SQL & PL/SQL :: Passing Bind Char Correctly

I tried google and most of bind example using number instead of character, ?

I have the code as below:

variable oldId varchar2(5)
exec :oldId :='&1'
variable newId varchar2(5)
exec :newId :='&2'

[Code]...

Since in TAB1, ID is defined as varchar2(5), and I have defined both oldId and newId as varchar2(5) in my code, I wonder why it doesn't work.

It returned this error:

DECLARE
*
ERROR at line 1: ORA-01722: invalid number ORA-06512: at line xx

After supply the value for oldId(val2) and newId(val1)

SQL > @thisScript val1 val2

I use dbms_output.put_line to print the command executed, it seems it is doing

UPDATE TABLE1 SET ID = val1 WHERE ID = val2

without the single quote ('val1'), ('val2'). But if I put in single quotes, the output statement

UPDATE TABLE1 SET ID = ':newId' WHERE ID = ':oldId'

The same bind logic works in SQL prompt

SQL > variable oldId varchar2(5);
SQL > variable newId varchar2(5);
SQL > exec :oldId :='val2'
SQL > exec :newId :='val1'
SQL > UPDATE TABLE1 SET ID = :newId WHERE ID = :oldId;
1 row updated.

I am wondering what should I do then? With or without single quotes?