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From: wcs@ho95e.ATT.COM (Bill.Stewart)
Newsgroups: comp.unix.questions
Subject: Re: why does this script... ( : and # )
Message-ID: <1500@ho95e.ATT.COM>
Date: Tue, 23-Jun-87 14:48:32 EDT
Article-I.D.: ho95e.1500
Posted: Tue Jun 23 14:48:32 1987
Date-Received: Thu, 25-Jun-87 03:54:13 EDT
References: <913@rtech.UUCP> <7062@mimsy.UUCP> <767@bsu-cs.UUCP>
Reply-To: wcs@ho95e.UUCP (46133-Bill.Stewart,2G218,x0705,)
Organization: AT&T Bell Labs 46133, Holmdel, NJ
Lines: 32
Keywords: #!, csh, ksh

In article <767@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
: In several articles strategies for ensuring execution by a specific shell
: are discussed.
: Larry Wall's terrific installation scripts for patch and rn use the
: equivalent of following line to force execution by sh:
:      export PATH || (echo "OOPS, trying again..."; sh $0; kill $$)
: There ought to be a way of doing the same thing for other shells.  The
: penalty is the extra fork (or is it TWO extra forks?).

I generally write all my scripts in Bourne Shell, because I know they'll work
(relatively) portably.  When it's important to use ksh, I use the following:
	if [ "$RANDOM" = "$RANDOM" ] 
	then ## recovery script to feed itself to ksh
		ksh -c "$0 $*"
		exit "$?"
	fi

The simpler approach of checking $SHELL doesn't work; SHELL is exported,
and some programs will drop into /bin/sh by default.  ksh updates $RANDOM each
time it uses the value, so "$RANDOM" = "$RANDOM" will never be true in ksh, and
will (presumably) always be true in other shells.

Writing a good recovery script is hard; if the arguments to the program contain
white space or metacharacters, the $* will trash them.  Sometimes it matters.

Is there a good equivalent of this technique for csh?  I suppose one could
separate the program in to a csh file and a Bourne-shell front-end,
	:
	exec csh -c "$0.csh $*"
but it would be much nicer to have something self-contained.
-- 
# Bill Stewart, AT&T Bell Labs 2G-202, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs