What is a .un~ file or or why does Vim in the Terminal make the .un~ file?

Solution 1:

When you edit and save files, Vim creates a file with the same name as the original file and an un~ extension at the end.

Vim 7.3 contains a new feature persistent undo, that is, undo information won't be lost when quitting Vim and be stored in a file that ends with .un~. You have set the undofile option, so Vim creates an undo file when saving the original file. You can stop Vim from creating the backup file, by clearing the option:

:set noundofile

Note that, by default this option is turned off. You have explicitly enabled the undofile option in one of the initialization files. If you want your undofiles to be stored only in a particular directory, you can point the undodir option to a directory that will contain all your aggregated undofiles.

Source: http://vimhelp.appspot.com/vim_faq.txt.html#faq-7.2

Solution 2:

Took me a while to find where to actually put the :set noundofile command. I am new and I’ll just reply with how I made it not do backups.

1. Open vim.
2. Type in command mode :e $HOME/.vimrc
3. Write :set noundofile
4. Save & quit: :wq

Solution 3:

Another possible way to avoid vim creating undo files everywhere is set set the undodir to some existing directory, e.g.

if has('persistent_undo')         "check if your vim version supports
  set undodir=$HOME/.vim/undo     "directory where the undo files will be stored
  set undofile                    "turn on the feature
endif

snagged from here