Passing parameters into an in-line psql invocation

Hi:

I'd like to pass a parameter into an inline psql call that itself calls an sql script, something like...

psql mydb -c "\i thesqlscript foo"

Where"foo" is the value I want to pass in.

Just as good would be the ability to sniff out an environment variable from within the sql script ("thesqlscript" in the example above).  In perl, I would use $ENV{VARNAME}.  Is there something like that in Postgres SQL?

V8.3.4 on Linux (upgrading to v9 very soon).

Thanks for any ideas !