Shebang "#!" starts and "!#" ends?

Question

I have been using shebang #! for a while and today came across a use case that confuses me.

There are several ways to run scala script like

#!/usr/bin/env scala 
println("hello world")

However, I came across this version of shebang

#!/bin/sh
exec scala "$0" "$@"
!#
println("hello world")

Looks like this solution basically invokes the bash first, the run exec scala "$0" "$@" where $0 denotes the current file name and $@ is input arguments in a positional array.

My question is does that mean everything between #! and !# could be executed in bash,

#!/bin/sh
exec scala "$0" "$@"
echo "oh Yeah"
!#
println("hello world")

It did not error out but neither gave me the "oh Yeah" in the stdout, can anyone explain to me what is going on here?

---

update: after realizing !# is a scala thing, I downloaded the source code of scala and realized it only appears in the comment of ScriptRunner.scala part written by Lex Spoon.

Answer 1

The !# line has no meaning to the shell.

The #!/bin/sh line means that the script is executed by /bin/sh. The exec scala "$0" "$@" line invokes scala, passing the name of the script and its arguments to the scala command. Since exec doesn't return, the shell doesn't see the rest of the script.

I don't know Scala, but my educated guess is that the Scala interpreter itself treats everything from the #! line to the !# line as a comment. It then starts executing with the Scala statement println("hello world").

In short the !# is Scala syntax, not shell syntax (but the Scala syntax is designed to let it be used like this in a shell script).

In a quick look at the Scala Language Specification, I haven't found out how this is defined. It's mentioned, but not explained, in this question. It's likely, as chepner 3's comment suggests, that it's a hack in the Scala interpreter rather than part of the actual language syntax.

som-snytt found the code in the Scala interpreter that implements this here:

object ScriptSourceFile {
  /** Length of the script header from the given content, if there is one.
   *  The header begins with "#!" or "::#!" and ends with a line starting
   *  with "!#" or "::!#".
   */
...

But I wonder whether it's documented.