ed is the original UNIX text editor, and it’s the most basic you can work with. It’s also very rarely used, if ever used, by most people.
Run it by typing ed. This starts an interactive session. Enter in write mode by typing a on a single line, and press enter. Then type everything you want, and once you are done, write just a dot (.) on a line and press enter.
Now type w followed by a file name to save the buffer to a file. It will return the number of bytes written to the file.
You can then press q to quit.
You can edit a file with ed by invoking it with the file name: ed <filename>. When you press a to add, you add content to the bottom of the file.
Inside an ed session you can type ,p to print the current file content.