There are many ways to append text to files on Linux, but to add a lot of lines, a script is likely one of your better options, especially if you need to do this repeatedly.

Choose a Bash or Python script if you need to perform a more complex substitution across multiple files or with additional logic. 6 Ways to Find and Replace Text in the Linux Terminal© Provided ...

The most common use cases of sed include operations like selecting text, substituting text, modifying an original file, adding lines to text, or deleting lines from the text. It can be used from the ...

That means if your script is careful to exit, you can add as much “garbage” to the end of it as you like. The shell will never look at it, so it’s possible to store the payload there.