How to use Vim to merge files

The pre-requisite to all operations detailed below is to make sure that all participating buffers are in diff mode (see :h start-vimdiff). I use tpope/vim-unimpaired that has the normal mode alias yod to toggle :diffthis on and off easily for a buffer.

Useful normal mode commands (and an extra)

• do - Get changes from other window into the current window.

• dp - Put the changes from current window into the other window.

• ]c - Jump to the next change.

• [c - Jump to the previous change.

• zo - Open folded lines.

• zc - Close folded lines.

• zr - Unfold both files completely.

• zm - Fold both files completely.

• Ctrlww - change window.

• :only | wq - quit other windows, and save them while at it.

Quirks to watch for

• Both do and dp work if you are on a block of change (or just one line under a single line of change) in Normal mode, but not in Visual mode.

• The undo command will only work in the buffer that was changed, so if you use dp and change your mind, you need to switch to the other buffer to undo.

• :diffupdate will re-scan the files for changes (Vim can get confused, and show bogus stuff).

Visual mode and finer grained control

When selecting lines of text in Visual mode, you must use the normal commands:

• :'<,'>diffget and
• :'<,'>diffput.

For example:

1. Enter Visual mode and mark some text/lines.

2. Then type :diffput to push the selected lines to the other file or :diffget to get the selected lines from the other file.

To belabor the point: This means that if there is a block of changes consisting of multiple lines, then selecting a subset of lines and issueing :diffput will only apply those changes in the other buffer.

(:diffget and :diffput also accept ranges, see :h copy-diffs for more.)

Compare two buffers inside Vim

If you load up two files in splits (:vs or :sp), you can do :diffthis on each window and achieve a diff of files that were already loaded in buffers.

:diffoff can be used to turn off the diff mode.

This Vimcasts post and video show this in practice.

How to apply all changes between buffers

Get all changes from a buffer to the current one:
:%diffget <buffer-number>

Put all changes from current buffer into another:
:%diffput <buffer-number>

(:% is a range to select the entire file; see :h :%. :ls will show currently opened buffers.)