Diagnosing Vim Undofile Issues When Undo History Does Not Persist

Recognizable condition

You edit a file, make changes, save and quit Vim. When you reopen the same file, :undolist shows no entries and the undo (u) and redo (Ctrl+R) commands behave as if the editing session never existed. The undofile option may appear to be set, yet no undo history is retained.

Short cause/diagnostic table

Symptom	Likely cause
:undolist empty after restart	undofile not enabled or undodirectory missing/unwritable
E850: Cannot create undo file	Insufficient permissions or missing directory for undofile
Undofile exists but undo/redo do nothing	Corrupted undo file or swap‑file interference
Swap file warnings at startup	Existing swap file prevents undofile handling
Undo works for some files but not others	Modeline overriding user undofile setting

Ordered checks

1. Verify Vim’s current settings:

   :set undofile?
   :set undodirectory?

   Run these commands in Vim’s command‑line mode. Note the values; they should show undofile and a directory path.

2. Check the undo directory from the shell:

   ls -ld $HOME/.vim/undo   # replace with the path from :set undodirectory?

   Ensure the directory exists, is owned by you, and has write permission (e.g., drwx------).

3. Look for error messages when writing the undo file:

   :verbose set undofile?

   If Vim reports E850: Cannot create undo file, note the path it tried to use.

4. Detect swap‑file interference:

   :sw

   If a swap file is listed for the current buffer, Vim may skip undofile processing.

5. Inspect the undo file after editing:

   ls -l $HOME/.vim/undo/*   # adjust path
   stat $HOME/.vim/undo/
   

   The file should appear after the first save and its size should change after further edits.

6. Test persistence:

   1. Open a test file: vim test.txt
   2. Make a change, save (:w) and quit (:q).
   3. Reopen the file and run :undolist.
   4. Undo (u) and redo (Ctrl+R) should revert/reapply the change.

Fixes tied to findings

Undofile not enabled or directory missing

• Add to your ~/.vimrc (or init.vim for Neovim):

set undofile
set undodirectory=$HOME/.vim/undo//

• Create the directory and restrict access:

mkdir -p $HOME/.vim/undo
chmod 700 $HOME/.vim/undo

• Restart Vim and repeat the test.

Permission error (E850)

• Confirm the directory is writable by your user:

touch $HOME/.vim/undo/testwrite && rm $HOME/.vim/undo/testwrite

• If the directory is on a shared or network mount, move it to a local filesystem (e.g., $HOME/.vim/undo).
• Update undodirectory accordingly.

Corrupted undo file

• Remove the suspect undo file:

rm $HOME/.vim/undo/

• Edit the file again; Vim will create a fresh undo file.
• If corruption recurs, check for hardware issues or abrupt power loss during Vim’s write.

Swap‑file interference

• List and delete stale swap files:

:ls
:bd!   # delete buffer associated with swap

• Or set a dedicated swap directory to avoid collisions:

set directory=$HOME/.vim/swap//

• Create $HOME/.vim/swap with chmod 700.

Modeline overriding user setting

• Search for modelines in the file:

:set modeline?
:verbose set undofile?

• If modeline is on and a line like vim: set noundofile exists, either remove the modeline or add set modelines=0 to your vimrc to disable them.

Escalation criteria

• After applying the relevant fix, if :undolist remains empty, start Vim with a minimal configuration to rule out plugin interference:

vim -u NONE -N test.txt

• If the problem persists only with specific file types, check filetype‑specific plugins or ftplugin scripts that may undo undofile.
• For persistent corruption, consider upgrading Vim (the undo file format changed after Vim 7.4.850) or reporting the issue with the exact Vim version (:version) and OS details.