sbt ~rtu
...
[success] Total time: 3 s, completed Dec 7, 2016 12:55:04 PM
4. Waiting for source changes... (press enter to interrupt)
I edit build.sbt to bump the scalatest version, and sbt reacts to the edit.
...
[info] downloading https://repo1.maven.org/maven2/org/scalatest/scalatest_2.11/3.0.1/scalatest_2.11-3.0.1.jar ...
[info] [SUCCESSFUL ] org.scalatest#scalatest_2.11;3.0.1!scalatest_2.11.jar(bundle) (10136ms)
[info] downloading https://repo1.maven.org/maven2/org/scalactic/scalactic_2.11/3.0.1/scalactic_2.11-3.0.1.jar ...
[info] [SUCCESSFUL ] org.scalactic#scalactic_2.11;3.0.1!scalactic_2.11.jar(bundle) (1000ms)
[info] Done updating.
...
[success] Total time: 14 s, completed Dec 7, 2016 12:55:30 PM
5. Waiting for source changes... (press enter to interrupt)
Yey!