Diff mode is a major mode used for the output of M-x diff and other similar commands. This kind of output is called a patch, because it can be passed to the patch command to automatically apply the specified changes. To select Diff mode manually, type M-x diff-mode.
The changes specified in a patch are grouped into hunks, which are contiguous chunks of text that contain one or more changed lines. Hunks can also include unchanged lines to provide context for the changes. Each hunk is preceded by a hunk header, which specifies the old and new line numbers at which the hunk occurs. Diff mode highlights each hunk header, to distinguish it from the actual contents of the hunk.
You can edit a Diff mode buffer like any other buffer. (If it is
read-only, you need to make it writable first. See Misc Buffer.)
Whenever you change a hunk, Diff mode attempts to automatically
correct the line numbers in the hunk headers, to ensure that the patch
remains “correct”. To disable automatic line number correction,
change the variable
Diff mode treats each hunk as an “error message”, similar to Compilation mode. Thus, you can use commands such as C-x ' to visit the corresponding source locations. See Compilation Mode.
In addition, Diff mode provides the following commands to navigate, manipulate and apply parts of patches:
This command has a side effect: it refines the hunk you move to, highlighting its changes with better granularity. To disable this feature, type M-x diff-auto-refine-mode to toggle off the minor mode Diff Auto-Refine mode. To disable Diff Auto Refine mode by default, add this to your init file (see Hooks):
(add-hook 'diff-mode-hook (lambda () (diff-auto-refine-mode -1)))
diff-hunk-prev). Like M-n, this has the side-effect of refining the hunk you move to, unless you disable Diff Auto-Refine mode.
diff-apply-hunk). With a prefix argument of C-u, revert this hunk.
diff-refine-hunk). This allows you to see exactly which parts of each changed line were actually changed.
diff-ediff-patch). See Ediff.
diff-restrict-view). See Narrowing. With a prefix argument of C-u, restrict the view to the current file of a multiple-file patch. To widen again, use C-x n w (
diff-split-hunk). This is for manually editing patches, and only works with the unified diff format produced by the -u or --unified options to the diff program. If you need to split a hunk in the context diff format produced by the -c or --context options to diff, first convert the buffer to the unified diff format with C-c C-u.
diff-unified->context). With a prefix argument, convert only the text within the region.
diff-context->unified). With a prefix argument, convert unified format to context format. When the mark is active, convert only the text within the region.
diff-add-change-log-entries-other-window). This creates a skeleton of the log of changes that you can later fill with the actual descriptions of the changes. C-x 4 a itself in Diff mode operates on behalf of the current hunk's file, but gets the function name from the patch itself. This is useful for making log entries for functions that are deleted by the patch.
Patches sometimes include trailing whitespace on modified lines, as an unintentional and undesired change. There are two ways to deal with this problem. Firstly, if you enable Whitespace mode in a Diff buffer (see Useless Whitespace), it automatically highlights trailing whitespace in modified lines. Secondly, you can use the command M-x diff-delete-trailing-whitespace, which searches for trailing whitespace in the lines modified by the patch, and removes that whitespace in both the patch and the patched source file(s). This command does not save the modifications that it makes, so you can decide whether to save the changes (the list of modified files is displayed in the echo area). With a prefix argument, it tries to modify the original source files rather than the patched source files.