[NTLUG:Discuss] Shell script help

[. Daniel]

This should probably be real easy for someone with experience.  It'd be 
pretty easy to write in C, so I'm a little embarassed that I find this so 
challenging.  Here's the scenario:

GNOME's Nautilus has user-scripts.  This, I thought, would be a terrific 
vehicle for me to be able to create a "powertoy" (to borrow an expression 
from Microsoft) that I once used frequently when I used Windows.  In this 
case, the tool would let me right-click on a folder and "Launch Command 
Prompt Here" where it would open a command shell window with the current 
working directory was set to the location of the folder indicated.  So 
that's what I want to do here.

What I have to work with is the environment variable:
$NAUTILUS_SCRIPT_SELECTED_FILE_PATHS

My script, as it is, kinda works.  It consists of two lines:

#!/bin/sh
cd $NAUTILUS_SCRIPT_SELECTED_FILE_PATHS
gnome-terminal --geometry=132x25

Since Nautilus does not determine whether the file indicated is a file or a 
folder, the script is available always.  So I would like the behavior to 
be:

If it's a folder, change to that directory.  If it's a file, change to the 
directory that file is in.

In my mind, the logic would go something like:

if filename is a file then
  trim filename back to the last folder name
endif
chgdir filename

The trouble is that I can't figure out a way to trim the filename off the 
end of full path list.  

So my request to any and all shell script wizards is "how do I trim a 
filename off the end of a pathlist?"  e.g. make 
"/home/username/pictures/dsc00054.jpg" into "/home/username/pictures"