# Vim documentation: undo

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    *undo.txt*      For Vim version 7.3f.  Last change: 2010 Jul 20

    		  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|
    2. Two ways of undo		|undo-two-ways|
    3. Undo blocks			|undo-blocks|
    4. Undo branches		|undo-branches|
    5. Undo persistence		|undo-persistence|
    6. Remarks about undo		|undo-remarks|

    ==============================================================================

    1. Undo and redo commands				*undo-commands*

    <Undo>		or					*undo* *<Undo>* *u*
    u			Undo [count] changes.  {Vi: only one level}

    							*:u* *:un* *:undo*
    :u[ndo]			Undo one change.  {Vi: only one level}

    								*E830*
    :u[ndo] {N}		Jump to after change number {N}.  See |undo-branches|
    			for the meaning of {N}.  {not in Vi}

    							*CTRL-R*
    CTRL-R			Redo [count] changes which were undone.  {Vi: redraw
    			screen}

    							*:red* *:redo* *redo*
    :red[o]			Redo one change which was undone.  {Vi: no redo}

    							*U*
    U			Undo all latest changes on one line.  {Vi: while not
    			moved off of it}

    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.
    			{not in Vi}

    This is most useful when you need to prompt the user halfway 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 an "u" command will undo the delete command and the previous
    change.

    To do the opposite, break a change into two undo blocks, 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 breaks undo.  Even when the new value
    is equal to the old value.

    ==============================================================================

    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   time
    				4      10	 10:34:11
    				18     4	 11:01:46

    			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 "time" column is the time this change was made.
    			For more details use the |undotree()| function.

    							*g-*
    g-			Go to older text state.  With a count repeat that many
    			times.  {not in Vi}

    							*: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 laste 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.  {not in Vi}

    							*: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.

    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 crypted.  The
    same key and method is used. |encryption|

    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.
    		{not in Vi}

    :rundo {file}	Read undo history from {file}.
    		{not in Vi}

    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, noting to undo"
    	There is no undo information not 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 = &undolevels
    	:set undolevels=-1
    	:exe "normal a \<BS>\<Esc>"
    	:let &undolevels = old_undolevels
    	:unlet old_undolevels

    Marks for the buffer ('a to 'z) are also saved and restored, together with the
    text.  {Vi does this a little bit different}

    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!" {not in Vi}.  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.

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