Comparing and Merging Files - 7. Merging From a Common Ancestor

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# 7. Merging From a Common Ancestor

When two people have made changes to copies of the same file, `diff3` can produce a merged output that contains both sets of changes together with warnings about conflicts.

One might imagine programs with names like `diff4` and `diff5` to compare more than three files simultaneously, but in practice the need rarely arises. You can use `diff3` to merge three or more sets of changes to a file by merging two change sets at a time.

`diff3` can incorporate changes from two modified versions into a common preceding version. This lets you merge the sets of changes represented by the two newer files. Specify the common ancestor version as the second argument and the two newer versions as the first and third arguments, like this:

    diff3 mine older yours

You can remember the order of the arguments by noting that they are in alphabetical order.

You can think of this as subtracting older from yours and adding the result to mine, or as merging into mine the changes that would turn older into yours. This merging is well-defined as long as mine and older match in the neighborhood of each such change. This fails to be true when all three input files differ or when only older differs; we call this a conflict. When all three input files differ, we call the conflict an overlap.

`diff3` gives you several ways to handle overlaps and conflicts. You can omit overlaps or conflicts, or select only overlaps, or mark conflicts with special ``<<<<<<<'` and ``>>>>>>>'` lines.

`diff3` can output the merge results as an `ed` script that that can be applied to the first file to yield the merged output. However, it is usually better to have `diff3` generate the merged output directly; this bypasses some problems with `ed`.

## 7.1 Selecting Which Changes to Incorporate

You can select all unmerged changes from older to yours for merging into mine with the ``-e'` or ``--ed'` option. You can select only the nonoverlapping unmerged changes with ``-3'` or ``--easy-only'`, and you can select only the overlapping changes with ``-x'` or ``--overlap-only'`.

The ``-e'`, ``-3'` and ``-x'` options select only unmerged changes, i.e. changes where mine and yours differ; they ignore changes from older to yours where mine and yours are identical, because they assume that such changes have already been merged. If this assumption is not a safe one, you can use the ``-A'` or ``--show-all'` option (see section 7.2 Marking Conflicts).

Here is the output of the command `diff3` with each of these three options (see section 6.1 A Third Sample Input File, for the complete contents of the files). Notice that ``-e'` outputs the union of the disjoint sets of changes output by ``-3'` and ``-x'`.

Output of ``diff3 -e lao tzu tao'`:

    11a

      -- The Way of Lao-Tzu, tr. Wing-tsit Chan
    .
    8c
      so we may see their result.
    .

Output of ``diff3 -3 lao tzu tao'`:

    8c
      so we may see their result.
    .

Output of ``diff3 -x lao tzu tao'`:

    11a

      -- The Way of Lao-Tzu, tr. Wing-tsit Chan
    .

## 7.2 Marking Conflicts

`diff3` can mark conflicts in the merged output by bracketing them with special marker lines. A conflict that comes from two files A and B is marked as follows:

    <<<<<<< A
    lines from A
    =======
    lines from B
    >>>>>>> B

A conflict that comes from three files A, B and C is marked as follows:

    <<<<<<< A
    lines from A
    ||||||| B
    lines from B
    =======
    lines from C
    >>>>>>> C

The ``-A'` or ``--show-all'` option acts like the ``-e'` option, except that it brackets conflicts, and it outputs all changes from older to yours, not just the unmerged changes. Thus, given the sample input files (see section 6.1 A Third Sample Input File), ``diff3 -A lao tzu tao'` puts brackets around the conflict where only ``tzu'` differs:

    <<<<<<< tzu
    =======
    The Way that can be told of is not the eternal Way;
    The name that can be named is not the eternal name.
    >>>>>>> tao

And it outputs the three-way conflict as follows:

    <<<<<<< lao
    ||||||| tzu
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!
    =======

      -- The Way of Lao-Tzu, tr. Wing-tsit Chan
    >>>>>>> tao

The ``-E'` or ``--show-overlap'` option outputs less information than the ``-A'` or ``--show-all'` option, because it outputs only unmerged changes, and it never outputs the contents of the second file. Thus the ``-E'` option acts like the ``-e'` option, except that it brackets the first and third files from three-way overlapping changes. Similarly, ``-X'` acts like ``-x'`, except it brackets all its (necessarily overlapping) changes. For example, for the three-way overlapping change above, the ``-E'` and ``-X'` options output the following:

    <<<<<<< lao
    =======

      -- The Way of Lao-Tzu, tr. Wing-tsit Chan
    >>>>>>> tao

If you are comparing files that have meaningless or uninformative names, you can use the ``-L label'` or ``--label=label'` option to show alternate names in the ``<<<<<<<'`, ``|||||||'` and ``>>>>>>>'` brackets. This option can be given up to three times, once for each input file. Thus ``diff3 -A -L X -L Y -L Z A B C'` acts like ``diff3 -A A B C'`, except that the output looks like it came from files named ``X'`, ``Y'` and ``Z'` rather than from files named ``A'`, ``B'` and ``C'`.

## 7.3 Generating the Merged Output Directly

With the ``-m'` or ``--merge'` option, `diff3` outputs the merged file directly. This is more efficient than using `ed` to generate it, and works even with non-text files that `ed` would reject. If you specify ``-m'` without an `ed` script option, ``-A'` (``--show-all'`) is assumed.

For example, the command ``diff3 -m lao tzu tao'` (see section 6.1 A Third Sample Input File for a copy of the input files) would output the following:

    <<<<<<< tzu
    =======
    The Way that can be told of is not the eternal Way;
    The name that can be named is not the eternal name.
    >>>>>>> tao
    The Nameless is the origin of Heaven and Earth;
    The Named is the mother of all things.
    Therefore let there always be non-being,
      so we may see their subtlety,
    And let there always be being,
      so we may see their result.
    The two are the same,
    But after they are produced,
      they have different names.
    <<<<<<< lao
    ||||||| tzu
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!
    =======

      -- The Way of Lao-Tzu, tr. Wing-tsit Chan
    >>>>>>> tao

## 7.4 How `diff3` Merges Incomplete Lines

With ``-m'`, incomplete lines (see section 16. Incomplete Lines) are simply copied to the output as they are found; if the merged output ends in an conflict and one of the input files ends in an incomplete line, succeeding ``|||||||'`, ``======='` or ``>>>>>>>'` brackets appear somewhere other than the start of a line because they are appended to the incomplete line.

Without ``-m'`, if an `ed` script option is specified and an incomplete line is found, `diff3` generates a warning and acts as if a newline had been present.

## 7.5 Saving the Changed File

Traditional Unix `diff3` generates an `ed` script without the trailing ``w'` and and ``q'` commands that save the changes. System V `diff3` generates these extra commands. GNU `diff3` normally behaves like traditional Unix `diff3`, but with the ``-i'` option it behaves like System V `diff3` and appends the ``w'` and ``q'` commands.

The ``-i'` option requires one of the `ed` script options ``-AeExX3'`, and is incompatible with the merged output option ``-m'`.

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