How is the undo tree used in Vim?

This answer says:

> Vim's undo/redo system is unbeatable. Type something, undo, type something else, and you can still get back the first thing you typed because Vim uses an undo tree rather than a stack. In almost every other program, the history of the first thing you typed is lost in this circumstance.

This is the first I hear of this. How can I backtrack along the tree?

See also :h undo-redo, which lists all the commands and their usage.

There are two ways to traverse the undo tree. One is to go "back in time". g+ and g- will traverse all of the nodes in the tree in chronological or reverse-chronological order (which can be a bit confusing, because it can jump arbitrarily between undo branches, but if you do g- long enough you'll always get where you need to go eventually). :earlier and :later take a time descriptor like 7m or 1h; again this can jump you arbitrarily between undo branches.

The other way is to jump to specific nodes in the tree using :undo n where n is a number of an action. (All actions, i.e. text additions, deletions, replacements, are numbered sequentially as you do them.) You can look up the number of the actions on the leaves of the undo tree via :undolist. This will let you jump between branches easily. You can then use u and Ctrl-R to move up and down that branch.

There are some good examples in the Vim help. The best way to figure out how this works is to play with it a bit.

I'm a bit late to the party,
but I figured I'd mention that I wrote an undo tree visualization plugin for Vim :
https://github.com/sjl/gundo.vim

Personally I found that graphing the tree like this was the only way I could make sense of it.

This page explains everything you need to know:

http://vimdoc.sourceforge.net/htmldoc/usr_32.html

If you're using vim, you can navigate through the undo tree using:

• u: (undo) move back in the undo tree
• Ctrl+R: (redo) move forward in the undo tree

Other ways of bringing document back or forward in time:

• :earlier 15m: move back in time 15 minutes
• :later 15m: move front in time 15 minutes

I'm aware this question has been answered, but I thought I'd add an example.

Create a new file and type:

this is a line

undol will display the undo tree. At this point you haven't undone anything

:undol

number changes  when               saved
     1       1  14:50:36

now press ESC and modify the line to:

this is a old line

switch to normal mode and press u (undo), this should remove "old". If you check undol, at this point you still have only one branch.

now modify the line so it says:

this is a new line

Now :undol shows:

number changes  when               saved
     2       2  87 seconds ago
     3       2  3 seconds ago

You can switch to the first branch by typing

:u 2

this will move you to the end of the branch associated with number 2. You can move along this branch with g+ and g-. At this point g+ will do nothing (you are at the leaf). If you press g- “old" will be removed (you are traversing the first undo tree). That is if you remove “old” with g- and press g+ again, “old" will be redone.

If you type

:u 3

You will jump to the leaf of the second undo branch and it will read:

this is a new line

A lot of this is summed up here:

http://vim.wikia.com/wiki/Using_undo_branches

Besides using gundo.vim I like to mention g+ and g-

The package undotree is written in pure vimscript so no requirement.

And add this to your vimrc before it is too late:

  set nobackup
  set noswapfile
  set nowritebackup
  set undolevels=10000         " use many levels of undo
  set history=10000    " After nocompatible

  if has('persistent_undo')
    set undodir=$HOME/.vim/undo
    set undofile 
  endif