[Techtalk] Shell expansion and quote removal nightmare

[Brenda Bell]

Could someone either point me to a good discussion of what happens with 
quotes in shell scripts (#!/bin/sh) or tell me how I can solve the 
problem described below?  (The last time I wrote a shell script, the OS 
was called System V and the Unix Programmer's Manuals were published by 
AT&T -- yes, I'm older than God :(

I'm trying to write a script that collects a bunch of data and 
dynamically constructs the arguments for a tar command... like either of 
the following:

     tar --create --file test.tar list-of-files
     tar --create --after-date "valid-date-here" --file test.tar 
list-of-files

Note that the tar command may or may not use the --after-date option 
depending on input to the script.  Also note that the argument for 
--after-date comes from a file and is in the default format you get from 
date.

I incorrectly assumed I could construct a shell variable TARPARM1 whose 
value is either "" or "--after-date 'valid-date-here'" and execute tar 
${TARPARM1}.

I've tried various combinations of quotes and escapes and can't come up 
with code that satisfies both scenarios.  I have test code that echoes 
the command line and it look right.  When I replace the echo with tar, I 
either have quotes where I don't want them or missing quotes where 
they're required.

What the heck am I missing here?