sbt components

sbt runner

An sbt build is executed using the sbt runner, also called "sbt-the-shell-script" to distinguish from other components. It's important to note is that sbt runner is designed to run any version of sbt.

Specifying sbt version with project/build.properties

The sbt runner executes a subcomponent called sbt launcher, which reads project/build.properties to determine the sbt version for the build, and downloads the artifacts if they haven't been cached:

sbt.version=2.0.0-alpha7

This means that:

Anyone who checkouts your build would get the same sbt version, regardless of sbt runner they may have installed on their machines.
The change of sbt version can be tracked in a version control system, like git.

sbtn (`sbt --client`)

sbtn (native thin client) is a subcomponent of the sbt runner, called when you pass --client flag to the sbt runner, and is used to send commands to the sbt server. It is called sbtn because it is compiled to native code using GraalVM native-image. The protocol between sbtn and sbt server is stable enough that it should work between most recent versions of sbt.

sbt server

The sbt server is the actual build tool whose version is specified using project/build.properties. The sbt server acts as a cashier to take commands from sbtn and editors.

Coursier

The sbt server runs Couriser as a subcomponent to resolve Scala library, Scala compiler, and any other library dependencies your build needs.

Zinc

Zinc is the incremental compiler for Scala, developed and maintained by sbt project. An often overlooked aspect of Zinc is that Zinc provides a stable API to invoke any modern versions of Scala compiler. Combined with the fact that Coursier can resolve any Scala version, with sbt we can invoke any modern versions of Scala just by writing a single line build.sbt:

scalaVersion := "3.3.1"

sbt --client

This should display something like the following:

$ sbt --client
[info] server was not detected. starting an instance
[info] welcome to sbt 2.0.0-alpha7 (Azul Systems, Inc. Java 1.8.0_352)
[info] loading project definition from /private/tmp/bar/project
[info] loading settings for project bar from build.sbt ...
[info] set current project to bar (in build file:/private/tmp/bar/)
[info] sbt server started at local:///Users/eed3si9n/.sbt/2.0.0-alpha7/server/d0ac1409c0117a949d47/sock
[info] started sbt server
[info] terminate the server with `shutdown`
[info] disconnect from the server with `exit`
sbt:bar>

Running sbt with no command line arguments starts sbt shell. sbt shell has a command prompt (with tab completion and history!).

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

sbt:bar> compile

To compile again, press up arrow and then enter.

To leave sbt shell, type exit or use Ctrl-D (Unix) or Ctrl-Z (Windows).

Batch mode using sbtn

You can also run sbt in batch mode:

sbt --client compile
sbt --client testOnly TestA

$ sbt --client compile
> compile

Shutting down sbt server

Run the following to shutdown all sbt servers on your machine:

sbt shutdownall

Or the following to shutdown just the current one:

sbt --client shutdown

The Book of sbt