Is there a list of all the graphical applications' names so that I can start them from the command line in a terminal window?
No, there is no comprehensive list. You can find desktop applications mixed in with the programs in /usr/bin, /usr/local/bin, and /usr/X11R6/bin.
When I use the xmessage command, I get an error message or an unexpected result. Why?
Remember to use the -buttons option and to enclose the comma-separated list of quoted button labels and the quoted message text as arguments.
The sample Nautilus script won't work with files that have spaces in the filename. How can I fix this problem?
In the interest of keeping the script simple for illustrative purposes, we did not use techniques for accommodating filenames with spaces or other special characters. The website http://g-scripts.sourceforge.net/ provides freely copyable script code that enables Nautilus scripts to work with filenames containing spaces and other special characters.