Forcing /bin/sh in a script under V/386 3.2 Korn shell

[bob at wyse.wyse.com]

In Article 14223 of comp.unix.questions ned at mcc.com (Ned Nowotny)
writes:
<  In article <14463 at bfmny0.UUCP> tneff at bfmny0.UUCP (Tom Neff) writes:
<  >My thanks to all who have responded via mail or news posting to my
<  >question about forcing the Korn shell (KSH) to run a script using the
<  >Bourne shell (SH) instead of itself.  There have been no completely
<  >satisfactory answers, but I will summarize the State Of What Is Known.
<  >
<  ... Lines Deleted ...
<  There is some missing history here.
<  
<  In the old days (around Version 6 or 7 and before), the Bourne shell
<  did not have a comment character, but it did have the null command, ":".
<  Now, the null command was useful in several ways ...
 ----lines also deleted----

My apologies as I did not see the original summary that Tom Neff put
out, which may have already included the solution I am sending for
getting ksh to exec sh.  Also, this is my first posting, so if I
have neglected some portion of netiquette, please let me know
via e-mail  :-).  I have also supplied some additional stuff
about uses of the colon command.

As noted, using the colon command for comments is a problem because
of the possible side effects.  I came across the recommendation, in
_Introducing the UNIX System_ by McGilton and Morgan, that it
is best to place single quotes around the arguments to prevent
these side effects.

The colon command is also useful for evaluation of variables, as in
the following:

	: ${CWDIR=`pwd`}

which will set CWDIR to the current directory if CWDIR is not already
set.  If the colon command were not used the shell would then
substitute this name and attempt to execute it as a commend.

With respect to the original question of Tom Neff's, perhaps the
following would help.  It depends on the fact that both _sh_ and
_ksh_ will execute the similar code, while _ksh_ adds some "extras", such
as RANDOM, which _sh_ will treat differently.  In _ksh_ each reference
to RANDOM will cause the value of RANDOM to change.

	if [ "$RANDOM" -eq "$RANDOM" ]
	then
	   # what you want sh to do
	else
	   # exec sh on this script
	   exec /bin/sh `whence $0` "$@"
	fi

The _whence_ builtin generates the full pathname of the current file
(assuming it is in the path) and _sh_ is used to run it.  The original
arguments must also be supplied to this new invocation.  Using the quoted
$@ above (if I understand it correctly), will guarantee that the
form of the arguments on the original command line will be passed
to the new script invocation in exactly the same form.

I picked up the general idea from some _ksh_ examples I saw once,
in _The Wizards Grabbag_ in UNIXWorld, but I am afraid I do not
remember the author, nor do I have the original at hand.  I have
used the method and found that it works as advertised.  (The
original was used to guarantee that _ksh_ was run on the script,
which contained _ksh_ specific code.)

Hope this proves useful :-)
Bob McGowan
Wyse Technology, San Jose, CA
..!uunet!wyse!bob
bob at wyse.com