ksh script sqlplus < file.sql problem

hi

i am trying to do the following line in ksh script:
sqlplus <user>/<pass>@<instance> < file.sql

file .sql = UPDATE <table_name> SET <param_name> = $environment_parameter

the problem is that the sql file does not recognize the env_param (only ksh do)

how do i introduce between the sql file to the env_param
or redirect the input to the sqlplus in another way (pipe ,xargs etc)

p.s if the sql file do not contain env_param (only hardcoded parameters ),the line in the script runs well

please help
thanks

10x