I want to have a shell script pop up menus and yes/no dialog boxes if the user 
is running X.

Several tools in the R3 contrib/ area were developed to satisfy these needs:
yorn pops up a yes/no box, xmessage displays a string, etc. There are several
versions of these tools; few, if any, have made it to the R4 contrib/ area,
though they may still be available on various archive sites.

XScript, a collection of X shell scripts, is on csc.canberra.edu.au under
/pub/motif/xscript and also on ftp.x.org; it includes several stand-alone X
applications which can be embedded in shell scripts. XScript requires
tclMotif 1.0 or later.

In addition, Richard Hesketh (rlh2@ukc.ac.uk) has posted the xmenu package to
comp.sources.x ("v08i008: xmenu") for 1-of-n choices.  [7/90]

Two versions of XPrompt have been posted to comp.sources.x, the latter being
an unauthorized rewrite. [R. Forsman (thoth@reef.cis.ufl.edu), 1/91]

There is a version of XMenu available from comp.sources.x; it is being worked
on and will likely be re-released.

xp-1.1.tar.Z, xpick-1.1.tar.Z and xzap-1.1.tar.Z on ftp.x.org's R5contrib/
are tools by Gerry.Tomlinson@newcastle.ac.UK which act as X versions of the
simple display and choice-making tools in K&P. [4/92]