If you run into a problem where a Scala shell script won’t run on MacOS — it hangs indefinitely without doing anything — hopefully this bug report will help. The solution is to change this line at the beginning of the Scala shell script:
exec scala -savecompiled "$0" "$@"
exec scala -nocompdaemon -savecompiled "$0" "$@"
I just had this problem with Scala 2.12.x and Java 8 running on MacOS 10.14.4, and I can confirm that adding
-nocompdaemon solved the problem for me.