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From: brian@ncrcan.UUCP
Newsgroups: comp.unix.questions
Subject: Re: Trick question
Message-ID: <334@ncrcan.UUCP>
Date: Mon, 19-Oct-87 18:27:48 EDT
Article-I.D.: ncrcan.334
Posted: Mon Oct 19 18:27:48 1987
Date-Received: Tue, 20-Oct-87 05:46:15 EDT
References: <666@portofix.liu.se>
Reply-To: brian@ncrcan.UUCP (Brian Onn)
Organization: NCR Canada Ltd., Mississauga, Ontario
Lines: 50

In article <666@portofix.liu.se> kjepo@portofix.liu.se (Kjell Post) writes:
>A little trick question for you all: what's happening here?
>
>	% cat > \*
>        date
>        ^D
>        % chmod +x \*
>	% \*
>        Tue Oct 13 03:13:08 MET 1987
>        %
>	% cat > +
>        date
>        ^D
>        % chmod +x +
>        % +
>        $                         -- entered Bourne-shell
>

This one's easy.  When the cshell goes to exec a program, it checks to see if
it has a valid magic number and uses fork/exec if so.  Otherwise it checks to
see if the first character is '#' and if so assumes that the file is a cshell
script and executes it directly.  If not, the cshell assumes that the 
file is a Bourne shell script and passes the file off to the Bourne shell to
be executed.  The file '+' looks like a Bourne shell script to the cshell, and
so the chell says:

		/bin/sh +

the '+' being the filename the cshell wants to execute.  Now in the Bourne
shell, one can turn on or off certain options using - or + in front of the
option letters.  Your example above also produces the same results if you
use '-' instead of '+'.  'sh -' is documented, 'sh +' is not.

So the Bourne shell thinks you want to turn off some option, but you
just didn't supply the option, or possibly this is the explicit opposite to
'sh -' (I haven't looked up the source, and especially don't want to dive into
the horrendouse Bourne Shell code).  If 'sh -' means execute the .profile,
maybe 'sh +' explicitly means not to?

Also, you can use any number of '+'s and '-'s, and you will end up in the
Bourne shell.  This may add more weight to the possibility of an opposite to
'sh -', as it seems to indicate how the command line in 'sh' in parsed for
arguments.  Anyone else care to comment more on this.

-- 

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