Hi,
you can create a script file that will deine a UNIX env parameter and call for it in the sql block :

---------------- CUT HERE ----------------------------
#!/bin/csh -fb
set param ="3"

sqlplus system/manager@orcl << EOF | tee /tmp/$$TMP_FILE
select some_value from some_table where some_param = $param
EOF
---------------- CUT HERE ----------------------------

** If you are selecting from v$ views you should use v\$:
select * from v\$session;
because the OS will think it is a UNIX variable.