On Wed, Mar 01, 2006 at 12:15:11PM +0100, Schwerzmann, Stephan wrote:
> 
>   troubles start with specific option  --optP  aimed at 'neutralizing' 
> an environment variable; that is make tool work as if the env.var would
> not be set but without messing with the env.var itself.
> 
>   reading from the doco, it tells to use  --optP ""  (2x doublequotes)
> 
>   interactive invocations of   tool --optP ""   are successful
> 
>   to bring doublequotes into the factored out string, I change the
> assignment to use single quotes (no evaluations needed in there)

Ah, the real problem you have is that you don't want quotes, what you
are trying to generate is a zero length argument string.

> TOOL='tool --optA --optB --optP "" '
> ${TOOL} --optX | ...

So the above generates an argument that is "" (not what you want).

When ${TOOL} is split into tokens (using the separators in $IFS) adjacent
IFS whitespace is treated as a single separator, so cannot be used to
generate an empty argument.  If you add a non-whitespace character to IFS
you can generate empty arguments, however you are likely to find that the
shells are buggy in this area. However, as an example:

$ IFS=' :'
$ x='a b:c::d'
$ set -- $x
$ echo $#
5
$ echo x${3}y
xcy
$ echo x${4}y
xy
$ echo x${5}y
xdy

However you are probably better of escaping all the other $, " and ' in the
line and using:
$ eval ${TOOL} ...


	David

-- 
David Laight: david@l8s.co.uk