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, 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 #!
, and if
they told the kernel which shell ran the script. And it was so, and the people
were confused.
For the #! looked like a "comment." Though
the kernel could see the #! 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
#! /bin/sh
or:
#! /bin/csh
as the first
line of your script. #!
/bin/csh -f is also helpful on occasion, and
it's usually faster because csh won't read
your .cshrc file (Section 3.3).
— CT