Show All online SID Database Oracle 11 in Linux SuSe and Find Database file I'm new with Oracle DBA in my new office, and they gave me a development server that already have a lot SID database online in SuSe (I checked with lsnrctl status)

My questions are :

How to check what SID instance that really live/online, because turns out when I checked again with ps ax|grep ora, only 3 of databases live/online?



If only 3 SID databases that really online like in 1st question, how do I know where all the 3 SID the database file position that already live/online? what do I have to type to know it?

I try to login sqlplus sys as sysdba when I checked with select name from v$database; I was logging in 1 SID database. Continuing from 2nd question, How do I logout from current SID database and then switch  to other 2 SID database?
(example in MongoDB, I type use tutorial to enter tutorial database and type use dbdevelopment to switch to database dbdevelopment)
cat /etc/oratab

As a DBA, you need to learn the Oracle Data Dictionary.


&quot;One of the most important parts of an Oracle database is its data dictionary, which is a read-only set of tables that provides information about the database&quot;
SQL&gt;select name from v$datafile;




# set environment
. oraenv &lt;&lt;&lt; SID1; 
# login as sys user (IPC)
sqlplus / as sysdba
# list datafiles
SQL&gt;select name from v$datafile;
SQL&gt;exit
# repeat for SID2 and SID3

Side note.
One Oracle Database System (SID) can serve multiple databases (schemas). It is expensive, in terms of resources, to have multiple systems running.
# create database 1
SQL&gt;create user DB_OWNER_1 identified by 'pass';
SQL&gt;create table db_owner_1.t (x char(1));
# create database 2
SQL&gt;create user DB_OWNER_2 identified by 'pass';
SQL&gt;create table db_owner_2.t (x char(1));


# login as 'app user A' who has privileges to manipulate tables (DML) in database 1.
sqlplus appuserA/pass@TNS_ALIAS_1 
SQL&gt;alter session set current_schema = &quot;DB_OWNER_1&quot;
SQL&gt;insert into t values ('x');

# login as 'app user B' who has privileges to manipulate tables (DML) in database 2.
sqlplus appuserB/pass@TNS_ALIAS_2 
SQL&gt;alter session set current_schema = &quot;DB_OWNER_2&quot;
SQL&gt;insert into t values ('x');

You now have two applications operating on two different databases served by the one and same Oracle Database System.
Best of luck!