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+Generating patches with -p
+When "git-diff-index", "git-diff-tree", or "git-diff-files" are run
+with a '-p' option, "git diff" without the '--raw' option, or
+"git log" with the "-p" option, they
+do not produce the output described above; instead they produce a
+patch file. You can customize the creation of such patches via the
+GIT_EXTERNAL_DIFF and the GIT_DIFF_OPTS environment variables.
+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 `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>
+3. TAB, LF, double quote and backslash characters in pathnames
+ are represented as `\t`, `\n`, `\"` and `\\`, respectively.
+ If there is need for such substitution then the whole
+ pathname is put in double quotes.
+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.
+combined diff format
+"git-diff-tree", "git-diff-files" and "git-diff" can take '-c' or
+'--cc' option to produce 'combined diff'. For showing a merge commit
+with "git log -p", this is the default 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 '-c' option is used):
+ diff --combined file
+or like this (when '--cc' option is used):
+ diff --c 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
+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 for apply. 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 last file,
+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 nor file2). Also two 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").