That is not what you have to do. You are changing the variable part into a hard-coded string. This hard-coded string will be returned in each row selected, hence every generated commandline has exactly the same text as filename.

If you look at my code, you see I select the value for @filename from each row out of the column "physical_device_name". This value has the full path included, so you can just run the code and leave the SELECT part of my code unchanged.