VIM: diff

## DIFF

    *diff.txt*      For Vim version 9.1.  Last change: 2025 Sep 15

    		  VIM REFERENCE MANUAL    by Bram Moolenaar

    				*diff* *vimdiff* *gvimdiff* *diff-mode*
    This file describes the |+diff| feature: Showing differences between two to
    eight versions of the same file.

    The basics are explained in section |08.7| of the user manual.

    1. Starting diff mode		|start-vimdiff|
    2. Viewing diffs		|view-diffs|
    3. Jumping to diffs		|jumpto-diffs|
    4. Copying diffs		|copy-diffs|
    5. Diff anchors			|diff-anchors|
    6. Diff options			|diff-options|

    1. Starting diff mode					*start-vimdiff*

    The easiest way to start editing in diff mode is with the "vimdiff" command.
    This starts Vim as usual, and additionally sets up for viewing the differences
    between the arguments.

    	vimdiff file1 file2 [file3 [file4]]

    This is equivalent to:

    	vim -d file1 file2 [file3 [file4]]

    You may also use "gvimdiff" or "vim -d -g".  The GUI is started then.
    You may also use "viewdiff" or "gviewdiff".  Vim starts in readonly mode then.
    "r" may be prepended for restricted mode (see |-Z|).

    The second and following arguments may also be a directory name.  Vim will
    then append the file name of the first argument to the directory name to find
    the file.

    By default an internal diff library will be used.  When 'diffopt' or
    'diffexpr' has been set an external "diff" command will be used.  This only
    works when such a diff program is available.

    Diffs are local to the current tab page |tab-page|.  You can't see diffs with
    a window in another tab page.  This does make it possible to have several
    diffs at the same time, each in their own tab page.

    What happens is that Vim opens a window for each of the files.  This is like
    using the |-O| argument.  This uses vertical splits.  If you prefer horizontal
    splits add the |-o| argument:

    	vimdiff -o file1 file2 [file3 [file4]]

    If you always prefer horizontal splits include "horizontal" in 'diffopt'.

    In each of the edited files these options are set:

    	'diff'		on
    	'scrollbind'	on
    	'cursorbind'	on
    	'scrollopt'	includes "hor"
    	'wrap'		off, or leave as-is if 'diffopt' includes "followwrap"
    	'foldmethod'	"diff"
    	'foldcolumn'	value from 'diffopt', default is 2

    These options are set local to the window.  When editing another file they are
    reset to the global value.
    The options can still be overruled from a modeline when re-editing the file.
    However, 'foldmethod' and 'wrap' won't be set from a modeline when 'diff' is
    set.
    See `:diffoff` for an easy way to revert the options.

    The differences shown are actually the differences in the buffer.  Thus if you
    make changes after loading a file, these will be included in the displayed
    diffs.  You might have to do ":diffupdate" now and then, not all changes are
    immediately taken into account, especially when using an external diff command.

    In your .vimrc file you could do something special when Vim was started in
    diff mode.  You could use a construct like this:

    	if &diff
    	   setup for diff mode
    	else
    	   setup for non-diff mode
    	endif

    While already in Vim you can start diff mode in three ways.

    							*E98*
    :diffs[plit] {filename}					*:diffs* *:diffsplit*
    		Open a new window on the file {filename}.  The options are set
    		as for "vimdiff" for the current and the newly opened window.
    		Also see 'diffexpr'.

    							*:difft* *:diffthis*
    :difft[his]	Make the current window part of the diff windows.  This sets
    		the options like for "vimdiff".

    :diffp[atch] {patchfile}			 *E816* *:diffp* *:diffpatch*
    		Use the current buffer, patch it with the diff found in
    		{patchfile} and open a buffer on the result.  The options are
    		set as for "vimdiff".
    		{patchfile} can be in any format that the "patch" program
    		understands or 'patchexpr' can handle.
    		Note that {patchfile} should only contain a diff for one file,
    		the current file.  If {patchfile} contains diffs for other
    		files as well, the results are unpredictable.  Vim changes
    		directory to /tmp to avoid files in the current directory
    		accidentally being patched.  But it may still result in
    		various ".rej" files to be created.  And when absolute path
    		names are present these files may get patched anyway.
    		Using the "patch" command is not allowed in |restricted-mode|.

    To make these commands use a vertical split, prepend |:vertical|.  Examples:

    	:vert diffsplit main.c
    	:vert diffpatch /tmp/diff

    If you always prefer a vertical split include "vertical" in 'diffopt'.

    							*E96*
    There can be up to eight buffers with 'diff' set.

    Since the option values are remembered with the buffer, you can edit another
    file for a moment and come back to the same file and be in diff mode again.

    							*:diffo* *:diffoff*
    :diffo[ff]	Switch off diff mode for the current window.  Resets related
    		options also when 'diff' was not set.

    :diffo[ff]!	Switch off diff mode for the current window and in all windows
    		in the current tab page where 'diff' is set.  Resetting
    		related options only happens in a window that has 'diff' set,
    		if the current window does not have 'diff' set then no options
    		in it are changed.
    		Hidden buffers are also removed from the list of diff'ed
    		buffers.

    The `:diffoff` command resets the relevant options to the values they had when
    using `:diffsplit`, `:diffpatch`, `:diffthis`. or starting Vim in diff mode.
    When using `:diffoff` twice the last saved values are restored.
    Otherwise they are set to their default value:

    	'diff'		off
    	'scrollbind'	off
    	'cursorbind'	off
    	'scrollopt'	without "hor"
    	'wrap'		on, or leave as-is if 'diffopt' includes "followwrap"
    	'foldmethod'	"manual"
    	'foldcolumn'	0

    'foldenable' will most-likely be reset to off.  That is when 'foldmethod' is
    is restored to "manual".  The folds themselves are not cleared but they should
    not show up, resetting 'foldenable' is the best way to do that.

    2. Viewing diffs						*view-diffs*

    The effect is that the diff windows show the same text, with the differences
    highlighted.  When scrolling the text, the 'scrollbind' option will make the
    text in other windows to be scrolled as well.  With vertical splits the text
    should be aligned properly.

    The alignment of text will go wrong when:
    - 'wrap' is on, some lines will be wrapped and occupy two or more screen
      lines
    - folds are open in one window but not another
    - 'scrollbind' is off
    - changes have been made to the text
    - "filler" is not present in 'diffopt', deleted/inserted lines makes the
      alignment go wrong

    All the buffers edited in a window where the 'diff' option is set will join in
    the diff.  This is also possible for hidden buffers.  They must have been
    edited in a window first for this to be possible.  To get rid of the hidden
    buffers use `:diffoff!`.

    					*:DiffOrig* *diff-original-file*
    Since 'diff' is a window-local option, it's possible to view the same buffer
    in diff mode in one window and "normal" in another window.  It is also
    possible to view the changes you have made to a buffer since the file was
    loaded.  Since Vim doesn't allow having two buffers for the same file, you
    need another buffer.  This command is useful:
    	 command DiffOrig vert new | set bt=nofile | r ++edit # | 0d_
    		\ | diffthis | wincmd p | diffthis
    (this is in |defaults.vim|).  Use ":DiffOrig" to see the differences between
    the current buffer and the file it was loaded from.

    A buffer that is unloaded cannot be used for the diff.  But it does work for
    hidden buffers.  You can use ":hide" to close a window without unloading the
    buffer.  If you don't want a buffer to remain used for the diff do ":set
    nodiff" before hiding it.

    						*:dif* *:diff* *:diffupdate*
    :dif[fupdate][!]		Update the diff highlighting and folds.

    Vim attempts to keep the differences updated when you make changes to the
    text.  This mostly takes care of inserted and deleted lines.  Changes within a
    line and more complicated changes do not cause the differences to be updated.
    To force the differences to be updated use:

    	:diffupdate

    If the ! is included Vim will check if the file was changed externally and
    needs to be reloaded.  It will prompt for each changed file, like `:checktime`
    was used.

    Vim will show filler lines for lines that are missing in one window but are
    present in another.  These lines were inserted in another file or deleted in
    this file.  Removing "filler" from the 'diffopt' option will make Vim not
    display these filler lines.

    Folds are used to hide the text that wasn't changed.  See |folding| for all
    the commands that can be used with folds.

    The context of lines above a difference that are not included in the fold can
    be set with the 'diffopt' option.  For example, to set the context to three
    lines:

    	:set diffopt=filler,context:3

    The diffs are highlighted with these groups:

    |hl-DiffAdd|	DiffAdd		Added (inserted) lines.  These lines exist in
    				this buffer but not in another.
    |hl-DiffChange|	DiffChange	Changed lines.
    |hl-DiffText|	DiffText	Changed text inside a Changed line.  Exact
    				behavior depends on the `inline:` setting in
    				'diffopt'.
    				With `inline:` set to "simple", Vim finds the
    				first character that is different, and the
    				last character that is different (searching
    				from the end of the line).  The text in
    				between is highlighted.  This means that parts
    				in the middle that are still the same are
    				highlighted anyway.  The 'diffopt' flags
    				"iwhite" and "icase" are used here.
    				With `inline:` set to "char" or "word", Vim
    				uses the internal diff library to perform a
    				detailed diff between the changed blocks and
    				highlight the exact difference between the
    				two.  Will respect any 'diffopt' flag that
    				affects internal diff.
    				Not used when `inline:` is set to "none".
    |hl-DiffTextAdd|  DiffTextAdd	Added text inside a Changed line.  Similar to
    				DiffText, but used when there is no
    				corresponding text in other buffers.  Not used
    				when `inline:` is set to "simple" or "none".
    |hl-DiffDelete|	DiffDelete	Deleted lines.  Also called filler lines,
    				because they don't really exist in this
    				buffer.

    3. Jumping to diffs					*jumpto-diffs*

    Two commands can be used to jump to diffs:
    								*[c*
    	[c		Jump backwards to the previous start of a change.
    			When a count is used, do it that many times.
    								*]c*
    	]c		Jump forwards to the next start of a change.
    			When a count is used, do it that many times.

    It is an error if there is no change for the cursor to move to.

    4. Diff copying			*copy-diffs* *E99* *E100* *E101* *E102* *E103*
    								*merge*
    There are two commands to copy text from one buffer to another.  The result is
    that the buffers will be equal within the specified range.

    							*:diffg* *:diffget*
    :[range]diffg[et] [bufspec]
    		Modify the current buffer to undo difference with another
    		buffer.  If [bufspec] is given, that buffer is used.  If
    		[bufspec] refers to the current buffer then nothing happens.
    		Otherwise this only works if there is one other buffer in diff
    		mode.
    		See below for [range].

    						*:diffpu* *:diffput* *E793*
    :[range]diffpu[t] [bufspec]
    		Modify another buffer to undo difference with the current
    		buffer.  Just like ":diffget" but the other buffer is modified
    		instead of the current one.
    		When [bufspec] is omitted and there is more than one other
    		buffer in diff mode where 'modifiable' is set this fails.
    		See below for [range].

    							*do*
    [count]do	Same as ":diffget" without range.  The "o" stands for "obtain"
    		("dg" can't be used, it could be the start of "dgg"!). Note:
    		this doesn't work in Visual mode.
    		If you give a [count], it is used as the [bufspec] argument
    		for ":diffget".

    							*dp*
    [count]dp	Same as ":diffput" without range.  Note: this doesn't work in
    		Visual mode.
    		If you give a [count], it is used as the [bufspec] argument
    		for ":diffput".

    When no [range] is given, the diff at the cursor position or just above it is
    affected.  There can be deleted lines below the last line of the buffer.  When
    the cursor is on the last line in the buffer and there is no diff above this
    line, and no [range] is given, the diff below the cursor position will be used
    instead.

    When [range] is used, Vim tries to only put or get the specified lines.  When
    there are deleted lines, they will be used if they are between the lines
    specified by [range].

    To be able to put or get those lines to/from another buffer in a [range] it's
    allowed to use 0 and the last line number plus one.  This command gets all
    diffs from the other buffer:

    	:0,$+1diffget

    Note that deleted lines are displayed, but not counted as text lines.  You
    can't move the cursor into them.  To fill the deleted lines with the lines
    from another buffer use ":diffget" on the line below them.
    								*E787*
    When the buffer that is about to be modified is read-only and the autocommand
    that is triggered by |FileChangedRO| changes buffers the command will fail.
    The autocommand must not change buffers.

    The [bufspec] argument above can be a buffer number, a pattern for a buffer
    name or a part of a buffer name.  Examples:

    	:diffget		Use the other buffer which is in diff mode
    	:diffget 3		Use buffer 3
    	:diffget v2		Use the buffer which matches "v2" and is in
    				diff mode (e.g., "file.c.v2")

    5. Diff anchors						*diff-anchors*

    Diff anchors allow you to control where the diff algorithm aligns and
    synchronize text across files.  Each anchor matches each other in each file,
    allowing you to control the output of a diff.

    This is useful when a change involves complicated edits.  For example, if a
    function was moved to another location and further edited.  By default, the
    algorithm aims to create the smallest diff, which results in that entire
    function being considered to be deleted and added on the other side, making it
    hard to see what the actual edit on it was.  You can use diff anchors to pin
    that function so the diff algorithm will align based on it.

    To use it, set anchors using 'diffanchors' which is a comma-separated list of
    {address} in each file, and then add "anchor" to 'diffopt'.  Internally, Vim
    splits each file up into sections split by the anchors.  It performs the diff
    on each pair of sections separately before merging the results back.

    Setting 'diffanchors' will update the diff immediately.  If an anchor is tied
    to a mark, and you change what the mark is pointed to, you need to manually
    call |:diffupdate| afterwards to get the updated diff results.

    Example:

    Let's say we have the following files, side-by-side.  We are interested in the
    change that happened to the function `foo()`, which was both edited and moved.

    File A:
    	int foo() {
    	    int n = 1;
    	    return n;
    	}

    	int g = 1;

    	int bar(int a) {
    	    a *= 2;
    	    a += 3;
    	    return a;
    	}
    <File B:
    	int bar(int a) {
    	    a *= 2;
    	    a += 3;
    	    return a;
    	}

    	int foo() {
    	    int n = 999;
    	    return n;
    	}

    	int g = 1;

    A normal diff will usually align the diff result as such:

    	int foo() {     |----------------
    	    int n = 1;  |----------------
    	    return n;   |----------------
    	}               |----------------
    	                |----------------
    	int g = 1;      |----------------
    	                |----------------
    	int bar(int a) {|int bar(int a) {
    	    a *= 2;     |    a *= 2;
    	    a += 3;     |    a += 3;
    	    return a;   |    return a;
    	}               |}
    	----------------|
    	----------------|int foo() {
    	----------------|    int n = 999;
    	----------------|    return n;
    	----------------|}
    	----------------|
    	----------------|int g = 1;

    What we want is to instead ask the diff to align on `foo()`:

    	----------------|int bar(int a) {
    	----------------|    a *= 2;
    	----------------|    a += 3;
    	----------------|    return a;
    	----------------|}
    	----------------|
    	int foo() {     |int foo() {
    	    int n = 1;  |    int n = 999;
    	    return n;   |    return n;
    	}               |}
    	                |
    	int g = 1;      |int g = 1;
    	                |----------------
    	int bar(int a) {|----------------
    	    a *= 2;     |----------------
    	    a += 3;     |----------------
    	    return a;   |----------------
    	}               |----------------

    Below are some ways of setting diff anchors to get the above result.  In each
    example, 'diffopt' needs to have `anchor` set for this to take effect.

    Marks: Set the |'a| mark on the `int foo()` lines in each file first before
    setting the anchors:
    	set diffanchors='a

    Pattern: Specify the anchor using a |pattern| (see |:/|).  Here, we make sure
    to always start search from line 1 for consistency:
    	set diffanchors=1/int\ foo(/

    Selection: Use visual mode to select the entire `foo()` function body in each
    file.  Here, we use two anchors.  This does a better job of making sure only
    the function bodies are anchored against each other but not the lines after
    it.  Note the `'>+1` below.  The "+1" is necessary as we want the split to
    happen below the last line of the function, not above:
    	set diffanchors='<,'>+1

    Manually set two anchors using line numbers via buffer-local options:
    	setlocal diffanchors=1,5
    	wincmd w
    	setlocal diffanchors=7,11

    6. Diff options						*diff-options*

    Also see 'diffopt' and the "diff" item of 'fillchars'.

    					    *diff-slow* *diff_translations*
    For very long lines, the diff syntax highlighting might be slow, especially
    since it tries to match all different kind of localisations. To disable
    localisations and speed up the syntax highlighting, set the global variable
    g:diff_translations to zero:

        let g:diff_translations = 0

    After setting this variable, reload the syntax script:

        set syntax=diff

    FINDING THE DIFFERENCES					*diff-diffexpr*

    The 'diffexpr' option can be set to use something else than the internal diff
    support or the standard "diff" program to compare two files and find the
    differences. *E959*

    When 'diffexpr' is empty, Vim uses this command to find the differences
    between file1 and file2:

    	diff file1 file2 > outfile

    The ">" is replaced with the value of 'shellredir'.

    The output of "diff" must be a normal "ed" style diff or a unified diff.  A
    context diff will NOT work.  For a unified diff no context lines can be used.
    Using "diff -u" will NOT work, use "diff -U0".

    This example explains the format that Vim expects for the "ed" style diff:

    	1a2
    	> bbb
    	4d4
    	< 111
    	7c7
    	< GGG

    	> ggg

    The "1a2" item appends the line "bbb".
    The "4d4" item deletes the line "111".
    The "7c7" item replaces the line "GGG" with "ggg".

    When 'diffexpr' is not empty, Vim evaluates it to obtain a diff file in the
    format mentioned.  These variables are set to the file names used:

    	v:fname_in		original file
    	v:fname_new		new version of the same file
    	v:fname_out		where to write the resulting diff file

    Additionally, 'diffexpr' should take care of "icase" and "iwhite" in the
    'diffopt' option.  'diffexpr' cannot change the value of 'lines' and
    'columns'.

    The advantage of using a function call without arguments is that it is faster,
    see |expr-option-function|.

    Example (this does almost the same as 'diffexpr' being empty):

    	set diffexpr=MyDiff()
    	function MyDiff()
    	   let opt = ""
    	   if &diffopt =~ "icase"
    	     let opt = opt .. "-i "
    	   endif
    	   if &diffopt =~ "iwhite"
    	     let opt = opt .. "-b "
    	   endif
    	   silent execute "!diff -a --binary " .. opt .. v:fname_in .. " " .. v:fname_new ..
    		\  " > " .. v:fname_out
    	   redraw!
    	endfunction

    The "-a" argument is used to force comparing the files as text, comparing as
    binaries isn't useful.  The "--binary" argument makes the files read in binary
    mode, so that a CTRL-Z doesn't end the text on DOS.

    The `redraw!` command may not be needed, depending on whether executing a
    shell command shows something on the display or not.

    If the 'diffexpr' expression starts with s: or |<SID>|, then it is replaced
    with the script ID (|local-function|). Example:
    		set diffexpr=s:MyDiffExpr()
    		set diffexpr=<SID>SomeDiffExpr()
    Otherwise, the expression is evaluated in the context of the script where the
    option was set, thus script-local items are available.

    						*E810* *E97*
    Vim will do a test if the diff output looks alright.  If it doesn't, you will
    get an error message.  Possible causes:
    -  The "diff" program cannot be executed.
    -  The "diff" program doesn't produce normal "ed" style diffs (see above).
    -  The 'shell' and associated options are not set correctly.  Try if filtering
       works with a command like ":!sort".
    -  You are using 'diffexpr' and it doesn't work.
    If it's not clear what the problem is set the 'verbose' option to one or more
    to see more messages.

    The self-installing Vim for MS-Windows includes a diff program.  If you don't
    have it you might want to download a diff.exe.  For example from
    http://gnuwin32.sourceforge.net/packages/diffutils.htm.

    USING PATCHES					*diff-patchexpr*

    The 'patchexpr' option can be set to use something else than the standard
    "patch" program.

    When 'patchexpr' is empty, Vim will call the "patch" program like this:

    	patch -o outfile origfile < patchfile

    This should work fine with most versions of the "patch" program.  Note that a
    CR in the middle of a line may cause problems, it is seen as a line break.

    If the default doesn't work for you, set the 'patchexpr' to an expression that
    will have the same effect.  These variables are set to the file names used:

    	v:fname_in		original file
    	v:fname_diff		patch file
    	v:fname_out		resulting patched file

    The advantage of using a function call without arguments is that it is faster,
    see |expr-option-function|.

    Example (this does the same as 'patchexpr' being empty):

    	set patchexpr=MyPatch()
    	function MyPatch()
    	   :call system("patch -o " .. v:fname_out .. " " .. v:fname_in ..
    	   \  " < " .. v:fname_diff)
    	endfunction

    Make sure that using the "patch" program doesn't have unwanted side effects.
    For example, watch out for additionally generated files, which should be
    deleted.  It should just patch the file and nothing else.
       Vim will change directory to "/tmp" or another temp directory before
    evaluating 'patchexpr'.  This hopefully avoids that files in the current
    directory are accidentally patched.  Vim will also delete files starting with
    v:fname_in and ending in ".rej" and ".orig".

    If the 'patchexpr' expression starts with s: or |<SID>|, then it is replaced
    with the script ID (|local-function|). Example:
    		set patchexpr=s:MyPatchExpr()
    		set patchexpr=<SID>SomePatchExpr()
    Otherwise, the expression is evaluated in the context of the script where the
    option was set, thus script-local items are available.

    DIFF FUNCTION EXAMPLES				*diff-func-examples*

    Some examples for using the |diff()| function to compute the diff indices
    between two Lists of strings are below.

      " some lines are changed
      :echo diff(['abc', 'def', 'ghi'], ['abx', 'rrr', 'xhi'], {'output': 'indices'})
       [{'from_idx': 0, 'from_count': 3, 'to_idx': 0, 'to_count': 3}]

      " few lines added at the beginning
      :echo diff(['ghi'], ['abc', 'def', 'ghi'], {'output': 'indices'})
       [{'from_idx': 0, 'from_count': 0, 'to_idx': 0, 'to_count': 2}]

      " few lines removed from the beginning
      :echo diff(['abc', 'def', 'ghi'], ['ghi'], {'output': 'indices'})
       [{'from_idx': 0, 'from_count': 2, 'to_idx': 0, 'to_count': 0}]

      " few lines added in the middle
      :echo diff(['abc', 'jkl'], ['abc', 'def', 'ghi', 'jkl'], {'output': 'indices'})
       [{'from_idx': 1, 'from_count': 0, 'to_idx': 1, 'to_count': 2}]

      " few lines removed in the middle
      :echo diff(['abc', 'def', 'ghi', 'jkl'], ['abc', 'jkl'], {'output': 'indices'})
       [{'from_idx': 1, 'from_count': 2, 'to_idx': 1, 'to_count': 0}]

      " few lines added at the end
      :echo diff(['abc'], ['abc', 'def', 'ghi'], {'output': 'indices'})
       [{'from_idx': 1, 'from_count': 0, 'to_idx': 1, 'to_count': 2}]

      " few lines removed from the end
      :echo diff(['abc', 'def', 'ghi'], ['abc'], {'output': 'indices'})
       [{'from_idx': 1, 'from_count': 2, 'to_idx': 1, 'to_count': 0}]

      " disjointed changes
      :echo diff(['ab', 'def', 'ghi', 'jkl'], ['abc', 'def', 'ghi', 'jk'], {'output': 'indices', 'context': 0})
       [{'from_idx': 0, 'from_count': 1, 'to_idx': 0, 'to_count': 1},
        {'from_idx': 3, 'from_count': 1, 'to_idx': 3, 'to_count': 1}]

      " disjointed changes with context length 1
      :echo diff(['ab', 'def', 'ghi', 'jkl'], ['abc', 'def', 'ghi', 'jk'], {'output': 'indices', 'context': 1})
       [{'from_idx': 0, 'from_count': 4, 'to_idx': 0, 'to_count': 4}]

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

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