Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!oliveb!mipos3!omepd!merlyn
From: merlyn@iwarp.intel.com (Randal Schwartz)
Newsgroups: comp.unix.questions
Subject: Re: Forcing /bin/sh in a script under V/386 3.2 Korn shell
Message-ID: <4692@omepd.UUCP>
Date: 24 Jul 89 23:34:15 GMT
References: <2318@wyse.wyse.com> <799@jonlab.UUCP>
Sender: news@omepd.UUCP
Reply-To: merlyn@iwarp.intel.com (Randal Schwartz)
Organization: Stonehenge; netaccess via Intel, Hillsboro, Oregon, USA
Lines: 45
In-reply-to: jon@jonlab.UUCP (Jon H. LaBadie)

In article <799@jonlab.UUCP>, jon@jonlab (Jon H. LaBadie) writes:
[lotsa stuff deleted]
| Combining these comments and using the conditional operator rather than
| an if statement; plus quoting $0 on general principles, we have:
| 
|   [ "${RANDOM}" = "${RANDOM}" ] || exec /bin/sh -c "${0}" ${@:+"${@}"}
| 
|   # what you want sh to do
| 
| Now a question for Bob.  Can you let me know the need for the
| 
| 	`whence $0`
| 
| construction in your solution.  On inspection, it seems that $0 would
| be sufficient as the script was invoked that way to begin with.  But
| I am certain I am overlooking some subtle situation in which that
| is not the case.

I can think of a subtle situation: the $ENV file of ksh sets a
different searchpath, or maybe even an alias, that would not be picked
up by the corresponding /bin/sh started later.

But also, you DON'T want the -c on there.  The command line would then
consist of just the command name, not the arguments.  (The shell
tosses arguments on a -c command line...)

Thus, a more secure initial line would be something like:

[ "X$RANDOM" = "X$RANDOM" ] || exec /bin/sh "`whence \"$0\"`" ${@+"$@"}

Note the "X"'s on the test... if $RANDOM is not set, the previous
example could raise havoc in the test.  I also don't believe the ":"
is required in the ${@..} construct, since the colon merely selects
between testing for $@ set, and $@ "set and non null", and the bug
comes about when $@ is *not* set (such as when there are no
arguments).

My solution also allows for a command name with whitespace in it :-)

Just another shell hacker,
-- 
/== Randal L. Schwartz, Stonehenge Consulting Services (503)777-0095 ====\
| on contract to Intel, Hillsboro, Oregon, USA                           |
| merlyn@iwarp.intel.com ...!uunet!iwarp.intel.com!merlyn	         |
\== Cute Quote: "Welcome to Oregon... Home of the California Raisins!" ==/