Ways for making a Scala script standalone in Linux?

Question

Programming Scala says:

Invoking scripts with scala is tedious when you use these scripts frequently. On Windows and Unix-like systems, you can create standalone Scala scripts that don’t require you to use the scala script-file-name invocation. For Unix-like systems, the following example demonstrates how to make an executable script. Remember that you have to make the permissions executable, e.g., chmod +x secho:

#!/bin/sh
# src/main/scala/progscala2/toolslibs/secho
exec scala "$0" "$@"
!#
print("You entered: ")
args.toList foreach { s => printf("%s ", s) }
println

Here is how you might use it:

$ secho Hello World
You entered: Hello World

I was wondering what it means by

# src/main/scala/progscala2/toolslibs/secho
exec scala "$0" "$@"
!#

in sh?

Can I make a Scala script standalone as

#!/path/to/scala
print("You entered: ")
args.toList foreach { s => printf("%s ", s) }
println

on Linux?

Thanks!

Answer 1

In sh,

# src/main/scala/progscala2/toolslibs/secho

is a comment, ignored;

exec scala "$0" "$@"

causes the current process to replace itself with scala, and all the shell parameters as arguments;

!#

has no effect because the shell has stopped reading the script at this point.

However Scala handles script files by ignoring a “header” at the start of the file; that header starts at a line beginning with #! (or ::#! on Windows) and ends with a line matching either !# (::!#) or /env. The header can be a single line, so both

#!/path/to/scala
!#
print("You entered: ")
args.toList foreach { s => printf("%s ", s) }
println

and

#!/usr/bin/env scala
print("You entered: ")
args.toList foreach { s => printf("%s ", s) }
println

work (the latter requires scala to be on the PATH). The latter corresponds to the documentation.

(!# is the symmetric of #!, and is used in Scala to close the “header” opened by #!, in a fashion similar to /* and */ for comments in C.)