This piece of code could lead to extreme disruption of a system if someone would manually change the OUTPUT_DIR variable. Inadvertently losing files or corrupting the entire system if else.

I would recommend instead to keep it as an arg clear-output and limited only to .mp3 files, maybe? or since there is already a transfer.sh file that fulfill that porpoise, just keep it outside the main code?

I'll PR in just about it takes me to change some other stuff (copying and rm cache could be done sequentially in between each download since otherwise it takes the double amount of space to maintain a copy of cache, plus output dir could be specified as an arg)