Literally, you cannot INSERT (in other words, grow a file by adding something to the middle). You can only append to files, or overwrite what's in the middle. Disk operating systems don't grow files from the middle. So the commonly used solution is to read the file one line at a time, writing out to a new file one line at a time... when you get to the part where you want to insert, write out the new data, and then continue writing the remainder of the old data to the new file. When finished, replace the old file with the new one. This process is slow for big files with lots of 'inserts'. This is where databases make sense.

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