How to become another User in SQLPlus

A DBA frequently needs to become another user to test something or verify a problem.  Short of having to gain acess to that users password, we are asked can I su to that account, sort of like root does on unix.

This is an 'su.sql' script I use:
 

whenever sqlerror exit

column password new_value pw

declare
    l_passwd varchar2(45);
begin
    select password into l_passwd
      from sys.dba_users
     where username = upper('&1');
end;
/

select password
  from sys.dba_users
 where username = upper( '&1' )
/

alter user &1 identified by Hello;
connect &1/hello
alter user &1 identified by values '&pw';
show user
whenever sqlerror continue

it starts by testing your access to the sys.dba_users table -- if that fails -- it exits SQLPlus.  If zero rows returned -- it exits SQLPlus.

It then selects the 'password' from the dba_users table and stuffs it into a macro variable "&pw"

We alter the user you want to become to have a known password (if that fails, we exit).

We 'fix' their password back after loggin in as them....

Note, you need to have access to dba_users and the alter user privelege.