Once upon a time, there was the Bourne shell. Since there was only "the" shell, there was no trouble deciding how to run a script: run it with the shell. It worked, and everyone was happy.
Along came progress, and wrote another shell. The people thought this was good, for now they could choose their own shell. So some chose the one, and some the other, and they wrote shell scripts and were happy. But one day someone who used the "other" shell ran a script by someone who used the "other other" shell, and alas! it bombed spectacularly. The people wailed and called upon their Guru for help.
"Well," said the Guru, "I see the problem. The one shell and the
other are not compatible. We need to make sure that the shells know
which other shell to use to run each script. And lo! the one shell
has a `comment' called
:, and the other a true comment called
#. I hereby decree that henceforth, the one shell will run
scripts that start with
:, and the other those that start with
#." And it was so, and the people were happy.
But progress was not finished. This time he noticed that only
shells ran scripts, and thought that if the kernel too could run
scripts, that this would be good, and the people would be happy.
So he wrote more code, and now the kernel could run scripts,
but only if they began with the magic incantation:
told the kernel which shell ran the script. And it was so, and
the people were confused.
#! looked like a "comment." Though the kernel could see
#! and run a shell, it would not do so unless certain magic
bits were set. And if the incantation were mispronounced, that
too could stop the kernel, which, after all, was not omniscient.
And so the people wailed, but alas!, the Guru did not respond.
And so it was, and still it is today.
Anyway, you will get best results from a 4BSD machine by using