Ah guys, He wants to change the value of username and password once they're substituted in the command!

So 'root' | 'password' has to be replaced with 'rooter' | 'new-pass'.

He's trying to build an interactive client based app(I guess), And wants the logins to change :/

@OP, Is this correct? If yes, Then why don't you take the values from user rather than trying to change them in mid program?