The answer to this relies somewhat on the platform that you are using.

Unix

ASE Versions Before 12.0

This applies to Unix and variants, Linux included. Install the various releases of software into logical places within your filesystem. I like to store all application software below a single directory for ease of maintenance, choose something like /sw. I know that some are keen on /opt and others /usr/local. It is all down to preference and server usage. If you have both Oracle and Sybase on the same server you might want /sw/sybase or /opt/sybase. Be a little careful here if your platform is Linux or FreeBSD. The standard installation directories for Sybase on those platforms is /opt/sybase. Finally, have a directory for the release, say ASE11_9_2 or simply 11.9.2 if you only ever have Sybase ASE running on this server. A little imagination is called for!

So, now you have a directory such as /sw/sybase/ASE/11.9.2 (my preferred choice :-), and some software installed under the directories, what now? In the most minimal form, that is all you need. Non of the environment variables are essential. You could quite successfully run