The io.open() function is used to open the file on SD card for subsequent reading or writing. After the script is done with the file manipulation io.close() function should be used.
filename full path to the file starting from the SD card root directory. This function can't create a new file in non-existing directory.
mode supported mode strings are:
"r" read access. File must exist beforehand. The read pointer is located at the beginning of file. This is the default mode if is omitted.
"w" write access. File is opened or created (if it didn't exist) and truncated (all existing file contents are lost).
"a" write access. File is opened or created (if it didn't exist) and write pointer is located at the end of the file. The existing file contents are preserved.
<file object> if file was successfully opened.
nil if file could not be opened.