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undo.txt      For Vim version 8.0.  Last change: 2014 May 24


                  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, the line where
                        the latest change was made. U itself also counts as
                        a change, and thus U undoes a previous U.
                        {Vi: while not moved off of the last modified line}

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 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 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  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.  {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 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.  {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.  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 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. 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.
                {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, 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 = &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.

 vim:tw=78:ts=8:ft=help:norl:

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