sql commands

Find version of Oracle database instance 

SELECT * FROM V$VERSION

or

SELECT version FROM V$INSTANCE

Find the current ORACLE_SID you are logged in 

sqlplus sys as sysdba;

select name from v$database;
select instance from v$thread;
select ora_database_name from dual;

Find the current schema name you are logged in 

select sys_context('userenv','instance_name') from dual; 
select user from dual; 
select * from global_name; 

Find location of alert logs, alert log file name will be alert_ORACLE_SID.log

show parameter background;  

To find out the location of diagnostic info like trace , logs etc. 

select * from v$diag_info;