Selecting G-code files from SD with serial
You can send the G Codes to select and print a file over the serial connection. 


List the files


  M20

Select a file name


  M23 filename.gco

Start/Resue the print


  M24



See https://reprap.org/wiki/G-code for more information M20 through M39 are the ones for interfacing with SD Card files.