Bourne Shell Comments Problem

[Chris Torek]

In article <1133 at usfvax2.EDU> ssi at usfvax2.EDU (Ssi) writes:
>The problem is just that. Placing a '#' there means you want to
>run the script in the csh shell.

Indeed.  As you mortals say :-), what goes around, comes around.

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Path: maryland!umcp-cs!chris
From: chris at umcp-cs.UUCP (Chris Torek)
Newsgroups: net.unix-wizards
Subject: Re: # sign in shell/cshell scripts...
Message-ID: <995 at umcp-cs.UUCP>
Date: 17 Apr 86 20:30:30 GMT
Date-Received: 17 Apr 86 20:30:30 GMT
References: <237 at Shasta.ARPA>
Reply-To: chris at maryland.UUCP (Chris Torek)
Organization: University of Maryland, Dept. of Computer Sci.
Lines: 57

In article <237 at Shasta.ARPA> ALEX at SU-SIERRA.ARPA writes:
>	I'm encountering weird problems with sh (and sometimes csh)
>dying on some shell scripts depending on whether or not my first line in
>the script is a comment like:
># This script does this (but not that)

Once upon a time, there was the 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 shells.  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 use 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 must ensure 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: `#!', and
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.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1415)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at mimsy.umd.edu

And a recent addendum: System III and System V, and systems derived
from them, usually do *not* support `#!'.  Alas, if you use it, you
may get the wrong shell.  Of course, System III and System V do not
come with the C shell either.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris