I assume you execute this in a SQL*Plus session, in this case you can do it in the following way (ignore the "def 2=", it is there just to show the result):
SQL> def 2="create_abc_controlfile"
SQL> col dbid new_value dbid
SQL> select trim(dbid) dbid from v$database;
DBID
----------------------------------------
288523517
1 row selected.
SQL> prompt backing up the control file into file '&2._&dbid'
backing up the control file into file 'create_abc_controlfile_288523517'
If you don't want to see the result of the query in your spool file (if any) then just put it between "set termout off"/"set termout on".