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I have found the following kind of shebang in the RosettaCode page:

--() { :; }; exec db2 -txf "$0"

It works for Db2, and a similar thing for Postgres. However, I do not understand the whole line.

I know the double dash is a comment in SQL, and after that it calls the Db2 executable with some parameters passing the file itself as file. But what about the parenthesis, the curly brakets, the colon and semi-colon, and how can replace a real shebang #! ?

https://rosettacode.org/wiki/Multiline_shebang#PostgreSQL

Related: Which shell interpreter runs a script with no shebang?

The script does not have a shebang/hashbang/#! line, simply because a double dash is not #!.

However, the script will be executed by a shell (see above linked question and answers), and in that shell, if - is a valid character in a function name, the line declares a shell function called -- that does nothing (well, it runs :, which does nothing) and which is never called.

The function, in the more common multi-line notation (just to make it more obvious what it looks like, as its odd name kinda obscures the fact that it's in fact a function):

-- () {

  :

}

The sole purpose of the function definition is to have a line that is valid in a shell script and at the same time a valid SQL command (a comment). This sort of code is called a polyglot.

After declaring the bogus shell function, the script, when executed by a shell script interpreter, uses exec to replace the current shell with the process resulting from running db2 -txf "$0", which would be the same as using db2 -txf on the pathname of the script from the command line.

This trick would probably not work reliably on systems where dash or other ash-based shells, yash, the Bourne shell, ksh88 or ksh93 is used as /bin/sh, as these shell do not accept functions whose name contains dashes.

Also related:

• Shell valid function name characters


• Will it be bad that a function or script name contains dash `-` instead of underline `_`?

I suppose the following would also work (not really tested):

--() { exec db2 -txf "$0"; }; --

As @Kusalananda has already said, that trick is broken and it won't work in all shells.

Here is my take at doing it portably:

--/.. 2>/dev/null; exec db2 -txf "$0"

The first command should fail even if a file/directory named -- exists in the current directory and any errors will be shut up by the 2>/dev/null; the shell will then proceed with the second command, the exec.