SQL & PL/SQL

I'm trying to run update statements from a sql file where the data will be inputed by the user from the command prompt.

i tried putting this in my script
update table_1 set column_1 = &variable where column_2 = 'X';
commit;
exit;
/

when i ran it it didn't put enter value for variable it just put a blank line and i inputted the variable and the table was updated. but no prompt saying
enter value for variable came up.
how can i add that prompt so the user can know what to do?