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+Generating patch text with -p
+-----------------------------
+
+Running
+linkgit:git-diff[1],
+linkgit:git-log[1],
+linkgit:git-show[1],
+linkgit:git-diff-index[1],
+linkgit:git-diff-tree[1], or
+linkgit: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 linkgit:git[1]), and the `diff` attribute (see linkgit: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
+ linkgit: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
+ linkgit: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 linkgit:git-diff[1] or
+linkgit: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").