When a text editor is saving a file, what very often happens is that it saves into a temporary file, empties (truncates) the original file, and copies the contents from the temporary file into the original file. It does this to not alter the permissions, ownership and other meta-data on the original file.
This means that from the perspective of
tail, the file is truncated (emptied), so it starts showing the last line of the file as the editor is rewriting it (which is why it displays the whole file).
To add lines to the file in such a way that your
tail command always shows only the last line of the file and nothing else, you will have to make sure to append lines to the file. Since text editors generally rewrite the file, you may have to do this using other means.
A very basic way of appending data to a file is by using
This would make
cat append to the file
Entrie.txt. It would wait for input from you, and you would be able to type directly into the file from the terminal. The current line is written to the end of the file as soon as you press Enter. To stop entering data, press Ctrl+D on an empty line.
This obviously does not give you any opportunity to correct what you have written on previous lines once you press Enter though.
Another alternative would be to edit the new lines in a separate file, say
additions.txt, with your editor. Save that file and then do
cat additions.txt >>Entrie.txt
This would add the full contents of
additions.txt to the end of the file