The sqlplus executable will be installed in the bin/ subdirectory under the Oracle Home directory for the database you're working with. Setting up the Environment Variables on the database server is usually done using the oraenv shell script.
The trick, of course, is that this, too, is installed in the bin/ subdirectory of an Oracle Home directory!
Your .bashrc or .bash_profile should, at the very least, set the ORACLE_SID and PATH Environment Variables and, if you want, make a call to oraenv to set up the rest.
# Use db1
See Database Administrator's Reference