Opens a file on the SD card. If the file is opened for writing, it will be created if it doesn’t already exist (but the directory containing it must already exist).
SD.open(filepath)
SD.open(filepath, mode)
filepath: the name of the file to open, which can include directories (delimited by forward-slashes, /).mode (optional): the mode in which to open the file. Mode can be FILE_READ (open the file for reading, starting at the beginning of the file) or FILE_WRITE (open the file for reading and writing, starting at the end of the file).A File object referring to the opened file; if the file couldn’t be opened, this object will evaluate to false in a boolean context, i.e. you can test the return value with “if (f)”.