19c以后TWO_TASK和LOCAL环境变量又变得有用了~~

  sqlplus user/password@pdb1

  sqlplus user/password

If you are in a Unix environment, you can normally set 2 environment variables:
ORACLE_HOME
ORACLE_SID


that will connect you to a local database identified by that home and sid. If you wanted to be able to connect to a remote database or a database on that machine via SQLNet, you can also set:

TWO_TASK = <tnsconnect string>

The setting of TWO_TASK overrides the ORACLE_SID when set. You will not connecting to a local database with two_task but rather using sqlnet to connect to a remote database.

So, instead of:

$ sqlplus scott/tiger@some_db
I can:

$ setenv TWO_TASK some_db
$ sqlplus scott/tiger


and the @some_db is implied.

TWO TASK
TWO_TASK (or LOCAL on Windows) is an environment variable used to specifies the default remote Oracle service. Connections will be made to a remote database (via SQL*Net) without specifying a service name (tnsnames.ora entry). It will appear to programs that they connect to a local database, while in fact they are routed to a remote database.

This environment variable is equivalent to the LOCAL registry entry on the Windows platforms. If both ORACLE_SID and TWO_TASK or LOCAL are defined, TWO_TASK or LOCAL takes precedence.

Usage
Windows:

set LOCAL=oraservice1
Unix/ Linux:

$ export TWO_TASK=oraservice1
or

$ setenv TWO_TASK oraservice1
Where 'oraservice1' is defined in the local TNSNAMES.ORA file or in an Names/LDAP server.

Example
One would normally connect to a remote database by specifying a service name (connect string). for example:

$ sqlplus scott/tiger@connect_str
By setting TWO_TASK you can omit the connect string (it is implied):


$ TWO_TASK=connect_str; export TWO_TASK
$ sqlplus scott/tiger