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*undo.txt*      For Vim version 9.1.  Last change: 2022 Jun 02

VIM REFERENCE MANUAL    by Bram Moolenaar

Undo and redo						*undo-redo*

The basics are explained in section |02.5| of the user manual.

==============================================================================
1. Undo and redo commands				*undo-commands*

<Undo>		or					*undo* *<Undo>* *u*
u			Undo [count] changes.

*:u* *:un* *:undo*
:u[ndo]			Undo one change.
								*E830*
:u[ndo] {N}		Jump to after change number {N}.  See |undo-branches|
			for the meaning of {N}.

*CTRL-R*
CTRL-R			Redo [count] changes which were undone.

*:red* *:redo* *redo*
:red[o]			Redo one change which was undone.

*U*
U			Undo all latest changes on one line, the line where
			the latest change was made. |U| itself also counts as
			a change, and thus |U| undoes a previous |U|.

The last changes are remembered.  You can use the undo and redo commands above
to revert the text to how it was before each change.  You can also apply the
changes again, getting back the text before the undo.

The "U" command is treated by undo/redo just like any other command.  Thus a
"u" command undoes a "U" command and a 'CTRL-R' command redoes it again.  When
mixing "U", "u" and 'CTRL-R' you will notice that the "U" command will
restore the situation of a line to before the previous "U" command.  This may
be confusing.  Try it out to get used to it.
The "U" command will always mark the buffer as changed.  When "U" changes the
buffer back to how it was without changes, it is still considered changed.
Use "u" to undo changes until the buffer becomes unchanged.

==============================================================================
2. Two ways of undo					*undo-two-ways*

How undo and redo commands work depends on the 'u' flag in 'cpoptions'.
There is the Vim way ('u' excluded) and the Vi-compatible way ('u' included).
In the Vim way, "uu" undoes two changes.  In the Vi-compatible way, "uu" does
nothing (undoes an undo).

'u' excluded, the Vim way:
You can go back in time with the undo command.  You can then go forward again
with the redo command.  If you make a new change after the undo command,
the redo will not be possible anymore.

'u' included, the Vi-compatible way:
The undo command undoes the previous change, and also the previous undo
command.  The redo command repeats the previous undo command.  It does NOT
repeat a change command, use "." for that.

Examples	Vim way			Vi-compatible way	~
"uu"		two times undo		no-op
"u CTRL-R"	no-op			two times undo

Rationale:  Nvi uses the "." command instead of CTRL-R.  Unfortunately, this
	    is not Vi compatible.  For example "dwdwu." in Vi deletes two
	    words, in Nvi it does nothing.

==============================================================================
3. Undo blocks						*undo-blocks*

One undo command normally undoes a typed command, no matter how many changes
that command makes.  This sequence of undo-able changes forms an undo block.
Thus if the typed key(s) call a function, all the commands in the function are
undone together.

If you want to write a function or script that doesn't create a new undoable
change but joins in with the previous change use this command:

*:undoj* *:undojoin* *E790*
:undoj[oin]		Join further changes with the previous undo block.
			Warning: Use with care, it may prevent the user from
			properly undoing changes.  Don't use this after undo
			or redo.

This is most useful when you need to prompt the user halfway through a change.
For example in a function that calls |getchar()|.  Do make sure that there was
a related change before this that you must join with.

This doesn't work by itself, because the next key press will start a new
change again.  But you can do something like this: >

:undojoin | delete

After this a "u" command will undo the delete command and the previous
change.
					*undo-break* *undo-close-block*
To do the opposite, use a new undo block for the next change, in Insert mode
use CTRL-G u.  This is useful if you want an insert command to be undoable in
parts.  E.g., for each sentence.  |i_CTRL-G_u|

Setting the value of 'undolevels' also closes the undo block.  Even when the
new value is equal to the old value.  Use `g:undolevels` to explicitly read
and write only the global value of 'undolevels'. In |Vim9| script: >
	&g:undolevels = &g:undolevels
In legacy script: >
	let &g:undolevels = &g:undolevels

Note that the similar-looking assignment `let &undolevels=&undolevels` does not
preserve the global option value of 'undolevels' in the event that the local
option has been set to a different value.  For example: >
	" Start with different global and local values for 'undolevels'.
	let &g:undolevels = 1000
	let &l:undolevels = 2000
	" This assignment changes the global option to 2000:
	let &undolevels = &undolevels

==============================================================================
4. Undo branches				*undo-branches* *undo-tree*

Above we only discussed one line of undo/redo.  But it is also possible to
branch off.  This happens when you undo a few changes and then make a new
change.  The undone changes become a branch.  You can go to that branch with
the following commands.

This is explained in the user manual: |usr_32.txt|.

*:undol* *:undolist*
:undol[ist]		List the leafs in the tree of changes.  Example:
			   number changes  when               saved ~
			       88      88  2010/01/04 14:25:53
			      108     107  08/07 12:47:51
			      136      46  13:33:01             7
			      166     164  3 seconds ago

The "number" column is the change number.  This number
			continuously increases and can be used to identify a
			specific undo-able change, see |:undo|.
			The "changes" column is the number of changes to this
			leaf from the root of the tree.
			The "when" column is the date and time when this
			change was made.  The four possible formats are:
			    N seconds ago
			    HH:MM:SS             hour, minute, seconds
			    MM/DD HH:MM:SS       idem, with month and day
			    YYYY/MM/DD HH:MM:SS  idem, with year
			The "saved" column specifies, if this change was
			written to disk and which file write it was. This can
			be used with the |:later| and |:earlier| commands.
			For more details use the |undotree()| function.

*g-*
g-			Go to older text state.  With a count repeat that many
			times.
							*:ea* *:earlier*
:earlier {count}	Go to older text state {count} times.
:earlier {N}s		Go to older text state about {N} seconds before.
:earlier {N}m		Go to older text state about {N} minutes before.
:earlier {N}h		Go to older text state about {N} hours before.
:earlier {N}d		Go to older text state about {N} days before.

:earlier {N}f		Go to older text state {N} file writes before.
			When changes were made since the last write
			":earlier 1f" will revert the text to the state when
			it was written.  Otherwise it will go to the write
			before that.
			When at the state of the first file write, or when
			the file was not written, ":earlier 1f" will go to
			before the first change.

*g+*
g+			Go to newer text state.  With a count repeat that many
			times.
							*:lat* *:later*
:later {count}		Go to newer text state {count} times.
:later {N}s		Go to newer text state about {N} seconds later.
:later {N}m		Go to newer text state about {N} minutes later.
:later {N}h		Go to newer text state about {N} hours later.
:later {N}d		Go to newer text state about {N} days later.

:later {N}f		Go to newer text state {N} file writes later.
			When at the state of the last file write, ":later 1f"
			will go to the newest text state.

Note that text states will become unreachable when undo information is cleared
for 'undolevels'.

Don't be surprised when moving through time shows multiple changes to take
place at a time.  This happens when moving through the undo tree and then
making a new change.

EXAMPLE

Start with this text:
	one two three ~

Delete the first word by pressing "x" three times:
	ne two three ~
	e two three ~
	 two three ~

Now undo that by pressing "u" three times:
	e two three ~
	ne two three ~
	one two three ~

Delete the second word by pressing "x" three times:
	one wo three ~
	one o three ~
	one  three ~

Now undo that by using "g-" three times:
	one o three ~
	one wo three ~
	 two three ~

You are now back in the first undo branch, after deleting "one".  Repeating
"g-" will now bring you back to the original text:
	e two three ~
	ne two three ~
	one two three ~

Jump to the last change with ":later 1h":
	one  three ~

And back to the start again with ":earlier 1h":
	one two three ~

Note that using "u" and CTRL-R will not get you to all possible text states
while repeating "g-" and "g+" does.

==============================================================================
5. Undo persistence		*undo-persistence* *persistent-undo*

When unloading a buffer Vim normally destroys the tree of undos created for
that buffer.  By setting the 'undofile' option, Vim will automatically save
your undo history when you write a file and restore undo history when you edit
the file again.

The 'undofile' option is checked after writing a file, before the BufWritePost
autocommands.  If you want to control what files to write undo information
for, you can use a BufWritePre autocommand: >
	au BufWritePre /tmp/* setlocal noundofile

Vim saves undo trees in a separate undo file, one for each edited file, using
a simple scheme that maps filesystem paths directly to undo files. Vim will
detect if an undo file is no longer synchronized with the file it was written
for (with a hash of the file contents) and ignore it when the file was changed
after the undo file was written, to prevent corruption.  An undo file is also
ignored if its owner differs from the owner of the edited file, except when
the owner of the undo file is the current user.  Set 'verbose' to get a
message about that when opening a file.

Undo files are normally saved in the same directory as the file.  This can be
changed with the 'undodir' option.

When the file is encrypted, the text in the undo file is also encrypted.  The
same key and method is used. |encryption|

Note that text properties are not stored in the undo file.  You can restore
text properties so long as a buffer is loaded, but you cannot restore them
from an undo file.  Rationale: It would require the associated text property
types to be defined in exactly the same was as before, which cannot be
guaranteed.

You can also save and restore undo histories by using ":wundo" and ":rundo"
respectively:
							*:wundo* *:rundo*
:wundo[!] {file}
		Write undo history to {file}.
		When {file} exists and it does not look like an undo file
		(the magic number at the start of the file is wrong), then
		this fails, unless the ! was added.
		If it exists and does look like an undo file it is
		overwritten. If there is no undo-history, nothing will be
		written.
		Implementation detail: Overwriting happens by first deleting
		the existing file and then creating a new file with the same
		name. So it is not possible to overwrite an existing undofile
		in a write-protected directory.

:rundo {file}	Read undo history from {file}.

You can use these in autocommands to explicitly specify the name of the
history file.  E.g.: >

au BufReadPost * call ReadUndo()
	au BufWritePost * call WriteUndo()
	func ReadUndo()
	  if filereadable(expand('%:h') .. '/UNDO/' .. expand('%:t'))
	    rundo %:h/UNDO/%:t
	  endif
	endfunc
	func WriteUndo()
	  let dirname = expand('%:h') .. '/UNDO'
	  if !isdirectory(dirname)
	    call mkdir(dirname)
	  endif
	  wundo %:h/UNDO/%:t
	endfunc

You should keep 'undofile' off, otherwise you end up with two undo files for
every write.

You can use the |undofile()| function to find out the file name that Vim would
use.

Note that while reading/writing files and 'undofile' is set most errors will
be silent, unless 'verbose' is set.  With :wundo and :rundo you will get more
error messages, e.g., when the file cannot be read or written.

NOTE: undo files are never deleted by Vim.  You need to delete them yourself.

Reading an existing undo file may fail for several reasons:
*E822*	It cannot be opened, because the file permissions don't allow it.
*E823*	The magic number at the start of the file doesn't match.  This usually
	means it is not an undo file.
*E824*	The version number of the undo file indicates that it's written by a
	newer version of Vim.  You need that newer version to open it.  Don't
	write the buffer if you want to keep the undo info in the file.
"File contents changed, cannot use undo info"
	The file text differs from when the undo file was written.  This means
	the undo file cannot be used, it would corrupt the text.  This also
	happens when 'encoding' differs from when the undo file was written.
*E825*  The undo file does not contain valid contents and cannot be used.
*E826*  The undo file is encrypted but decryption failed.
*E827*  The undo file is encrypted but this version of Vim does not support
	encryption.  Open the file with another Vim.
*E832*  The undo file is encrypted but 'key' is not set, the text file is not
	encrypted.  This would happen if the text file was written by Vim
	encrypted at first, and later overwritten by not encrypted text.
	You probably want to delete this undo file.
"Not reading undo file, owner differs"
	The undo file is owned by someone else than the owner of the text
	file.  For safety the undo file is not used.

Writing an undo file may fail for these reasons:
*E828*	The file to be written cannot be created.  Perhaps you do not have
	write permissions in the directory.
"Cannot write undo file in any directory in 'undodir'"
	None of the directories in 'undodir' can be used.
"Will not overwrite with undo file, cannot read"
	A file exists with the name of the undo file to be written, but it
	cannot be read.  You may want to delete this file or rename it.
"Will not overwrite, this is not an undo file"
	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:

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