I'm trying to run a cron job which calls a shell script that runs several queries against an Oracle database. I'm getting emails telling me that there are environment variables which are not set.

I would like to run the shell script in an environment which is as close as possible to how cron runs the script (except that I would actually like to see stdout and stderr, rather than having them echo to an email message).

I'm guessing that something like

should pretty well do the trick, but I may be missing something.

Also, is there a way to lauch a crontab entry manually, as if cron had just fired off? (yes, I know that I can set cron to fire off every minute, but that destroys my work flow).

This page explains how to set env vars in your cron job, and which ones are (usually) set by default.
http://linux.die.net/man/5/crontab
The easiest soln is not to set them in the crontab, because you may have several different programs in there.
For a given job, set them at the top of the script.
Always use complete absolute paths to any cmds/files referenced, as the default cron env is minimal (see manpages).