You cannot do this just by e.g. write, that will either append or overwrite depending on how you opened the file.

You wish to insert a line in the middle, moving everything else "downward". You can only do this one of two ways:

1. Read whole file into memory array/list, insert line, write whole array/list back to file.

2. Open existing file for read line at a time, open new, distinct file for output line at a time, loop reading/writing, insert extra line into output at appropriate point. On successful conclusion, delete original file, rename new file to old file. Personally I like this way (no memory usage, no potential "partial overwrite" of file), though #1 is less code to write.