> What should I do to make ruby interpreter know where to find the file
all
> the time.

> ENV["RUBYLIB"] is valid only for the session, right?

ENV is how you refer to environment variables within Ruby. You can set
this variable permanently on your system for all future runs of the
interpreter. 

How you do this depends on your system. On Windows it's usually
right-click on my computer -> advanced -> environment variables. On
linux it depends on your shell. It's usually something like 
(Bash)
 set RUBYLIB=...
 export RUBYLIB
or on CSH:
 setenv RUBYLIB ...