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set trimspool [SQL*Plus]

set trimspool on
set trimspool off
If trimspool is set to on, it will remove trailing blanks in spooled files.
See also trimout which does the same thing to the output to the console (terminal).

Storing settings across sessions

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