How to programmatically insert a string at the end of some file?

Question

How to silently (in background) create a new line at the end of a specific file, say /path/to/file.txt (which is not necessarily a visiting file and is not necessarily already open in emacs) and insert some string (say xyz) in that new line and then to save the file after?

Answer 1

You can use the append-to-file and write-region functions, for example,

~ $ echo hello > file.txt
~ $ emacs --batch --eval '(append-to-file "xyz\n" nil "file.txt")'
~ $ cat file.txt
hello
xyz
~ $

See also (info "(elisp) Writing to Files").