Comparing and Merging Files - 2. diff Output Formats

Go to the first, previous, next, last section, table of contents.

# 2. `diff` Output Formats

`diff` has several mutually exclusive options for output format. The following sections describe each format, illustrating how `diff` reports the differences between two sample input files.

## 2.1 Two Sample Input Files

Here are two sample files that we will use in numerous examples to illustrate the output of `diff` and how various options can change it.

This is the file ``lao'`:

    The Way that can be told of is not the eternal Way;
    The name that can be named is not the eternal name.
    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 outcome.
    The two are the same,
    But after they are produced,
      they have different names.

This is the file ``tzu'`:

    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 outcome.
    The two are the same,
    But after they are produced,
      they have different names.
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!

In this example, the first hunk contains just the first two lines of ``lao'`, the second hunk contains the fourth line of ``lao'` opposing the second and third lines of ``tzu'`, and the last hunk contains just the last three lines of ``tzu'`.

## 2.2 Showing Differences Without Context

The "normal" `diff` output format shows each hunk of differences without any surrounding context. Sometimes such output is the clearest way to see how lines have changed, without the clutter of nearby unchanged lines (although you can get similar results with the context or unified formats by using 0 lines of context). However, this format is no longer widely used for sending out patches; for that purpose, the context format (see section 2.3.1 Context Format) and the unified format (see section 2.3.2 Unified Format) are superior. Normal format is the default for compatibility with older versions of `diff` and the Posix standard.

### 2.2.1 Detailed Description of Normal Format

The normal output format consists of one or more hunks of differences; each hunk shows one area where the files differ. Normal format hunks look like this:

    change-command
    < from-file-line
    < from-file-line...
    ---
    > to-file-line
    > to-file-line...

There are three types of change commands. Each consists of a line number or comma-separated range of lines in the first file, a single character indicating the kind of change to make, and a line number or comma-separated range of lines in the second file. All line numbers are the original line numbers in each file. The types of change commands are:

``lar'`
  Add the lines in range r of the second file after line l of the first file. For example, ``8a12,15'` means append lines 12--15 of file 2 after line 8 of file 1; or, if changing file 2 into file 1, delete lines 12--15 of file 2.
``fct'`
  Replace the lines in range f of the first file with lines in range t of the second file. This is like a combined add and delete, but more compact. For example, ``5,7c8,10'` means change lines 5--7 of file 1 to read as lines 8--10 of file 2; or, if changing file 2 into file 1, change lines 8--10 of file 2 to read as lines 5--7 of file 1.
``rdl'`
  Delete the lines in range r from the first file; line l is where they would have appeared in the second file had they not been deleted. For example, ``5,7d3'` means delete lines 5--7 of file 1; or, if changing file 2 into file 1, append lines 5--7 of file 1 after line 3 of file 2.

### 2.2.2 An Example of Normal Format

Here is the output of the command ``diff lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files). Notice that it shows only the lines that are different between the two files.

    1,2d0
    < The Way that can be told of is not the eternal Way;
    < The name that can be named is not the eternal name.
    4c2,3
    < The Named is the mother of all things.
    ---
    > The named is the mother of all things.
    >
    11a11,13
    > They both may be called deep and profound.
    > Deeper and more profound,
    > The door of all subtleties!

## 2.3 Showing Differences in Their Context

Usually, when you are looking at the differences between files, you will also want to see the parts of the files near the lines that differ, to help you understand exactly what has changed. These nearby parts of the files are called the context.

GNU `diff` provides two output formats that show context around the differing lines: context format and unified format. It can optionally show in which function or section of the file the differing lines are found.

If you are distributing new versions of files to other people in the form of `diff` output, you should use one of the output formats that show context so that they can apply the diffs even if they have made small changes of their own to the files. `patch` can apply the diffs in this case by searching in the files for the lines of context around the differing lines; if those lines are actually a few lines away from where the diff says they are, `patch` can adjust the line numbers accordingly and still apply the diff correctly. See section 9.2 Applying Imperfect Patches, for more information on using `patch` to apply imperfect diffs.

### 2.3.1 Context Format

The context output format shows several lines of context around the lines that differ. It is the standard format for distributing updates to source code.

To select this output format, use the ``-C lines'`, ``--context[=lines]'`, or ``-c'` option. The argument lines that some of these options take is the number of lines of context to show. If you do not specify lines, it defaults to three. For proper operation, `patch` typically needs at least two lines of context.

#### 2.3.1.1 Detailed Description of Context Format

The context output format starts with a two-line header, which looks like this:

    *** from-file from-file-modification-time
    --- to-file to-file-modification time

You can change the header's content with the ``-L label'` or ``--label=label'` option; see section 2.3.4 Showing Alternate File Names.

Next come one or more hunks of differences; each hunk shows one area where the files differ. Context format hunks look like this:

    ***************
    *** from-file-line-range ****
      from-file-line
      from-file-line...
    --- to-file-line-range ----
      to-file-line
      to-file-line...

The lines of context around the lines that differ start with two space characters. The lines that differ between the two files start with one of the following indicator characters, followed by a space character:

``!'`
  A line that is part of a group of one or more lines that changed between the two files. There is a corresponding group of lines marked with ``!'` in the part of this hunk for the other file.
``+'`
  An "inserted" line in the second file that corresponds to nothing in the first file.
``-'`
  A "deleted" line in the first file that corresponds to nothing in the second file.

If all of the changes in a hunk are insertions, the lines of from-file are omitted. If all of the changes are deletions, the lines of to-file are omitted.

#### 2.3.1.2 An Example of Context Format

Here is the output of ``diff -c lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files). Notice that up to three lines that are not different are shown around each line that is different; they are the context lines. Also notice that the first two hunks have run together, because their contents overlap.

    *** lao	Sat Jan 26 23:30:39 1991
    --- tzu	Sat Jan 26 23:30:50 1991
    ***************
    *** 1,7 ****
    - The Way that can be told of is not the eternal Way;
    - The name that can be named is not the eternal name.
      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,
    --- 1,6 ----
      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,
    ***************
    *** 9,11 ****
    --- 8,13 ----
      The two are the same,
      But after they are produced,
        they have different names.
    + They both may be called deep and profound.
    + Deeper and more profound,
    + The door of all subtleties!

#### 2.3.1.3 An Example of Context Format with Less Context

Here is the output of ``diff --context=1 lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files). Notice that at most one context line is reported here.

    *** lao	Sat Jan 26 23:30:39 1991
    --- tzu	Sat Jan 26 23:30:50 1991
    ***************
    *** 1,5 ****
    - The Way that can be told of is not the eternal Way;
    - The name that can be named is not the eternal name.
      The Nameless is the origin of Heaven and Earth;
    ! The Named is the mother of all things.
      Therefore let there always be non-being,
    --- 1,4 ----
      The Nameless is the origin of Heaven and Earth;
    ! The named is the mother of all things.
    !
      Therefore let there always be non-being,
    ***************
    *** 11 ****
    --- 10,13 ----
        they have different names.
    + They both may be called deep and profound.
    + Deeper and more profound,
    + The door of all subtleties!

### 2.3.2 Unified Format

The unified output format is a variation on the context format that is more compact because it omits redundant context lines. To select this output format, use the ``-U lines'`, ``--unified[=lines]'`, or ``-u'` option. The argument lines is the number of lines of context to show. When it is not given, it defaults to three.

At present, only GNU `diff` can produce this format and only GNU `patch` can automatically apply diffs in this format. For proper operation, `patch` typically needs at least two lines of context.

#### 2.3.2.1 Detailed Description of Unified Format

The unified output format starts with a two-line header, which looks like this:

    --- from-file from-file-modification-time
    +++ to-file to-file-modification-time

You can change the header's content with the ``-L label'` or ``--label=label'` option; see See section 2.3.4 Showing Alternate File Names.

Next come one or more hunks of differences; each hunk shows one area where the files differ. Unified format hunks look like this:

    @@ from-file-range to-file-range @@
     line-from-either-file
     line-from-either-file...

The lines common to both files begin with a space character. The lines that actually differ between the two files have one of the following indicator characters in the left column:

``+'`
  A line was added here to the first file.
``-'`
  A line was removed here from the first file.

#### 2.3.2.2 An Example of Unified Format

Here is the output of the command ``diff -u lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files):

    --- lao	Sat Jan 26 23:30:39 1991
    +++ tzu	Sat Jan 26 23:30:50 1991
    @@ -1,7 +1,6 @@
    -The Way that can be told of is not the eternal Way;
    -The name that can be named is not the eternal name.
     The Nameless is the origin of Heaven and Earth;
    -The Named is the mother of all things.
    +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,
    @@ -9,3 +8,6 @@
     The two are the same,
     But after they are produced,
       they have different names.
    +They both may be called deep and profound.
    +Deeper and more profound,
    +The door of all subtleties!

### 2.3.3 Showing Which Sections Differences Are in

Sometimes you might want to know which part of the files each change falls in. If the files are source code, this could mean which function was changed. If the files are documents, it could mean which chapter or appendix was changed. GNU `diff` can show this by displaying the nearest section heading line that precedes the differing lines. Which lines are "section headings" is determined by a regular expression.

#### 2.3.3.1 Showing Lines That Match Regular Expressions

To show in which sections differences occur for files that are not source code for C or similar languages, use the ``-F regexp'` or ``--show-function-line=regexp'` option. `diff` considers lines that match the argument regexp to be the beginning of a section of the file. Here are suggested regular expressions for some common languages:

``^[A-Za-z_]'`
  C, C++, Prolog
``^('`
  Lisp
``^@\(chapter\|appendix\|unnumbered\|chapheading\)'`
  Texinfo

This option does not automatically select an output format; in order to use it, you must select the context format (see section 2.3.1 Context Format) or unified format (see section 2.3.2 Unified Format). In other output formats it has no effect.

The ``-F'` and ``--show-function-line'` options find the nearest unchanged line that precedes each hunk of differences and matches the given regular expression. Then they add that line to the end of the line of asterisks in the context format, or to the ``@@'` line in unified format. If no matching line exists, they leave the output for that hunk unchanged. If that line is more than 40 characters long, they output only the first 40 characters. You can specify more than one regular expression for such lines; `diff` tries to match each line against each regular expression, starting with the last one given. This means that you can use ``-p'` and ``-F'` together, if you wish.

#### 2.3.3.2 Showing C Function Headings

To show in which functions differences occur for C and similar languages, you can use the ``-p'` or ``--show-c-function'` option. This option automatically defaults to the context output format (see section 2.3.1 Context Format), with the default number of lines of context. You can override that number with ``-C lines'` elsewhere in the command line. You can override both the format and the number with ``-U lines'` elsewhere in the command line.

The ``-p'` and ``--show-c-function'` options are equivalent to ``-F'^[_a-zA-Z$]''` if the unified format is specified, otherwise ``-c -F'^[_a-zA-Z$]''` (see section 2.3.3.1 Showing Lines That Match Regular Expressions). GNU `diff` provides them for the sake of convenience.

### 2.3.4 Showing Alternate File Names

If you are comparing two files that have meaningless or uninformative names, you might want `diff` to show alternate names in the header of the context and unified output formats. To do this, use the ``-L label'` or ``--label=label'` option. The first time you give this option, its argument replaces the name and date of the first file in the header; the second time, its argument replaces the name and date of the second file. If you give this option more than twice, `diff` reports an error. The ``-L'` option does not affect the file names in the `pr` header when the ``-l'` or ``--paginate'` option is used (see section 4.2 Paginating `diff` Output).

Here are the first two lines of the output from ``diff -C2 -Loriginal -Lmodified lao tzu'`:

    *** original
    --- modified

## 2.4 Showing Differences Side by Side

`diff` can produce a side by side difference listing of two files. The files are listed in two columns with a gutter between them. The gutter contains one of the following markers:

white space
  The corresponding lines are in common. That is, either the lines are identical, or the difference is ignored because of one of the ``--ignore'` options (see section 1.2 Suppressing Differences in Blank and Tab Spacing).
``|'`
  The corresponding lines differ, and they are either both complete or both incomplete.
``<'`
  The files differ and only the first file contains the line.
``>'`
  The files differ and only the second file contains the line.
``('`
  Only the first file contains the line, but the difference is ignored.
``)'`
  Only the second file contains the line, but the difference is ignored.
``\'`
  The corresponding lines differ, and only the first line is incomplete.
``/'`
  The corresponding lines differ, and only the second line is incomplete.

Normally, an output line is incomplete if and only if the lines that it contains are incomplete; See section 16. Incomplete Lines. However, when an output line represents two differing lines, one might be incomplete while the other is not. In this case, the output line is complete, but its the gutter is marked ``\'` if the first line is incomplete, ``/'` if the second line is.

Side by side format is sometimes easiest to read, but it has limitations. It generates much wider output than usual, and truncates lines that are too long to fit. Also, it relies on lining up output more heavily than usual, so its output looks particularly bad if you use varying width fonts, nonstandard tab stops, or nonprinting characters.

You can use the `sdiff` command to interactively merge side by side differences. See section 8. Interactive Merging with `sdiff`, for more information on merging files.

## 2.5 Controlling Side by Side Format

The ``-y'` or ``--side-by-side'` option selects side by side format. Because side by side output lines contain two input lines, they are wider than usual. They are normally 130 columns, which can fit onto a traditional printer line. You can set the length of output lines with the ``-W columns'` or ``--width=columns'` option. The output line is split into two halves of equal length, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.

The ``--left-column'` option prints only the left column of two common lines. The ``--suppress-common-lines'` option suppresses common lines entirely.

### 2.5.1 An Example of Side by Side Format

Here is the output of the command ``diff -y -W 72 lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files).

    The Way that can be told of is n   <
    The name that can be named is no   <
    The Nameless is the origin of He        The Nameless is the origin of He
    The Named is the mother of all t   |    The named is the mother of all t
                                       >
    Therefore let there always be no        Therefore let there always be no
      so we may see their subtlety,           so we may see their subtlety,
    And let there always be being,          And let there always be being,
      so we may see their outcome.            so we may see their outcome.
    The two are the same,                   The two are the same,
    But after they are produced,            But after they are produced,
      they have different names.              they have different names.
                                       >    They both may be called deep and
                                       >    Deeper and more profound,
                                       >    The door of all subtleties!

## 2.6 Making Edit Scripts

Several output modes produce command scripts for editing from-file to produce to-file.

### 2.6.1 `ed` Scripts

`diff` can produce commands that direct the `ed` text editor to change the first file into the second file. Long ago, this was the only output mode that was suitable for editing one file into another automatically; today, with `patch`, it is almost obsolete. Use the ``-e'` or ``--ed'` option to select this output format.

Like the normal format (see section 2.2 Showing Differences Without Context), this output format does not show any context; unlike the normal format, it does not include the information necessary to apply the diff in reverse (to produce the first file if all you have is the second file and the diff).

If the file ``d'` contains the output of ``diff -e old new'`, then the command ``(cat d && echo w) | ed - old'` edits ``old'` to make it a copy of ``new'`. More generally, if ``d1'`, ``d2'`, ..., ``dN'` contain the outputs of ``diff -e old new1'`, ``diff -e new1 new2'`, ..., ``diff -e newN-1 newN'`, respectively, then the command ``(cat d1 d2 ... dN && echo w) | ed - old'` edits ``old'` to make it a copy of ``newN'`.

#### 2.6.1.1 Detailed Description of `ed` Format

The `ed` output format consists of one or more hunks of differences. The changes closest to the ends of the files come first so that commands that change the number of lines do not affect how `ed` interprets line numbers in succeeding commands. `ed` format hunks look like this:

    change-command
    to-file-line
    to-file-line...
    .

Because `ed` uses a single period on a line to indicate the end of input, GNU `diff` protects lines of changes that contain a single period on a line by writing two periods instead, then writing a subsequent `ed` command to change the two periods into one. The `ed` format cannot represent an incomplete line, so if the second file ends in a changed incomplete line, `diff` reports an error and then pretends that a newline was appended.

There are three types of change commands. Each consists of a line number or comma-separated range of lines in the first file and a single character indicating the kind of change to make. All line numbers are the original line numbers in the file. The types of change commands are:

``la'`
  Add text from the second file after line l in the first file. For example, ``8a'` means to add the following lines after line 8 of file 1.
``rc'`
  Replace the lines in range r in the first file with the following lines. Like a combined add and delete, but more compact. For example, ``5,7c'` means change lines 5--7 of file 1 to read as the text file 2.
``rd'`
  Delete the lines in range r from the first file. For example, ``5,7d'` means delete lines 5--7 of file 1.

#### 2.6.1.2 Example `ed` Script

Here is the output of ``diff -e lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files):

    11a
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!
    .
    4c
    The named is the mother of all things.

    .
    1,2d

### 2.6.2 Forward `ed` Scripts

`diff` can produce output that is like an `ed` script, but with hunks in forward (front to back) order. The format of the commands is also changed slightly: command characters precede the lines they modify, spaces separate line numbers in ranges, and no attempt is made to disambiguate hunk lines consisting of a single period. Like `ed` format, forward `ed` format cannot represent incomplete lines.

Forward `ed` format is not very useful, because neither `ed` nor `patch` can apply diffs in this format. It exists mainly for compatibility with older versions of `diff`. Use the ``-f'` or ``--forward-ed'` option to select it.

### 2.6.3 RCS Scripts

The RCS output format is designed specifically for use by the Revision Control System, which is a set of free programs used for organizing different versions and systems of files. Use the ``-n'` or ``--rcs'` option to select this output format. It is like the forward `ed` format (see section 2.6.2 Forward `ed` Scripts), but it can represent arbitrary changes to the contents of a file because it avoids the forward `ed` format's problems with lines consisting of a single period and with incomplete lines. Instead of ending text sections with a line consisting of a single period, each command specifies the number of lines it affects; a combination of the ``a'` and ``d'` commands are used instead of ``c'`. Also, if the second file ends in a changed incomplete line, then the output also ends in an incomplete line.

Here is the output of ``diff -n lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files):

    d1 2
    d4 1
    a4 2
    The named is the mother of all things.

    a11 3
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!

## 2.7 Merging Files with If-then-else

You can use `diff` to merge two files of C source code. The output of `diff` in this format contains all the lines of both files. Lines common to both files are output just once; the differing parts are separated by the C preprocessor directives `#ifdef name` or `#ifndef name`, `#else`, and `#endif`. When compiling the output, you select which version to use by either defining or leaving undefined the macro name.

To merge two files, use `diff` with the ``-D name'` or ``--ifdef=name'` option. The argument name is the C preprocessor identifier to use in the `#ifdef` and `#ifndef` directives.

For example, if you change an instance of `wait (&s)` to `waitpid (-1, &s, 0)` and then merge the old and new files with the ``--ifdef=HAVE_WAITPID'` option, then the affected part of your code might look like this:

        do {
    #ifndef HAVE_WAITPID
            if ((w = wait (&s)) < 0  &&  errno != EINTR)
    #else /* HAVE_WAITPID */
            if ((w = waitpid (-1, &s, 0)) < 0  &&  errno != EINTR)
    #endif /* HAVE_WAITPID */
                return w;
        } while (w != child);

You can specify formats for languages other than C by using line group formats and line formats, as described in the next sections.

### 2.7.1 Line Group Formats

Line group formats let you specify formats suitable for many applications that allow if-then-else input, including programming languages and text formatting languages. A line group format specifies the output format for a contiguous group of similar lines.

For example, the following command compares the TeX files ``old'` and ``new'`, and outputs a merged file in which old regions are surrounded by ``\begin{em}'`-``\end{em}'` lines, and new regions are surrounded by ``\begin{bf}'`-``\end{bf}'` lines.

    diff \
       --old-group-format='\begin{em}
    %<\end{em}
    ' \
       --new-group-format='\begin{bf}
    %>\end{bf}
    ' \
       old new

The following command is equivalent to the above example, but it is a little more verbose, because it spells out the default line group formats.

    diff \
       --old-group-format='\begin{em}
    %<\end{em}
    ' \
       --new-group-format='\begin{bf}
    %>\end{bf}
    ' \
       --unchanged-group-format='%=' \
       --changed-group-format='\begin{em}
    %<\end{em}
    \begin{bf}
    %>\end{bf}
    ' \
       old new

Here is a more advanced example, which outputs a diff listing with headers containing line numbers in a "plain English" style.

    diff \
       --unchanged-group-format='' \
       --old-group-format='-------- %dn line%(n=1?:s) deleted at %df:
    %<' \
       --new-group-format='-------- %dN line%(N=1?:s) added after %de:
    %>' \
       --changed-group-format='-------- %dn line%(n=1?:s) changed at %df:
    %<-------- to:
    %>' \
       old new

To specify a line group format, use `diff` with one of the options listed below. You can specify up to four line group formats, one for each kind of line group. You should quote format, because it typically contains shell metacharacters.

``--old-group-format=format'`
  These line groups are hunks containing only lines from the first file. The default old group format is the same as the changed group format if it is specified; otherwise it is a format that outputs the line group as-is.
``--new-group-format=format'`
  These line groups are hunks containing only lines from the second file. The default new group format is same as the the changed group format if it is specified; otherwise it is a format that outputs the line group as-is.
``--changed-group-format=format'`
  These line groups are hunks containing lines from both files. The default changed group format is the concatenation of the old and new group formats.
``--unchanged-group-format=format'`
  These line groups contain lines common to both files. The default unchanged group format is a format that outputs the line group as-is.

In a line group format, ordinary characters represent themselves; conversion specifications start with ``%'` and have one of the following forms.

``%<'`
  stands for the lines from the first file, including the trailing newline. Each line is formatted according to the old line format (see section 2.7.2 Line Formats).
``%>'`
  stands for the lines from the second file, including the trailing newline. Each line is formatted according to the new line format.
``%='`
  stands for the lines common to both files, including the trailing newline. Each line is formatted according to the unchanged line format.
``%%'`
  stands for ``%'`.
``%c'C''`
  where C is a single character, stands for C. C may not be a backslash or an apostrophe. For example, ``%c':''` stands for a colon, even inside the then-part of an if-then-else format, which a colon would normally terminate.
``%c'\O''`
  where O is a string of 1, 2, or 3 octal digits, stands for the character with octal code O. For example, ``%c'\0''` stands for a null character.
``Fn'`
  where F is a `printf` conversion specification and n is one of the following letters, stands for n's value formatted with F.
  ``e'`
    The line number of the line just before the group in the old file.
  ``f'`
    The line number of the first line in the group in the old file; equals e + 1.
  ``l'`
    The line number of the last line in the group in the old file.
  ``m'`
    The line number of the line just after the group in the old file; equals l + 1.
  ``n'`
    The number of lines in the group in the old file; equals l - f + 1.
  ``E, F, L, M, N'`
    Likewise, for lines in the new file.
  The `printf` conversion specification can be ``%d'`, ``%o'`, ``%x'`, or ``%X'`, specifying decimal, octal, lower case hexadecimal, or upper case hexadecimal output respectively. After the ``%'` the following options can appear in sequence: a ``-'` specifying left-justification; an integer specifying the minimum field width; and a period followed by an optional integer specifying the minimum number of digits. For example, ``%5dN'` prints the number of new lines in the group in a field of width 5 characters, using the `printf` format `"%5d"`.
``(A=B?T:E)'`
  If A equals B then T else E. A and B are each either a decimal constant or a single letter interpreted as above. This format spec is equivalent to T if A's value equals B's; otherwise it is equivalent to E. For example, ``%(N=0?no:%dN) line%(N=1?:s)'` is equivalent to ``no lines'` if N (the number of lines in the group in the the new file) is 0, to ``1 line'` if N is 1, and to ``%dN lines'` otherwise.

### 2.7.2 Line Formats

Line formats control how each line taken from an input file is output as part of a line group in if-then-else format.

For example, the following command outputs text with a one-column change indicator to the left of the text. The first column of output is ``-'` for deleted lines, ``|'` for added lines, and a space for unchanged lines. The formats contain newline characters where newlines are desired on output.

    diff \
       --old-line-format='-%l
    ' \
       --new-line-format='|%l
    ' \
       --unchanged-line-format=' %l
    ' \
       old new

To specify a line format, use one of the following options. You should quote format, since it often contains shell metacharacters.

``--old-line-format=format'`
  formats lines just from the first file.
``--new-line-format=format'`
  formats lines just from the second file.
``--unchanged-line-format=format'`
  formats lines common to both files.
``--line-format=format'`
  formats all lines; in effect, it sets all three above options simultaneously.

In a line format, ordinary characters represent themselves; conversion specifications start with ``%'` and have one of the following forms.

``%l'`
  stands for the the contents of the line, not counting its trailing newline (if any). This format ignores whether the line is incomplete; See section 16. Incomplete Lines.
``%L'`
  stands for the the contents of the line, including its trailing newline (if any). If a line is incomplete, this format preserves its incompleteness.
``%%'`
  stands for ``%'`.
``%c'C''`
  where C is a single character, stands for C. C may not be a backslash or an apostrophe. For example, ``%c':''` stands for a colon.
``%c'\O''`
  where O is a string of 1, 2, or 3 octal digits, stands for the character with octal code O. For example, ``%c'\0''` stands for a null character.
``Fn'`
  where F is a `printf` conversion specification, stands for the line number formatted with F. For example, ``%.5dn'` prints the line number using the `printf` format `"%.5d"`. See section 2.7.1 Line Group Formats, for more about printf conversion specifications.

The default line format is ``%l'` followed by a newline character.

If the input contains tab characters and it is important that they line up on output, you should ensure that ``%l'` or ``%L'` in a line format is just after a tab stop (e.g. by preceding ``%l'` or ``%L'` with a tab character), or you should use the ``-t'` or ``--expand-tabs'` option.

Taken together, the line and line group formats let you specify many different formats. For example, the following command uses a format similar to `diff`'s normal format. You can tailor this command to get fine control over `diff`'s output.

    diff \
       --old-line-format='< %l
    ' \
       --new-line-format='> %l
    ' \
       --old-group-format='%df%(f=l?:,%dl)d%dE
    %<' \
       --new-group-format='%dea%dF%(F=L?:,%dL)
    %>' \
       --changed-group-format='%df%(f=l?:,%dl)c%dF%(F=L?:,%dL)
    %<---
    %>' \
       --unchanged-group-format='' \
       old new

### 2.7.3 Detailed Description of If-then-else Format

For lines common to both files, `diff` uses the unchanged line group format. For each hunk of differences in the merged output format, if the hunk contains only lines from the first file, `diff` uses the old line group format; if the hunk contains only lines from the second file, `diff` uses the new group format; otherwise, `diff` uses the changed group format.

The old, new, and unchanged line formats specify the output format of lines from the first file, lines from the second file, and lines common to both files, respectively.

The option ``--ifdef=name'` is equivalent to the following sequence of options using shell syntax:

    --old-group-format='#ifndef name
    %<#endif /* not name */
    ' \
    --new-group-format='#ifdef name
    %>#endif /* name */
    ' \
    --unchanged-group-format='%=' \
    --changed-group-format='#ifndef name
    %<#else /* name */
    %>#endif /* name */
    '

You should carefully check the `diff` output for proper nesting. For example, when using the the ``-D name'` or ``--ifdef=name'` option, you should check that if the differing lines contain any of the C preprocessor directives ``#ifdef'`, ``#ifndef'`, ``#else'`, ``#elif'`, or ``#endif'`, they are nested properly and match. If they don't, you must make corrections manually. It is a good idea to carefully check the resulting code anyway to make sure that it really does what you want it to; depending on how the input files were produced, the output might contain duplicate or otherwise incorrect code.

The `patch` ``-D name'` option behaves just like the `diff` ``-D name'` option, except it operates on a file and a diff to produce a merged file; See section 14.4 Options to `patch`.

### 2.7.4 An Example of If-then-else Format

Here is the output of ``diff -DTWO lao tzu'` (see section 2.1 Two Sample Input Files, for the complete contents of the two files):

    #ifndef TWO
    The Way that can be told of is not the eternal Way;
    The name that can be named is not the eternal name.
    #endif /* not TWO */
    The Nameless is the origin of Heaven and Earth;
    #ifndef TWO
    The Named is the mother of all things.
    #else /* TWO */
    The named is the mother of all things.

    #endif /* TWO */
    Therefore let there always be non-being,
      so we may see their subtlety,
    And let there always be being,
      so we may see their outcome.
    The two are the same,
    But after they are produced,
      they have different names.
    #ifdef TWO
    They both may be called deep and profound.
    Deeper and more profound,
    The door of all subtleties!
    #endif /* TWO */

Go to the first, previous, next, last section, table of contents.