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Generating patch text with -p

Running git-diff[1], git-log[1], git-show[1], git-diff-index[1], git-diff-tree[1], or git-diff-files[1] with the -p option produces patch text. You can customize the creation of patch text via the GIT_EXTERNAL_DIFF and the GIT_DIFF_OPTS environment variables (see git[1]), and the diff attribute (see gitattributes[5]).

What the -p option produces is slightly different from the traditional diff format:

  1. It is preceded with a "git diff" header that looks like this:

    diff --git a/file1 b/file2

    The a/ and b/ filenames are the same unless rename/copy is involved. Especially, even for a creation or a deletion, /dev/null is not used in place of the a/ or b/ filenames.

    When rename/copy is involved, file1 and file2 show the name of the source file of the rename/copy and the name of the file that rename/copy produces, respectively.

  2. It is followed by one or more extended header lines:

    old mode <mode>
    new mode <mode>
    deleted file mode <mode>
    new file mode <mode>
    copy from <path>
    copy to <path>
    rename from <path>
    rename to <path>
    similarity index <number>
    dissimilarity index <number>
    index <hash>..<hash> <mode>

    File modes are printed as 6-digit octal numbers including the file type and file permission bits.

    Path names in extended headers do not include the a/ and b/ prefixes.

    The similarity index is the percentage of unchanged lines, and the dissimilarity index is the percentage of changed lines. It is a rounded down integer, followed by a percent sign. The similarity index value of 100% is thus reserved for two equal files, while 100% dissimilarity means that no line from the old file made it into the new one.

    The index line includes the blob object names before and after the change. The <mode> is included if the file mode does not change; otherwise, separate lines indicate the old and the new mode.

  3. Pathnames with "unusual" characters are quoted as explained for the configuration variable core.quotePath (see git-config[1]).

  4. All the file1 files in the output refer to files before the commit, and all the file2 files refer to files after the commit. It is incorrect to apply each change to each file sequentially. For example, this patch will swap a and b:

    diff --git a/a b/b
    rename from a
    rename to b
    diff --git a/b b/a
    rename from b
    rename to a
  5. Hunk headers mention the name of the function to which the hunk applies. See "Defining a custom hunk-header" in gitattributes[5] for details of how to tailor to this to specific languages.

Combined diff format

Any diff-generating command can take the -c or --cc option to produce a combined diff when showing a merge. This is the default format when showing merges with git-diff[1] or git-show[1]. Note also that you can give suitable --diff-merges option to any of these commands to force generation of diffs in specific format.

A "combined diff" format looks like this:

diff --combined describe.c
index fabadb8,cc95eb0..4866510
--- a/describe.c
+++ b/describe.c
@@@ -98,20 -98,12 +98,20 @@@
	return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
  }

- static void describe(char *arg)
 -static void describe(struct commit *cmit, int last_one)
++static void describe(char *arg, int last_one)
  {
 +	unsigned char sha1[20];
 +	struct commit *cmit;
	struct commit_list *list;
	static int initialized = 0;
	struct commit_name *n;

 +	if (get_sha1(arg, sha1) < 0)
 +		usage(describe_usage);
 +	cmit = lookup_commit_reference(sha1);
 +	if (!cmit)
 +		usage(describe_usage);
 +
	if (!initialized) {
		initialized = 1;
		for_each_ref(get_name);
  1. It is preceded with a "git diff" header, that looks like this (when the -c option is used):

    diff --combined file

    or like this (when the --cc option is used):

    diff --cc file
  2. It is followed by one or more extended header lines (this example shows a merge with two parents):

    index <hash>,<hash>..<hash>
    mode <mode>,<mode>..<mode>
    new file mode <mode>
    deleted file mode <mode>,<mode>

    The mode <mode>,<mode>..<mode> line appears only if at least one of the <mode> is different from the rest. Extended headers with information about detected contents movement (renames and copying detection) are designed to work with diff of two <tree-ish> and are not used by combined diff format.

  3. It is followed by two-line from-file/to-file header

    --- a/file
    +++ b/file

    Similar to two-line header for traditional unified diff format, /dev/null is used to signal created or deleted files.

    However, if the --combined-all-paths option is provided, instead of a two-line from-file/to-file you get a N+1 line from-file/to-file header, where N is the number of parents in the merge commit

    --- a/file
    --- a/file
    --- a/file
    +++ b/file

    This extended format can be useful if rename or copy detection is active, to allow you to see the original name of the file in different parents.

  4. Chunk header format is modified to prevent people from accidentally feeding it to patch -p1. Combined diff format was created for review of merge commit changes, and was not meant to be applied. The change is similar to the change in the extended index header:

    @@@ <from-file-range> <from-file-range> <to-file-range> @@@

    There are (number of parents + 1) @ characters in the chunk header for combined diff format.

Unlike the traditional unified diff format, which shows two files A and B with a single column that has - (minus — appears in A but removed in B), + (plus — missing in A but added to B), or " " (space — unchanged) prefix, this format compares two or more files file1, file2,…​ with one file X, and shows how X differs from each of fileN. One column for each of fileN is prepended to the output line to note how X’s line is different from it.

A - character in the column N means that the line appears in fileN but it does not appear in the result. A + character in the column N means that the line appears in the result, and fileN does not have that line (in other words, the line was added, from the point of view of that parent).

In the above example output, the function signature was changed from both files (hence two - removals from both file1 and file2, plus ++ to mean one line that was added does not appear in either file1 or file2). Also eight other lines are the same from file1 but do not appear in file2 (hence prefixed with +).

When shown by git diff-tree -c, it compares the parents of a merge commit with the merge result (i.e. file1..fileN are the parents). When shown by git diff-files -c, it compares the two unresolved merge parents with the working tree file (i.e. file1 is stage 2 aka "our version", file2 is stage 3 aka "their version").

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