RUBYOPT
Additional command-line options to Ruby; examined after real command-line options are parsed ($SAFE must be 0).
RUBYLIB
Additional search path for Ruby programs ($SAFE must be 0).
RUBYPATH
With -S option, search path for Ruby programs (defaults to PATH).
RUBYSHELL
Shell to use when spawning a process; if not set, will also check SHELL or COMSPEC. DLN_LIBRARY_PATH Search path for dynamically loaded modules.
RUBYLIB_PREFIX
(Windows only) Mangle the RUBYLIB search path by adding this prefix to each component.