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Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!seismo!mcnc!xanth!john
From: john@xanth.UUCP
Newsgroups: comp.unix.questions
Subject: Re: why does this script... ( : and # )
Message-ID: <1276@xanth.UUCP>
Date: Tue, 16-Jun-87 10:22:43 EDT
Article-I.D.: xanth.1276
Posted: Tue Jun 16 10:22:43 1987
Date-Received: Thu, 18-Jun-87 03:32:05 EDT
References: <913@rtech.UUCP>
Organization: Old Dominion University, Norfolk Va.
Lines: 47

In article <913@rtech.UUCP>, daveb@rtech.UUCP (Dave Brower) writes:
> It's clear you can't have a '#', because that implies csh on some kernels.
> You can't rely on '#!/bin/sh', because some kernels don't undertand it.
> (Yes, there are some that recoginize '#', but not '#!/...").

You say "kernels" above, but I hope you mean "distributions of UNIX
operating systems" or something like that.  The whole point of #! is
that the ->kernel<- understands it and runs the specified interpreter,
as opposed to just the # which the ->csh<- understands to mean "use
the csh", and the newer Bourne shells just consider a comment.

So yes, it's certainly true that there are plenty of versions of UNIX
out there where the kernel does not understand #! but the csh
understands # - all version of System V with the csh, for example.
(If there are System V kernels that do #!, please let me know!)

I think we should end this discussion with a quick summary of the
situation.  Here is a table describing which shell your script will be
run by, depending on what it begins with, and where you're running it
from.  The "sh" here is a Bourne shell that uses "#" as a comment; if
not, the situation's even worse.

script			r u n   f r o m
begins	  kernel knows #!		kernel does not know #!
with	  sh	      csh		sh		csh

#!	-- specified script --		sh		csh

#	  sh	      csh		sh              csh

other	  sh	      sh		sh		sh
(includ-
ing :)

So if you're writing a csh script, the best you can do is to start it
with #! /bin/csh and hope no one tries to run it from the Bourne shell
under a kernel that does not know #!.  If you're writing a Bourne
shell script, you should start it with something else, like a colon
alone on the line, as Dave suggests, and it will always be run by sh.
Only if you know the script will never go to a system that does not
know #! should you use #!/bin/sh .


-- 
John Owens		Old Dominion University - Norfolk, Virginia, USA
john@ODU.EDU		old arpa: john%odu.edu@RELAY.CS.NET
+1 804 440 4529		old uucp: {seismo,harvard,sun,hoptoad}!xanth!john