Undo Handling: Further Considerations and Clearing Mechanisms

A file exists with the name of the undo file to be written, but it does not start with the right magic number. You may want to delete this file or rename it. "Skipping undo file write, nothing to undo" There is no undo information to be written, nothing has been changed or 'undolevels' is negative. *E829* An error occurred while writing the undo file. You may want to try again. ============================================================================== 6. Remarks about undo *undo-remarks* The number of changes that are remembered is set with the 'undolevels' option. If it is zero, the Vi-compatible way is always used. If it is negative no undo is possible. Use this if you are running out of memory. *clear-undo* When you set 'undolevels' to -1 the undo information is not immediately cleared, this happens at the next change. To force clearing the undo information you can use these commands: > :let old_undolevels = &l:undolevels :setlocal undolevels=-1 :exe "normal a \<BS>\<Esc>" :let &l:undolevels = old_undolevels :unlet old_undolevels Note use of `&l:undolevels` to explicitly read the local value of 'undolevels' and the use of `:setlocal` to change only the local option (which takes precedence over the corresponding global option value). Saving the option value via the use of `&undolevels` is unpredictable; it reads either the local value (if one has been set) or the global value (otherwise). Also, if a local value has been set, changing the option via `:set undolevels` will change both the global and local values, requiring extra work to save and restore both values. Marks for the buffer ('a to 'z) are also saved and restored, together with the text. When all changes have been undone, the buffer is not considered to be changed. It is then possible to exit Vim with ":q" instead of ":q!". Note that this is relative to the last write of the file. Typing "u" after ":w" actually changes the buffer, compared to what was written, so the buffer is considered changed then. When manual |folding| is being used, the folds are not saved and restored. Only changes completely within a fold will keep the fold as it was, because the first and last line of the fold don't change. The numbered registers can also be used for undoing deletes. Each time you delete text, it is put into register "1. The contents of register "1 are shifted to "2, etc. The contents of register "9 are lost. You can now get back the most recent deleted text with the put command: '"1P'. (also, if the deleted text was the result of the last delete or copy operation, 'P' or 'p' also works as this puts the contents of the unnamed register). You can get back the text of three deletes ago with '"3P'. *redo-register* If you want to get back more than one part of deleted text, you can use a special feature of the repeat command ".". It will increase the number of the register used. So if you first do '"1P', the following "." will result in a '"2P'. Repeating this will result in all numbered registers being inserted. Example: If you deleted text with 'dd....' it can be restored with '"1P....'. If you don't know in which register the deleted text is, you can use the :display command. An alternative is to try the first register with '"1P', and if it is not what you want do 'u.'. This will remove the contents of the first put, and repeat the put command for the second register. Repeat the 'u.' until you got what you want. vim:tw=78:ts=8:noet:ft=help:norl:

This section covers further details about undo handling in Vim. It explains scenarios where undo file writing is skipped (e.g., no changes or negative 'undolevels'). It details using 'undolevels' option to control the depth of undo history and demonstrates methods to clear undo information using specific commands. The segment highlights saving and restoring buffer marks, the impact of changes on buffer state, and considerations for folding. It also describes utilizing numbered registers for undoing deletes, offering examples of restoring deleted text with put commands and the repeat command.