Can any one help me how to pass the value of result of one query to another query.

I to pass the value of result of 'select max(id) from a' into another query
like
update table
set col =<value from last query>
where ....;

updatestaging()
{
xx=`$ORACLE_HOME/bin/sqlplus -s $ORA_USER_PASS 2>/dev/null << EOF
     WHENEVER SQLERROR EXIT 1
     SET HEADING OFF SET FEEDBACK OFF SET VERIFY OFF SET PAGESIZE 0
       
     WHENEVER SQLERROR EXIT SQL.SQLCODE;
           
      update XX_IMPORT_DATA
      set file_id=< value from query)
      WHERE FILE_ID IS NULL;
      commit;
      exit
EOF`
}

just replace < value from query) with the query itself..

update XX_IMPORT_DATA
      set file_id=( select max(id) from a )
      WHERE FILE_ID IS NULL;