1. If you are creating a database manually you have to put the entry in ORATAB file.
If you are creating database using DBCA then the database entry will automatically
filed in oratab file.
2- Location of oratab file is /etc/oratab
3. Then edit the oratab file. It will look like below:
# A colon, ':', is used as the field terminator. A new line terminates
# the entry. Lines beginning with a pound sign, '#', are comments.
# Entries are of the form:
# The first and second fields are the system identifier and home
# directory of the database respectively. The third filed indicates
# to the dbstart utility that the database should , "Y", or should not,
# "N", be brought up at system boot time.
# Multiple entries with the same $ORACLE_SID are not allowed.
4. Then run the file: . .bash_profile or .profile
5. Then run . oraenv
6. Then enter the name of the database(instance name) you wanna open
7. This is all about running a oraenv file