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set serveroutput [SQL*pluS]

set serveroutput on
set serveroutput off

set serveroutput on size n
set serveroutput on format word_wrapped 
set serveroutput on format truncated 

set serveroutput on size unlimited
By default, SQL*PLUS doesn't read what a PL/SQL programm has written with dbms_output. With set serveroutput on, this behaviour is changed.
The size unlimited clause comes with Oracle 10g Release 2.

format wrapped

Another quirk is: lines printed with dbms_output that start with blanks are displayed wrong by SQL*Plus: the blanks are cut. To avoid this, use the set serveroutput on format wrapped option.

Storing settings across sessions

Settings (such as serveroutput) can be stored accross sessions with the glogin.sql and/or login.sql file.
See also store.

Misc

SQL*Plus calls dbms_output.enable when someone sets serveroutput on.
After enabling dbms_output, SQL*Plus calls dbms_output.get_line after each insert, update, delete, select statement or anonymous block.