[GLLUG] Appending text via shell script

[Mike Szumlinski]

I'm starting to make packages for installs here at work and from time  
to time I need to append text to a preference file rather than write  
over it.  It does have to be called through a shell script (part of  
the requirements of an OS X .pkg file).  I'm new to command line text  
manipulation and I'm not quite sure what tool/tools are going to work  
best for me.  Basically I'll want to search various plist files (XML)  
and append lines/delete lines from them from a shell script.  Anyone  
have a good idea what direction to point me in?

-Mike