how to test condition: variable set but not empty

Ok, I went into the archives and found this post:

<http://www.rosat.mpe-garching.mpg.de/cgi-bin/w3glimpse2html/procmail/2002-01/msg00164.html?77#mfs>

but I think it confused my little brain more than it helped.

Here's my situation:

At the top of my RC file, these variables are set:

SUBJECTTAG=""
FOOTER=""
DESTINATION=""
LISTREPLYTO=""

At the end of the RC file, I want to take 4 actions if, and only if, the
corresponding variable has been set to something other than ""

For example, assume for a minute that the procmail list had a dumb subject
tag like [procmail-users]

I would set the variable like this

        SUBJECTTAG="[procmail-users]"

and then I want to set the recipe (here's where I need help)

(It's the second line of the recipe, I think, where I might be going
wrong.  I don't really understand what it means, I just yanked it out of
the page above, probably incorrectly)

:0
* $ ! ${SUBJECTTAG+!}
{
        NEWSUB=`echo $SUBJECT | sed "s/$SUBJECTTAG//"`

        :0fhw
        |formail -i"Subject: $NEWSUB"
}


Thanks for any help

TjL



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