Any identifier can be prefixed with another $ to prevent execution without a certain parameter.
For example, using $$1 in a command means the command will only run if $1 is filled.
If the specified identifier does not exist, execution of commands will be halted.
You can also use $$?1 or $?1 which will try to fill the value with parameter one if it exists. If parameter one doesn't exist, it will ask for it. In the first case the parameter is necessary for the command to be executed, in the second case it isn't.
Will run the /op command if $1 is filled.