There is a simpler way - use (any) shell word splitting.
Let's create a script like that:
#!/bin/bash
readonly CONN="${RDS_ADMIN}@//${RDS_HOST}/${RDS_DBNAME}"
SCN=$(sqlplus -S $CONN << EOF
SET HEADING OFF
SELECT CURRENT_SCN FROM V\$DATABASE;
EOF
)
echo ">>>$SCN<<<"
SCN=$(echo $SCN)
echo ">>>$SCN<<<"
Here is the output of the script:
oracle@ctrl-dev|265$ ./t.sh
>>>
   87401840<<<
>>>87401840<<<
oracle@ctrl-dev|266$ 
 There is no external command required, the downside
is VAR=$(echo $VAR) construction which does not look nice.
 
                