No. You will need to be logged in as a user that has a shell and has admin privileges.

If you do not have NetInfo Manager, and your terminal is set to /dev/null your user account is dead in the water with regard to launching a terminal session.

You could try adding an admin user account to your machine, logging in as that new user and changing the settings for your normal account. If a default of /dev/null is set for new accounts, then that won't help.

You could try booting into single user mode (start up with the command and "s" keys held down, and try changing it from there.

Otherwise, you will probably need to acquire a copy of OS X, then reinstall the OS.

Good luck!