using sqlplus not PL/SQL

I have table t1 with columns c1,c2 and c3 that are vchar2.
Is it possible to write a single sqlplus (not pl/sql) statement to
- If c1 is not null, return all rows that match c1
        select * from t1 where c1='JJJ'; for an example

- Otherwise return all rows in t1
        select * from t1;

Thanks,