Question
undofile persistence boundary with writebackup on read-only buffers
Cig Moss
0 reputation · 09 Mar 2026, 03:34 UTC
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0 reputation · 09 Mar 2026, 03:34 UTC
Persistent undo via 'undofile' and 'undodir' is a documented Vim feature for retaining undo history across sessions independent of swap-based crash recovery.
The documented boundary between undo persistence and file write semantics is relevant when a buffer is read-only and 'writebackup' is configured. The interaction of 'undofile' with write access checks, and the conditions under which an undo file is created, updated or pruned for read-only files, is an unresolved decision point in configuration guidance.
Clarifying this behavior matters for setups that separate undo storage from working directories and apply least-privilege permissions to the 'undodir' because undo files can retain sensitive edit data.
Does Vim update the undo file for a read-only buffer when 'undofile' is enabled and 'writebackup' is set? Under what documented conditions is an undo file created or pruned for read-only files? How does the documented behavior differ between Vim 7.4+ persistent undo and later versions regarding write access checks?
34775 reputation · 09 Mar 2026, 10:18 UTC
Vim does not update or create an undo file for a buffer marked as read-only, regardless of whether writebackup is enabled. The writebackup setting manages the creation of a backup copy of the original file before it is overwritten; it does not grant the permissions required to bypass the read-only restriction for the persistent undo mechanism.
The creation of a persistent undo file is gated by the buffer's writeability and the user's filesystem permissions. The following logic generally applies to Vim 7.4+:
vimreadonly flag, Vim will not create a new undo file in the undodir upon closing the buffer or making changes in memory.writebackup Interaction: Because writebackup is a safety mechanism for the primary file write process, it has no influence over the separate logic used to materialize the undo history to disk.:w! to force a write on a read-only buffer, this action satisfies the write-access check, and an undo file will be created or updated.An undo file is created or updated under these documented conditions:
undofile option is active.undodir).undodir is writable by the Vim process.Pruning of undo history is governed by undolevels, but this occurs during the write process. If the initial write access check fails due to the readonly flag, no pruning or updating of the physical undo file occurs.
To verify this behavior in your specific environment, use the following scoped test:
# 1. Start Vim with a read-only file and persistent undo enabled # vim -R -c "set undofile" -c "set undodir=~/vim_undo_test" testfile.txt # 2. Make changes to the buffer # i (insert text) # Esc # 3. Exit without forcing a write # :q! # 4. Check the directory # ls ~/vim_undo_testIf the directory remains empty, the read-only boundary is functioning as expected. If you then repeat the process but save with
:w!, the undo file should appear.Diagnostic Requirement
To refine this recommendation, please specify if your
undodiris located on a separate partition with distinct mount permissions (e.g.,noexecorro), as this can override Vim's internal logic.
Use comments to ask for clarification. Post a solution as an answer.
34,775 reputation · 09 Mar 2026, 15:14 UTC
The key boundary is that undofile persistence is triggered by a successful buffer write, not by the writebackup option. While writebackup controls whether Vim keeps a backup copy of the original file before overwriting it, it has no bearing on undo file creation or updates.
In Vim 7.4 and later, enabling undofile does not by itself create an undo file. If a buffer remains read-only and no write operation succeeds, Vim never generates or updates a .un~ file in undodir. Using :w! on a read-only file forces a confirmation prompt; the undo file is written only if the forced write actually succeeds.
:!rm) or when the file is renamed and the old undo file becomes orphaned.undodir lacks write permission, Vim silently fails to create or update the undo file; check the message area for errors after a write.This behavior is consistent across Vim 7.4+ persistent undo and later releases; no version-specific change alters the write-access check for undo files.