt’s map, giving it the value "hello".

If you use the wrong value type, the build definition will not compile: 

name := 42  // will not compile

Settings must be separated by blank lines 

You can’t write a build.sbt like this: 

// will NOT compile, no blank lines
name := "hello"
version := "1.0"
scalaVersion := "2.10.3"

sbt needs some kind of delimiter to tell where one expression stops and the next begins.

.sbt files contain a list of Scala expressions, not a single Scala program. These expressions have to be split up and passed to the compiler individually.

Keys 

Types 

There are three flavors of key:

Built-in Keys 

The built-in keys are just fields in an object called Keys. A build.sbt implicitly has an import sbt.Keys._, so sbt.Keys.name can be referred to as name.

Custom Keys 

Custom keys may be defined with their respective creation methods: settingKey, taskKey, and inputKey. Each method expects the type of the value associated with the key as well as a description. The name of the key is taken from the val the key is assigned to. For example, to define a key for a new task called hello, 

lazy val hello = taskKey[Unit]("An example task")

Here we have used the fact that an .sbt file can contain vals and defs in addition to settings. All such definitions are evaluated before settings regardless of where they are defined in the file. vals and defs must be separated from settings by blank lines.

Note: Typically, lazy vals are used instead of vals to avoid initialization order problems.

Task vs Setting keys 

A TaskKey[T] is said to define a task. Tasks are operations such as compile or package. They may return Unit (Unit is Scala for void), or they may return a value related to the task, for example package is a TaskKey[File] and its value is the jar file it creates.

Each time you start a task execution, for example by typing compile at the interactive sbt prompt, sbt will re-run any tasks involved exactly once.

sbt’s map describing the project can keep around a fixed string value for a setting such as name, but it has to keep around some executable code for a task such as compile — even if that executable code eventually returns a string, it has to be re-run every time.

A given key always refers to either a task or a plain setting. That is, “taskiness” (whether to re-run each time) is a property of the key, not the value.

Defining tasks and settings 

Using :=, you can assign a value to a setting and a computation to a task. For a setting, the value will be computed once at project load time. For a task, the computation will be re-run each time the task is executed.

For example, to implement the hello task from the previous section, : 

hello := { println("Hello!") }

We already saw an example of defining settings when we defined the project’s name, 

name := "hello"

Types for tasks and settings 

From a type-system perspective, the Setting created from a task key is slightly different from the one created from a setting key. taskKey := 42 results in a Setting[Task[T]] while settingKey := 42 results in a Setting[T]. For most purposes this makes no difference; the task key still creates a value of type T when the task executes.

The T vs. Task[T] type difference has this implication: a setting can’t depend on a task, because a setting is evaluated only once on project load and is not re-run. More on this in more kinds of setting, coming up soon.

Keys in sbt interactive mode 

In sbt’s interactive mode, you can type the name of any task to execute that task. This is why typing compile runs the compile task. compile is a task key.

If you type the name of a setting key rather than a task key, the value of the setting key will be displayed. Typing a task key name executes the task but doesn’t display the resulting value; to see a task’s result, use show <task name> rather than plain <task name>. The convention for keys names is to use camelCase so that the command line name and the Scala identifiers are the same.

To learn more about any key, type inspect <keyname> at the sbt interactive prompt. Some of the information inspect displays won’t make sense yet, but at the top it shows you the setting’s value type and a brief description of the setting.

Imports in build.sbt 

You can place import statements at the top of build.sbt; they need not be separated by blank lines.

There are some implied default imports, as follows: 

import sbt._
import Process._
import Keys._

(In addition, if you have .scala files, the contents of any Build or Plugin objects in those files will be imported. More on that when we get to .scala build definition.)

Adding library dependencies 

To depend on third-party libraries, there are two options. The first is to drop jars in lib/ (unmanaged dependencies) and the other is to add managed dependencies, which will look like this in build.sbt: 

libraryDependencies += "org.apache.derby" % "derby" % "10.4.1.3"

This is how you add a managed dependency on the Apache Derby library, version 10.4.1.3.

The libraryDependencies key involves two complexities: += rather than :=, and the % method. += appends to the key’s old value rather than replacing it, this is explained in more kinds of setting. The % method is used to construct an Ivy module ID from strings, explained in Library dependencies.

We’ll skip over the details of library dependencies until later in the Getting Started Guide. There’s a whole page covering it later on.

Scopes 

This page describes scopes. It assumes you’ve read and understood the previous page, .sbt build definition.

The whole story about keys 

Previously we pretended that a key like name corresponded to one entry in sbt’s map of key-value pairs. This was a simplification.

In truth, each key can have an associated value in more than one context, called a “scope.”

Some concrete examples:

There is no single value for a given key name, because the value may differ according to scope.

However, there is a single value for a given scoped key.

If you think about sbt processing a list of settings to generate a key-value map describing the project, as discussed earlier, the keys in that key-value map are scoped keys. Each setting defined in the build definition (for example in build.sbt) applies to a scoped key as well.

Often the scope is implied or has a default, but if the defaults are wrong, you’ll need to mention the desired scope in build.sbt.

Scope axes 

A scope axis is a type, where each instance of the type can define its own scope (that is, each instance can have its own unique values for keys).

There are three scope axes:

Scoping by project axis 

If you put multiple projects in a single build, each project needs its own settings. That is, keys can be scoped according to the project.

The project axis can also be set to “entire build”, so a setting applies to the entire build rather than a single project. Build-level settings are often used as a fallback when a project doesn’t define a project-specific setting.

Scoping by configuration axis 

A configuration defines a flavor of build, potentially with its own classpath, sources, generated packages, etc. The configuration concept comes from Ivy, which sbt uses for managed dependencies Library Dependencies, and from MavenScopes.

Some configurations you’ll see in sbt:

By default, all the keys associated with compiling, packaging, and running are scoped to a configuration and therefore may work differently in each configuration. The most obvious examples are the task keys compile, package, and run; but all the keys which affect those keys (such as sourceDirectories or scalacOptions or fullClasspath) are also scoped to the configuration.

Scoping by task axis 

Settings can affect how a task works. For example, the packageSrc task is affected by the packageOptions setting.

To support this, a task key (such as packageSrc) can be a scope for another key (such as packageOptions).

The various tasks that build a package (packageSrc, packageBin, packageDoc) can share keys related to packaging, such as artifactName and packageOptions. Those keys can have distinct values for each packaging task.

Global scope 

Each scope axis can be filled in with an instance of the axis type (for example the task axis can be filled in with a task), or the axis can be filled in with the special value Global.

Global means what you would expect: the setting’s value applies to all instances of that axis. For example if the task axis is Global, then the setting would apply to all tasks.

Delegation 

A scoped key may be undefined, if it has no value associated with it in its scope.

For each scope, sbt has a fallback search path made up of other scopes. Typically, if a key has no associated value in a more-specific scope, sbt will try to get a value from a more general scope, such as the Global scope or the entire-build scope.

This feature allows you to set a value once in a more general scope, allowing multiple more-specific scopes to inherit the value.

You can see the fallback search path or “delegates” for a key using the inspect command, as described below. Read on.

Referring to scoped keys when running sbt 

On the command line and in interactive mode, sbt displays (and parses) scoped keys like this: 

{<build-uri>}<project-id>/config:intask::key

* can appear for each axis, referring to the Global scope.

If you omit part of the scoped key, it will be inferred as follows:

For more details, see Interacting with the Configuration System.

Examples of scoped key notation 

Inspecting scopes 

In sbt’s interactive mode, you can use the inspect command to understand keys and their scopes. Try inspect test:fullClasspath: 

$ sbt
> inspect test:fullClasspath
[info] Task: scala.collection.Seq[sbt.Attributed[java.io.File]]
[info] Description:
[info]  The exported classpath, consisting of build products and unmanaged and managed, internal and external dependencies.
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}default-aea33a/test:fullClasspath
[info] Dependencies:
[info]  test:exportedProducts
[info]  test:dependencyClasspath
[info] Reverse dependencies:
[info]  test:runMain
[info]  test:run
[info]  test:testLoader
[info]  test:console
[info] Delegates:
[info]  test:fullClasspath
[info]  runtime:fullClasspath
[info]  compile:fullClasspath
[info]  *:fullClasspath
[info]  {.}/test:fullClasspath
[info]  {.}/runtime:fullClasspath
[info]  {.}/compile:fullClasspath
[info]  {.}/*:fullClasspath
[info]  */test:fullClasspath
[info]  */runtime:fullClasspath
[info]  */compile:fullClasspath
[info]  */*:fullClasspath
[info] Related:
[info]  compile:fullClasspath
[info]  compile:fullClasspath(for doc)
[info]  test:fullClasspath(for doc)
[info]  runtime:fullClasspath

On the first line, you can see this is a task (as opposed to a setting, as explained in .sbt build definition). The value resulting from the task will have type scala.collection.Seq[sbt.Attributed[java.io.File]].

“Provided by” points you to the scoped key that defines the value, in this case {file:/home/hp/checkout/hello/}default-aea33a/test:fullClasspath (which is the fullClasspath key scoped to the test configuration and the {file:/home/hp/checkout/hello/}default-aea33a project).

“Dependencies” may not make sense yet; stay tuned for the next page.

You can also see the delegates; if the value were not defined, sbt would search through:

Try inspect fullClasspath (as opposed to the above example, inspect test:fullClasspath) to get a sense of the difference. Because the configuration is omitted, it is autodetected as compile. inspect compile:fullClasspath should therefore look the same as inspect fullClasspath.

Try inspect *:fullClasspath for another contrast. fullClasspath is not defined in the Global configuration by default.

Again, for more details, see Interacting with the Configuration System.

Referring to scopes in a build definition 

If you create a setting in build.sbt with a bare key, it will be scoped to the current project, configuration Global and task Global: 

name := "hello"

Run sbt and inspect name to see that it’s provided by {file:/home/hp/checkout/hello/}default-aea33a/*:name, that is, the project is {file:/home/hp/checkout/hello/}default-aea33a, the configuration is * (meaning global), and the task is not shown (which also means global).

build.sbt always defines settings for a single project, so the “current project” is the project you’re defining in that particular build.sbt. (For multi-project builds, each project has its own build.sbt.)

Keys have an overloaded method called in used to set the scope. The argument to in can be an instance of any of the scope axes. So for example, though there’s no real reason to do this, you could set the name scoped to the Compile configuration: 

name in Compile := "hello"

or you could set the name scoped to the packageBin task (pointless! just an example): 

name in packageBin := "hello"

or you could set the name with multiple scope axes, for example in the packageBin task in the Compile configuration: 

name in (Compile, packageBin) := "hello"

or you could use Global for all axes: 

name in Global := "hello"

(name in Global implicitly converts the scope axis Global to a scope with all axes set to Global; the task and configuration are already Global by default, so here the effect is to make the project Global, that is, define */*:name rather than {file:/home/hp/checkout/hello/}default-aea33a/*:name)

If you aren’t used to Scala, a reminder: it’s important to understand that in and := are just methods, not magic. Scala lets you write them in a nicer way, but you could also use the Java style: 

name.in(Compile).:=("hello")

There’s no reason to use this ugly syntax, but it illustrates that these are in fact methods.

When to specify a scope 

You need to specify the scope if the key in question is normally scoped. For example, the compile task, by default, is scoped to Compile and Test configurations, and does not exist outside of those scopes.

To change the value associated with the compile key, you need to write compile in Compile or compile in Test. Using plain compile would define a new compile task scoped to the current project, rather than overriding the standard compile tasks which are scoped to a configuration.

If you get an error like “Reference to undefined setting“, often you’ve failed to specify a scope, or you’ve specified the wrong scope. The key you’re using may be defined in some other scope. sbt will try to suggest what you meant as part of the error message; look for “Did you mean compile:compile?”

One way to think of it is that a name is only part of a key. In reality, all keys consist of both a name, and a scope (where the scope has three axes). The entire expression packageOptions in (Compile, packageBin) is a key name, in other words. Simply packageOptions is also a key name, but a different one (for keys with no in, a scope is implicitly assumed: current project, global config, global task).

More kinds of setting 

This page explains other ways to create a Setting, beyond the basic := method. It assumes you’ve read .sbt build definition and scopes.

Refresher: Settings 

Remember, a build definition creates a list of Setting, which is then used to transform sbt’s description of the build (which is a map of key-value pairs). A Setting is a transformation with sbt’s earlier map as input and a new map as output. The new map becomes sbt’s new state.

Different settings transform the map in different ways. Earlier, you read about the := method.

The Setting which := creates puts a fixed, constant value in the new, transformed map. For example, if you transform a map with the setting name := "hello" the new map has the string "hello" stored under the key name.

Settings must end up in the master list of settings to do any good (all lines in a build.sbt automatically end up in the list, but in a .scala file you can get it wrong by creating a Setting without putting it where sbt will find it).

Appending to previous values: += and ++= 

Assignment with := is the simplest transformation, but keys have other methods as well. If the T in SettingKey[T] is a sequence, i.e. the key’s value type is a sequence, you can append to the sequence rather than replacing it.

For example, the key sourceDirectories in Compile has a Seq[File] as its value. By default this key’s value would include src/main/scala. If you wanted to also compile source code in a directory called source (since you just have to be nonstandard), you could add that directory: 

sourceDirectories in Compile += new File("source")

Or, using the file() function from the sbt package for convenience: 

sourceDirectories in Compile += file("source")

(file() just creates a new File.)

You could use ++= to add more than one directory at a time: 

sourceDirectories in Compile ++= Seq(file("sources1"), file("sources2"))

Where Seq(a, b, c, ...) is standard Scala syntax to construct a sequence.

To replace the default source directories entirely, you use := of course: 

sourceDirectories in Compile := Seq(file("sources1"), file("sources2"))

Computing a value based on other keys’ values 

Reference the value of another task or setting by calling value on the key for the task or setting. The value method is special and may only be called in the argument to :=, +=, or ++=.

As a first example, consider defining the project organization to be the same as the project name. 

// name our organization after our project (both are SettingKey[String])
organization := name.value

Or, set the name to the name of the project’s directory: 

// name is a Key[String], baseDirectory is a Key[File]
// name the project after the directory it's inside
name := baseDirectory.value.getName

This transforms the value of baseDirectory using the standard getName method of java.io.File.

Using multiple inputs is similar. For example, 

name := "project " + name.value + " from " + organization.value + " version " + version.value

This sets the name in terms of its previous value as well as the organization and version settings.

Settings with dependencies 

In the setting name := baseDirectory.value.getName, name will have a dependency on baseDirectory. If you place the above in build.sbt and run the sbt interactive console, then type inspect name, you should see (in part): 

[info] Dependencies:
[info]  *:baseDirectory

This is how sbt knows which settings depend on which other settings. Remember that some settings describe tasks, so this approach also creates dependencies between tasks.

For example, if you inspect compile you’ll see it depends on another key compileInputs, and if you inspect compileInputs it in turn depends on other keys. Keep following the dependency chains and magic happens. When you type compile sbt automatically performs an update, for example. It Just Works because the values required as inputs to the compile computation require sbt to do the update computation first.

In this way, all build dependencies in sbt are automatic rather than explicitly declared. If you use a key’s value in another computation, then the computation depends on that key. It just works!

When settings are undefined 

Whenever a setting uses :=, +=, or ++= to create a dependency on itself or another key’s value, the value it depends on must exist. If it does not, sbt will complain. It might say “Reference to undefined setting“, for example. When this happens, be sure you’re using the key in the scope that defines it.

It’s possible to create cycles, which is an error; sbt will tell you if you do this.

Tasks based on other keys’ values 

You can compute values of some tasks or settings to define or append value for another task. It’s done by using Def.task and taskValue, as argument to :=, += or ++=.

As a first example, consider appending a source generator using the project base directory and compilation classpath. 

sourceGenerators in Compile += Def.task {
  myGenerator(baseDirectory.value, (managedClasspath in Compile).value)
}.taskValue

Tasks with dependencies 

As noted in .sbt build definition, task keys create a Setting[Task[T]] rather than a Setting[T] when you build a setting with :=, etc. Tasks can use settings as inputs, but settings cannot use tasks as inputs.

Take these two keys (from Keys): 

val scalacOptions = taskKey[Seq[String]]("Options for the Scala compiler.")
val checksums = settingKey[Seq[String]]("The list of checksums to generate and to verify for dependencies.")

(scalacOptions and checksums have nothing to do with each other, they are just two keys with the same value type, where one is a task.)

It is possible to compile a build.sbt that aliases scalacOptions to checksums, but not the other way. For example, this is allowed: 

// The scalacOptions task may be defined in terms of the checksums setting
scalacOptions := checksums.value

There is no way to go the other direction. That is, a setting key can’t depend on a task key. That’s because a setting key is only computed once on project load, so the task would not be re-run every time, and tasks expect to re-run every time. 

// The checksums setting may not be defined in terms of the scalacOptions task
checksums := scalacOptions.value

Appending with dependencies: += and ++= 

Other keys can be used when appending to an existing setting or task, just like they can for assigning with :=.

For example, say you have a coverage report named after the project, and you want to add it to the files removed by clean: 

cleanFiles += file("coverage-report-" + name.value + ".txt")

Library dependencies 

This page assumes you’ve already read the earlier Getting Started pages, in particular .sbt build definition, scopes, and more kinds of setting.

Library dependencies can be added in two ways:

Unmanaged dependencies 

Most people use managed dependencies instead of unmanaged. But unmanaged can be simpler when starting out.

Unmanaged dependencies work like this: add jars to lib and they will be placed on the project classpath. Not much else to it!

You can place test jars such as ScalaCheck, Specs2, and ScalaTest in lib as well.

Dependencies in lib go on all the classpaths (for compile, test, run, and console). If you wanted to change the classpath for just one of those, you would adjust dependencyClasspath in Compile or dependencyClasspath in Runtime for example.

There’s nothing to add to build.sbt to use unmanaged dependencies, though you could change the unmanagedBase key if you’d like to use a different directory rather than lib.

To use custom_lib instead of lib: 

unmanagedBase := baseDirectory.value / "custom_lib"

baseDirectory is the project’s root directory, so here you’re changing unmanagedBase depending on baseDirectory using the special value method as explained in more kinds of setting.

There’s also an unmanagedJars task which lists the jars from the unmanagedBase directory. If you wanted to use multiple directories or do something else complex, you might need to replace the whole unmanagedJars task with one that does something else, e.g. empty the list for Compile configuration regardless of the files in lib directory: 

unmanagedJars in Compile := Seq.empty[sbt.Attributed[java.io.File]]

Managed Dependencies 

sbt uses Apache Ivy to implement managed dependencies, so if you’re familiar with Ivy or Maven, you won’t have much trouble.

The libraryDependencies key 

Most of the time, you can simply list your dependencies in the setting libraryDependencies. It’s also possible to write a Maven POM file or Ivy configuration file to externally configure your dependencies, and have sbt use those external configuration files. You can learn more about that here.

Declaring a dependency looks like this, where groupId, artifactId, and revision are strings: 

libraryDependencies += groupID % artifactID % revision

or like this, where configuration can be a string or Configuration val: 

libraryDependencies += groupID % artifactID % revision % configuration

libraryDependencies is declared in Keys like this: 

val libraryDependencies = settingKey[Seq[ModuleID]]("Declares managed dependencies.")

The % methods create ModuleID objects from strings, then you add those ModuleID to libraryDependencies.

Of course, sbt (via Ivy) has to know where to download the module. If your module is in one of the default repositories sbt comes with, this will just work. For example, Apache Derby is in the standard Maven2 repository: 

libraryDependencies += "org.apache.derby" % "derby" % "10.4.1.3"

If you type that in build.sbt and then update, sbt should download Derby to ~/.ivy2/cache/org.apache.derby/. (By the way, update is a dependency of compile so there’s no need to manually type update most of the time.)

Of course, you can also use ++= to add a list of dependencies all at once: 

libraryDependencies ++= Seq(
  groupID % artifactID % revision,
  groupID % otherID % otherRevision
)

In rare cases you might find reasons to use := with libraryDependencies as well.

Getting the right Scala version with %% 

If you use groupID %% artifactID % revision rather than groupID % artifactID % revision (the difference is the double %% after the groupID), sbt will add your project’s Scala version to the artifact name. This is just a shortcut. You could write this without the %%: 

libraryDependencies += "org.scala-tools" % "scala-stm_2.11.1" % "0.3"

Assuming the scalaVersion for your build is 2.11.1, the following is identical (note the double %% after "org.scala-tools"): 

libraryDependencies += "org.scala-tools" %% "scala-stm" % "0.3"

The idea is that many dependencies are compiled for multiple Scala versions, and you’d like to get the one that matches your project to ensure binary compatibility.

The complexity in practice is that often a dependency will work with a slightly different Scala version; but %% is not smart about that. So if the dependency is available for 2.10.1 but you’re using scalaVersion := "2.10.4", you won’t be able to use %% even though the 2.10.1 dependency likely works. If %% stops working, just go see which versions the dependency is really built for, and hardcode the one you think will work (assuming there is one).

See Cross Building for some more detail on this.

Ivy revisions 

The revision in groupID % artifactID % revision does not have to be a single fixed version. Ivy can select the latest revision of a module according to constraints you specify. Instead of a fixed revision like "1.6.1", you specify "latest.integration", "2.9.+", or "[1.0,)". See the Ivy revisions documentation for details.

Resolvers 

Not all packages live on the same server; sbt uses the standard Maven2 repository by default. If your dependency isn’t on one of the default repositories, you’ll have to add a resolver to help Ivy find it.

To add an additional repository, use 

resolvers += name at location

with the special at between two strings.

For example: 

resolvers += "Sonatype OSS Snapshots" at "https://oss.sonatype.org/content/repositories/snapshots"

The resolvers key is defined in Keys like this: 

val resolvers = settingKey[Seq[Resolver]]("The user-defined additional resolvers for automatically managed dependencies.")

The at method creates a Resolver object from two strings.

sbt can search your local Maven repository if you add it as a repository: 

resolvers += "Local Maven Repository" at "file://"+Path.userHome.absolutePath+"/.m2/repository"

or, for convenience: 

resolvers += Resolver.mavenLocal

See Resolvers for details on defining other types of repositories.

Overriding default resolvers 

resolvers does not contain the default resolvers; only additional ones added by your build definition.

sbt combines resolvers with some default repositories to form externalResolvers.

Therefore, to change or remove the default resolvers, you would need to override externalResolvers instead of resolvers.

Per-configuration dependencies 

Often a dependency is used by your test code (in src/test/scala, which is compiled by the Test configuration) but not your main code.

If you want a dependency to show up in the classpath only for the Test configuration and not the Compile configuration, add % "test" like this: 

libraryDependencies += "org.apache.derby" % "derby" % "10.4.1.3" % "test"

You may also use the type-safe version of Test configuration as follows: 

libraryDependencies += "org.apache.derby" % "derby" % "10.4.1.3" % Test

Now, if you type show compile:dependencyClasspath at the sbt interactive prompt, you should not see the derby jar. But if you type show test:dependencyClasspath, you should see the derby jar in the list.

Typically, test-related dependencies such as ScalaCheck, Specs2, and ScalaTest would be defined with % "test".

There are more details and tips-and-tricks related to library dependencies on this page.

Multi-project builds 

This page introduces multiple projects in a single build.

Please read the earlier pages in the Getting Started Guide first, in particular you need to understand build.sbt before reading this page.

Multiple projects 

It can be useful to keep multiple related projects in a single build, especially if they depend on one another and you tend to modify them together.

Each sub-project in a build has its own source directories, generates its own jar file when you run package, and in general works like any other project.

A project is defined by declaring a lazy val of type Project. For example, : 

lazy val util = project

lazy val core = project

The name of the val is used as the project’s ID and base directory name. The ID is used to refer to the project at the command line. The base directory may be changed from the default using the in method. For example, the following is a more explicit way to write the previous example: 

lazy val util = project.in(file("util"))

lazy val core = project in file("core")

Dependencies 

Projects in the build can be completely independent of one another, but usually they will be related to one another by some kind of dependency. There are two types of dependencies: aggregate and classpath.

Aggregation 

Aggregation means that running a task on the aggregate project will also run it on the aggregated projects. For example, 

lazy val root = (project in file(".")).
  aggregate(util, core)

lazy val util = project

lazy val core = project

In the above example, the root project aggregates util and core. Start up sbt with two subprojects as in the example, and try compile. You should see that all three projects are compiled.

In the project doing the aggregating, the root project in this case, you can control aggregation per-task. For example, to avoid aggregating the update task: 

lazy val root = (project in file(".")).
  aggregate(util, core).
  settings(
    aggregate in update := false
  )

[...]

aggregate in update is the aggregate key scoped to the update task. (See scopes.)

Note: aggregation will run the aggregated tasks in parallel and with no defined ordering between them.

Classpath dependencies 

A project may depend on code in another project. This is done by adding a dependsOn method call. For example, if core needed util on its classpath, you would define core as: 

lazy val core = project.dependsOn(util)

Now code in core can use classes from util. This also creates an ordering between the projects when compiling them; util must be updated and compiled before core can be compiled.

To depend on multiple projects, use multiple arguments to dependsOn, like dependsOn(bar, baz).

Per-configuration classpath dependencies 

foo dependsOn(bar) means that the compile configuration in foo depends on the compile configuration in bar. You could write this explicitly as dependsOn(bar % "compile->compile").

The -> in "compile->compile" means “depends on” so "test->compile" means the test configuration in foo would depend on the compile configuration in bar.

Omitting the ->config part implies ->compile, so dependsOn(bar % "test") means that the test configuration in foo depends on the Compile configuration in bar.

A useful declaration is "test->test" which means test depends on test. This allows you to put utility code for testing in bar/src/test/scala and then use that code in foo/src/test/scala, for example.

You can have multiple configurations for a dependency, separated by semicolons. For example, dependsOn(bar % "test->test;compile->compile").

Default root project 

If a project is not defined for the root directory in the build, sbt creates a default one that aggregates all other projects in the build.

Because project hello-foo is defined with base = file("foo"), it will be contained in the subdirectory foo. Its sources could be directly under foo, like foo/Foo.scala, or in foo/src/main/scala. The usual sbt directory structure applies underneath foo with the exception of build definition files.

Any .sbt files in foo, say foo/build.sbt, will be merged with the build definition for the entire build, but scoped to the hello-foo project.

If your whole project is in hello, try defining a different version (version := "0.6") in hello/build.sbt, hello/foo/build.sbt, and hello/bar/build.sbt. Now show version at the sbt interactive prompt. You should get something like this (with whatever versions you defined): 

> show version
[info] hello-foo/*:version
[info]  0.7
[info] hello-bar/*:version
[info]  0.9
[info] hello/*:version
[info]  0.5

hello-foo/*:version was defined in hello/foo/build.sbt, hello-bar/*:version was defined in hello/bar/build.sbt, and hello/*:version was defined in hello/build.sbt. Remember the syntax for scoped keys. Each version key is scoped to a project, based on the location of the build.sbt. But all three build.sbt are part of the same build definition.

You may find it cleaner to put everything including settings in .scala files in order to keep all build definition under a single project directory, however. It’s up to you.

You cannot have a project subdirectory or project/*.scala files in the sub-projects. foo/project/Build.scala would be ignored.

At the sbt interactive prompt, type projects to list your projects and project <projectname> to select a current project. When you run a task like compile, it runs on the current project. So you don’t necessarily have to compile the root project, you could compile only a subproject.

You can run a task in another project by explicitly specifying the project ID, such as subProjectID/compile.

Common code 

The definitions in .sbt files are not visible in other .sbt files. In order to share code between .sbt files, define one or more Scala files in the project/ directory of the build root. This directory is also an sbt project, but for your build.

For example:

<root>/project/Common.scala: 

import sbt._
import Keys._

object Common {
  def text = "org.example"
}

<root>/build.sbt: 

organization := Common.text

See .scala Build Definition for details.

Using plugins 

Please read the earlier pages in the Getting Started Guide first, in particular you need to understand build.sbt and library dependencies, before reading this page.

What is a plugin? 

A plugin extends the build definition, most commonly by adding new settings. The new settings could be new tasks. For example, a plugin could add a codeCoverage task which would generate a test coverage report.

Declaring a plugin 

If your project is in directory hello, and you’re adding sbt-site plugin to the build definition, create hello/project/site.sbt and declare the plugin dependency by passing the plugin’s Ivy module ID to addSbtPlugin: 

addSbtPlugin("com.typesafe.sbt" % "sbt-site" % "0.7.0")

If you’re adding sbt-assembly, create hello/project/assembly.sbt with the following: 

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.11.2")

Not every plugin is located on one of the default repositories and a plugin’s documentation may instruct you to also add the repository where it can be found: 

resolvers += Resolver.sonatypeRepo("public")

Plugins usually provide settings that get added to a project to enable the plugin’s functionality. This is described in the next section.

Enabling and disabling auto plugins 

A plugin can declare that its settings be automatically added to the build definition, in which case you don’t have to do anything to add them.

As of sbt 0.13.5, there is a new auto plugins feature that enables plugins to automatically, and safely, ensure their settings and dependencies are on a project. Many auto plugins should have their default settings automatically, however some may require explicit enablement.

If you’re using an auto plugin that requires explicit enablement, then you you have to add the following to your build.sbt: 

lazy val util = (project in file("util")).
  enablePlugins(FooPlugin, BarPlugin).
  settings(
    name := "hello-util"
  )

The enablePlugins method allows projects to explicitly define the auto plugins they wish to consume.

Projects can also exclude plugins using the disablePlugins method. For example, if we wish to remove the IvyPlugin settings from util, we modify our build.sbt as follows: 

lazy val util = (project in file("util")).
  enablePlugins(FooPlugin, BarPlugin).
  disablePlugins(plugins.IvyPlugin).
  settings(
    name := "hello-util"
  )

Auto plugins should document whether they need to explicitly enabled. If you’re curious which auto plugins are enabled for a given project, just run the plugins command on the sbt console.

For example: 

> plugins
In file:/home/jsuereth/projects/sbt/test-ivy-issues/
        sbt.plugins.IvyPlugin: enabled in scala-sbt-org
        sbt.plugins.JvmPlugin: enabled in scala-sbt-org
        sbt.plugins.CorePlugin: enabled in scala-sbt-org
        sbt.plugins.JUnitXmlReportPlugin: enabled in scala-sbt-org

Here, the plugins output is showing that the sbt default plugins are all enabled. sbt’s default settings are provided via three plugins:

  1. CorePlugin: Provides the core parallelism controls for tasks
  2. IvyPlugin: Provides the mechanisms to publish/resolve modules.
  3. JvmPlugin: Provides the mechanisms to compile/test/run/package Java/Scala projects.

In addition, JUnitXmlReportPlugin provides an experimental support for generating junit-xml.

Older non-auto plugins often require settings to be added explictly, so that multi-project build could have different types of projects. The plugin documentation will indicate how to configure it, but typically for older plugins this involves adding the base settings for the plugin and customizing as necessary.

For example, for the sbt-site plugin, create site.sbt with the following content 

site.settings

to enable it for that project.

If the build defines multiple projects, instead add it directly to the project: 

// don't use the site plugin for the `util` project
lazy val util = (project in file("util"))

// enable the site plugin for the `core` project
lazy val core = (project in file("core")).
  settings(site.settings : _*)

Global plugins 

Plugins can be installed for all your projects at once by declaring them in ~/.sbt/0.13/plugins/. ~/.sbt/0.13/plugins/ is an sbt project whose classpath is exported to all sbt build definition projects. Roughly speaking, any .sbt or .scala files in ~/.sbt/0.13/plugins/ behave as if they were in the project/ directory for all projects.

You can create ~/.sbt/0.13/plugins//build.sbt and put addSbtPlugin() expressions in there to add plugins to all your projects at once. Because doing so would increase the dependency on the machine environment, this feature should be used sparingly. See Best Practices.

Available Plugins 

There’s a list of available plugins.

Some especially popular plugins are:

For more details, including ways of developing plugins, see Plugins. For best practices, see Plugins-Best-Practices.

Custom settings and tasks 

This page gets you started creating your own settings and tasks.

To understand this page, be sure you’ve read earlier pages in the Getting Started Guide, especially .build.sbt and more kinds of setting.

Defining a key 

Keys is packed with examples illustrating how to define keys. Most of the keys are implemented in Defaults.

Keys have one of three types. SettingKey and TaskKey are described in .sbt build definition. Read about InputKey on the Input Tasks page.

Some examples from Keys: 

val scalaVersion = settingKey[String]("The version of Scala used for building.")
val clean = taskKey[Unit]("Deletes files produced by the build, such as generated sources, compiled classes, and task caches.")

The key constructors have two string parameters: the name of the key ("scalaVersion") and a documentation string ("The version of scala used for building.").

Remember from .sbt build definition that the type parameter T in SettingKey[T] indicates the type of value a setting has. T in TaskKey[T] indicates the type of the task’s result. Also remember from .sbt build definition that a setting has a fixed value until project reload, while a task is re-computed for every “task execution” (every time someone types a command at the sbt interactive prompt or in batch mode).

Keys may be defined in a .sbt file, .scala file, or in a plugin. Any val found in a Build object in your .scala build definition files or any val found in a Plugin object from a plugin will be imported automatically into your .sbt files.

Implementing a task 

Once you’ve defined a key for your task, you’ll need to complete it with a task definition. You could be defining your own task, or you could be planning to redefine an existing task. Either way looks the same; use := to associate some code with the task key: 

val sampleStringTask = taskKey[String]("A sample string task.")

val sampleIntTask = taskKey[Int]("A sample int task.")

sampleStringTask := System.getProperty("user.home")

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

If the task has dependencies, you’d reference their value using value, as discussed in more kinds of setting.

The hardest part about implementing tasks is often not sbt-specific; tasks are just Scala code. The hard part could be writing the “meat” of your task that does whatever you’re trying to do. For example, maybe you’re trying to format HTML in which case you might want to use an HTML library (you would add a library dependency to your build definition and write code based on the HTML library, perhaps).

sbt has some utility libraries and convenience functions, in particular you can often use the convenient APIs in IO to manipulate files and directories.

Use plugins! 

If you find you have a lot of custom code, consider moving it to a plugin for re-use across multiple builds.

It’s very easy to create a plugin, as teased earlier and discussed at more length here.

This page has been a quick taste; there’s much much more about custom tasks on the Tasks page.

.scala build definition 

This page assumes you’ve read previous pages in the Getting Started Guide, especially .sbt build definition and more kinds of setting.

sbt is recursive 

build.sbt is so simple, it conceals how sbt really works. sbt builds are defined with Scala code. That code, itself, has to be built. What better way than with sbt?

The project directory is another project inside your project which knows how to build your project. The project inside project can (in theory) do anything any other project can do. Your build definition is an sbt project.

And the turtles go all the way down. If you like, you can tweak the build definition of the build definition project, by creating a project/project/ directory.

Here’s an illustration. 

hello/                  # your project's base directory

    Hello.scala         # a source file in your project (could be in
                        #   src/main/scala too)

    build.sbt           # build.sbt is part of the source code for the
                        #   build definition project inside project/

    project/            # base directory of the build definition project

        Build.scala     # a source file in the project/ project,
                        #   that is, a source file in the build definition

        build.sbt       # this is part of a build definition for a project
                        #   in project/project ; build definition's build
                        #   definition


        project/        # base directory of the build definition project
                        #   for the build definition

            Build.scala # source file in the project/project/ project

Don’t worry! Most of the time you are not going to need all that. But understanding the principle can be helpful.

By the way: any time files ending in .scala or .sbt are used, naming them build.sbt and Build.scala are conventions only. This also means that multiple files are allowed.

.scala source files in the build definition project 

.sbt files are merged into their sibling project directory. Looking back at the project layout: 

hello/                  # your project's base directory

    build.sbt           # build.sbt is part of the source code for the
                        #   build definition project inside project/

    project/            # base directory of the build definition project

        Build.scala     # a source file in the project/ project,
                        #   that is, a source file in the build definition

The Scala expressions in build.sbt are compiled alongside and merged with Build.scala (or any other .scala files in the project/ directory).

*.sbt files in the base directory for a project become part of the project build definition project also located in that base directory.

The .sbt file format is a convenient shorthand for adding settings to the build definition project.

Relating build.sbt to Build.scala 

To mix .sbt and .scala files in your build definition, you need to understand how they relate.

The following two files illustrate. First, if your project is in hello, create hello/project/Build.scala as follows: 

import sbt._
import Keys._

object HelloBuild extends Build {
  val sampleKeyA = settingKey[String]("demo key A")
  val sampleKeyB = settingKey[String]("demo key B")
  val sampleKeyC = settingKey[String]("demo key C")
  val sampleKeyD = settingKey[String]("demo key D")

  override lazy val settings = super.settings ++
    Seq(
      sampleKeyA := "A: in Build.settings in Build.scala",
      resolvers := Seq()
    )

  lazy val root = Project(id = "hello",
    base = file("."),
    settings = Seq(
      sampleKeyB := "B: in the root project settings in Build.scala"
    ))
}

Now, create hello/build.sbt as follows: 

sampleKeyC in ThisBuild := "C: in build.sbt scoped to ThisBuild"

sampleKeyD := "D: in build.sbt"

Start up the sbt interactive prompt. Type inspect sampleKeyA and you should see (among other things): 

[info] Setting: java.lang.String = A: in Build.settings in Build.scala
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}/*:sampleKeyA

and then inspect sampleKeyC and you should see: 

[info] Setting: java.lang.String = C: in build.sbt scoped to ThisBuild
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}/*:sampleKeyC

Note that the “Provided by” shows the same scope for the two values. That is, sampleKeyC in ThisBuild in a .sbt file is equivalent to placing a setting in the Build.settings list in a .scala file. sbt takes build-scoped settings from both places to create the build definition.

Now, inspect sampleKeyB: 

[info] Setting: java.lang.String = B: in the root project settings in Build.scala
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}hello/*:sampleKeyB

Note that sampleKeyB is scoped to the project ({file:/home/hp/checkout/hello/}hello) rather than the entire build ({file:/home/hp/checkout/hello/}).

As you’ve probably guessed, inspect sampleKeyD matches sampleKeyB: 

[info] Setting: java.lang.String = D: in build.sbt
[info] Provided by:
[info]  {file:/home/hp/checkout/hello/}hello/*:sampleKeyD

sbt appends the settings from .sbt files to the settings from Build.settings and Project.settings which means .sbt settings take precedence. Try changing Build.scala so it sets key sampleC or sampleD, which are also set in build.sbt. The setting in build.sbt should “win” over the one in Build.scala.

One other thing you may have noticed: sampleKeyC and sampleKeyD were available inside build.sbt. That’s because sbt imports the contents of your Build object into your .sbt files. In this case import HelloBuild._ was implicitly done for the build.sbt file.

In summary:

When to use .scala files 

In .scala files, you can write any Scala code, including top-level classes and objects. Also, there are no restrictions on blank lines, since they are standard .scala files.

The recommended approach is to define most configuration in .sbt files, using .scala files for task implementations or to share values, such as keys, across .sbt files.

The build definition project in interactive mode 

You can switch the sbt interactive prompt to have the build definition project in project/ as the current project. To do so, type reload plugins. 

> reload plugins
[info] Set current project to default-a0e8e4 (in build file:/home/hp/checkout/hello/project/)
> show sources
[info] ArrayBuffer(/home/hp/checkout/hello/project/Build.scala)
> reload return
[info] Loading project definition from /home/hp/checkout/hello/project
[info] Set current project to hello (in build file:/home/hp/checkout/hello/)
> show sources
[info] ArrayBuffer(/home/hp/checkout/hello/hw.scala)
>

As shown above, you use reload return to leave the build definition project and return to your regular project.

Reminder: it’s all immutable 

It would be wrong to think that the settings in build.sbt are added to the settings fields in Build and Project objects. Instead, the settings list from Build and Project, and the settings from build.sbt, are concatenated into another immutable list which is then used by sbt. The Build and Project objects are “immutable configuration” forming only part of the complete build definition.

In fact, there are other sources of settings as well. They are appended in this order:

Later settings override earlier ones. The entire list of settings forms the build definition.

Getting Started summary 

This page wraps up the Getting Started Guide.

To use sbt, there are a small number of concepts you must understand. These have some learning curve, but on the positive side, there isn’t much to sbt except these concepts. sbt uses a small core of powerful concepts to do everything it does.

If you’ve read the whole Getting Started series, now you know what you need to know.

sbt: The Core Concepts 

If any of this leaves you wondering rather than nodding, please ask for help, go back and re-read, or try some experiments in sbt’s interactive mode.

Good luck!

Advanced Notes 

Since sbt is open source, don’t forget you can check out the source code too!