Begin Backup

NON-CDB
note: if you work with CDB have to check pdb's datafiles

col name format a25
SELECT b.file#, t.name, b.status 
FROM v$backup b, v$tablespace t
where b.file#= t.TS#
order by 3;

to begin backup
alter database begin backup;
alter tablespace users begin backup;
or
select 'alter tablespace '||tablespace_name||' begin backup;' from dba_tablespaces;
or

select 'alter tablespace '||tablespace_name||' begin backup;' from cdb_tablespaces;