# Preface

This file documents the the GNU `diff`, `diff3`, `sdiff`, and `cmp` commands for showing the differences between text files and the `patch` command for using their output to update files.

Copyright (C) 1992, 1993, 1994 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

This file documents the the GNU `diff`, `diff3`, `sdiff`, and `cmp` commands for showing the differences between text files and the `patch` command for using their output to update files.

This is Edition 1.8, dated 1994-11-14, for `diffutils` 2.7.1 and `patch` 2.1.

- Preliminary information.
- What file comparison means.
- Formats for difference reports.
- Comparing files and directories.
- Making `diff` output prettier.
- Making `diff` smarter or faster.
- Formats for three-way difference reports.
- Merging from a common ancestor.
- Interactive merging with `sdiff`.
- Using `patch` to change old files into new ones.
- Tips for making patch distributions.
- How to run `cmp` and a summary of its options.
- How to run `diff` and a summary of its options.
- How to run `diff3` and a summary of its options.
- How to run `patch` and a summary of its options.
- How to run `sdiff` and a summary of its options.
- Lines that lack trailing newlines.
- If you think you've found a bug or other shortcoming.
- Index of concepts.

# Overview

Computer users often find occasion to ask how two files differ. Perhaps one file is a newer version of the other file. Or maybe the two files started out as identical copies but were changed by different people.

You can use the `diff` command to show differences between two files, or each corresponding file in two directories. `diff` outputs differences between files line by line in any of several formats, selectable by command line options. This set of differences is often called a diff or patch. For files that are identical, `diff` normally produces no output; for binary (non-text) files, `diff` normally reports only that they are different.

You can use the `cmp` command to show the offsets and line numbers where two files differ. `cmp` can also show all the characters that differ between the two files, side by side. Another way to compare two files character by character is the Emacs command `M-x compare-windows`. See Section Other Window of

### The GNU Emacs Manual

, for more information on that command.

You can use the `diff3` command to show differences among three files. When two people have made independent changes to a common original, `diff3` can report the differences between the original and the two changed versions, and can produce a merged file that contains both persons' changes together with warnings about conflicts.

You can use the `sdiff` command to merge two files interactively.

You can use the set of differences produced by `diff` to distribute updates to text files (such as program source code) to other people. This method is especially useful when the differences are small compared to the complete files. Given `diff` output, you can use the `patch` program to update, or patch, a copy of the file. If you think of `diff` as subtracting one file from another to produce their difference, you can think of `patch` as adding the difference to one file to reproduce the other.

This manual first concentrates on making diffs, and later shows how to use diffs to update files.

GNU `diff` was written by Mike Haertel, David Hayes, Richard Stallman, Len Tower, and Paul Eggert. Wayne Davison designed and implemented the unified output format. The basic algorithm is described in ``An O(ND) Difference Algorithm and its Variations'', Eugene W. Myers, Algorithmica Vol. 1 No. 2, 1986, pp. 251--266; and in ``A File Comparison Program'', Webb Miller and Eugene W. Myers, Software---Practice and Experience Vol. 15 No. 11, 1985, pp. 1025--1040. The algorithm was independently discovered as described in ``Algorithms for Approximate String Matching'', E. Ukkonen, Information and Control Vol. 64, 1985, pp. 100--118. GNU `diff3` was written by Randy Smith. GNU `sdiff` was written by Thomas Lord. GNU `cmp` was written by Torbjorn Granlund and David MacKenzie.

`patch` was written mainly by Larry Wall; the GNU enhancements were written mainly by Wayne Davison and David MacKenzie. Parts of this manual are adapted from a manual page written by Larry Wall, with his permission.

# What Comparison Means

There are several ways to think about the differences between two files. One way to think of the differences is as a series of lines that were deleted from, inserted in, or changed in one file to produce the other file. `diff` compares two files line by line, finds groups of lines that differ, and reports each group of differing lines. It can report the differing lines in several formats, which have different purposes.

GNU `diff` can show whether files are different without detailing the differences. It also provides ways to suppress certain kinds of differences that are not important to you. Most commonly, such differences are changes in the amount of white space between words or lines. `diff` also provides ways to suppress differences in alphabetic case or in lines that match a regular expression that you provide. These options can accumulate; for example, you can ignore changes in both white space and alphabetic case.

Another way to think of the differences between two files is as a sequence of pairs of characters that can be either identical or different. `cmp` reports the differences between two files character by character, instead of line by line. As a result, it is more useful than `diff` for comparing binary files. For text files, `cmp` is useful mainly when you want to know only whether two files are identical.

To illustrate the effect that considering changes character by character can have compared with considering them line by line, think of what happens if a single newline character is added to the beginning of a file. If that file is then compared with an otherwise identical file that lacks the newline at the beginning, `diff` will report that a blank line has been added to the file, while `cmp` will report that almost every character of the two files differs.

`diff3` normally compares three input files line by line, finds groups of lines that differ, and reports each group of differing lines. Its output is designed to make it easy to inspect two different sets of changes to the same file.

- Groups of differing lines.
- Suppressing differences in white space.
- Suppressing differences in blank lines.
- Suppressing differences in alphabetic case.
- Suppressing differences that match regular expressions.
- Summarizing which files are different.
- Comparing binary files or forcing text comparisons.

## Hunks

When comparing two files, `diff` finds sequences of lines common to both files, interspersed with groups of differing lines called hunks. Comparing two identical files yields one sequence of common lines and no hunks, because no lines differ. Comparing two entirely different files yields no common lines and one large hunk that contains all lines of both files. In general, there are many ways to match up lines between two given files. `diff` tries to minimize the total hunk size by finding large sequences of common lines interspersed with small hunks of differing lines.

For example, suppose the file `F` contains the three lines `a`, `b`, `c`, and the file `G` contains the same three lines in reverse order `c`, `b`, `a`. If `diff` finds the line `c` as common, then the command `diff F G` produces this output:

    1,2d0
    < a
    < b
    3a2,3
    > b
    > a

But if `diff` notices the common line `b` instead, it produces this output:

    1c1
    < a
    ---
    > c
    3c3
    < c
    ---
    > a

It is also possible to find `a` as the common line. `diff` does not always find an optimal matching between the files; it takes shortcuts to run faster. But its output is usually close to the shortest possible. You can adjust this tradeoff with the `--minimal` option (see diff Performance).

## Suppressing Differences in Blank and Tab Spacing

The `-b` and `--ignore-space-change` options ignore white space at line end, and considers all other sequences of one or more white space characters to be equivalent. With these options, `diff` considers the following two lines to be equivalent, where `$` denotes the line end:

    Here lyeth  muche rychnesse  in lytell space.   -- John Heywood$
    Here lyeth muche rychnesse in lytell space. -- John Heywood   $

The `-w` and `--ignore-all-space` options are stronger than `-b`. They ignore difference even if one file has white space where the other file has none. White space characters include tab, newline, vertical tab, form feed, carriage return, and space; some locales may define additional characters to be white space. With these options, `diff` considers the following two lines to be equivalent, where `$` denotes the line end and `^M` denotes a carriage return:

    Here lyeth  muche  rychnesse in lytell space.--  John Heywood$
      He relyeth much erychnes  seinly tells pace.  --John Heywood   ^M$

## Suppressing Differences in Blank Lines

The `-B` and `--ignore-blank-lines` options ignore insertions or deletions of blank lines. These options affect only lines that are completely empty; they do not affect lines that look empty but contain space or tab characters. With these options, for example, a file containing

    1.  A point is that which has no part.
    2.  A line is breadthless length.
    -- Euclid, The Elements, I

is considered identical to a file containing

    1.  A point is that which has no part.
    2.  A line is breadthless length.
    -- Euclid, The Elements, I

## Suppressing Case Differences

GNU `diff` can treat lowercase letters as equivalent to their uppercase counterparts, so that, for example, it considers `Funky Stuff`, `funky STUFF`, and `fUNKy stuFf` to all be the same. To request this, use the `-i` or `--ignore-case` option.

## Suppressing Lines Matching a Regular Expression

To ignore insertions and deletions of lines that match a regular expression, use the `-I regexp` or `--ignore-matching-lines=regexp` option. You should escape regular expressions that contain shell metacharacters to prevent the shell from expanding them. For example, `diff -I '^[0-9]'` ignores all changes to lines beginning with a digit.

However, `-I` only ignores the insertion or deletion of lines that contain the regular expression if every changed line in the hunk---every insertion and every deletion---matches the regular expression. In other words, for each nonignorable change, `diff` prints the complete set of changes in its vicinity, including the ignorable ones.

You can specify more than one regular expression for lines to ignore by using more than one `-I` option. `diff` tries to match each line against each regular expression, starting with the last one given.

## Summarizing Which Files Differ

When you only want to find out whether files are different, and you don't care what the differences are, you can use the summary output format. In this format, instead of showing the differences between the files, `diff` simply reports whether files differ. The `-q` and `--brief` options select this output format.

This format is especially useful when comparing the contents of two directories. It is also much faster than doing the normal line by line comparisons, because `diff` can stop analyzing the files as soon as it knows that there are any differences.

You can also get a brief indication of whether two files differ by using `cmp`. For files that are identical, `cmp` produces no output. When the files differ, by default, `cmp` outputs the byte offset and line number where the first difference occurs. You can use the `-s` option to suppress that information, so that `cmp` produces no output and reports whether the files differ using only its exit status (see Invoking cmp).

Unlike `diff`, `cmp` cannot compare directories; it can only compare two files.

## Binary Files and Forcing Text Comparisons

If `diff` thinks that either of the two files it is comparing is binary (a non-text file), it normally treats that pair of files much as if the summary output format had been selected (see Brief), and reports only that the binary files are different. This is because line by line comparisons are usually not meaningful for binary files.

`diff` determines whether a file is text or binary by checking the first few bytes in the file; the exact number of bytes is system dependent, but it is typically several thousand. If every character in that part of the file is non-null, `diff` considers the file to be text; otherwise it considers the file to be binary.

Sometimes you might want to force `diff` to consider files to be text. For example, you might be comparing text files that contain null characters; `diff` would erroneously decide that those are non-text files. Or you might be comparing documents that are in a format used by a word processing system that uses null characters to indicate special formatting. You can force `diff` to consider all files to be text files, and compare them line by line, by using the `-a` or `--text` option. If the files you compare using this option do not in fact contain text, they will probably contain few newline characters, and the `diff` output will consist of hunks showing differences between long lines of whatever characters the files contain.

You can also force `diff` to consider all files to be binary files, and report only whether they differ (but not how). Use the `--brief` option for this.

In operating systems that distinguish between text and binary files, `diff` normally reads and writes all data as text. Use the `--binary` option to force `diff` to read and write binary data instead. This option has no effect on a Posix-compliant system like GNU or traditional Unix. However, many personal computer operating systems represent the end of a line with a carriage return followed by a newline. On such systems, `diff` normally ignores these carriage returns on input and generates them at the end of each output line, but with the `--binary` option `diff` treats each carriage return as just another input character, and does not generate a carriage return at the end of each output line. This can be useful when dealing with non-text files that are meant to be interchanged with Posix-compliant systems.

If you want to compare two files byte by byte, you can use the `cmp` program with the `-l` option to show the values of each differing byte in the two files. With GNU `cmp`, you can also use the `-c` option to show the ASCII representation of those bytes. See Invoking cmp, for more information.

If `diff3` thinks that any of the files it is comparing is binary (a non-text file), it normally reports an error, because such comparisons are usually not useful. `diff3` uses the same test as `diff` to decide whether a file is binary. As with `diff`, if the input files contain a few non-text characters but otherwise are like text files, you can force `diff3` to consider all files to be text files and compare them line by line by using the `-a` or `--text` options.

# `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.

- Sample `diff` input files for examples.
- Showing differences without surrounding text.
- Showing differences with the surrounding text.
- Showing differences in two columns.
- Generating scripts for other programs.
- Merging files with if-then-else.

## 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`.

## 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 Context Format) and the unified format (see Unified Format) are superior. Normal format is the default for compatibility with older versions of `diff` and the Posix standard.

- A detailed description of normal output format.
- Sample output in the normal format.

### 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.

### An Example of Normal Format

Here is the output of the command `diff lao tzu` (see Sample diff Input, 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!

## 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 Imperfect, for more information on using `patch` to apply imperfect diffs.

- An output format that shows surrounding lines.
- A more compact output format that shows context.
- Showing which sections of the files differences are in.
- Showing alternate file names in context headers.

### 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.

- A detailed description of the context output format.
- Sample output in context format.
- Another sample with less context.

#### 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 Alternate 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.

#### An Example of Context Format

Here is the output of `diff -c lao tzu` (see Sample diff Input, 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!

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

Here is the output of `diff --context=1 lao tzu` (see Sample diff Input, 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!

### 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.

- A detailed description of unified format.
- Sample output in unified format.

#### 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 Alternate 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.

#### An Example of Unified Format

Here is the output of the command `diff -u lao tzu` (see Sample diff Input, 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!

### 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.

- Showing headings that match regular expressions.
- Showing headings of C functions.

#### 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 Context Format) or unified format (see 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.

#### 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 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 Specified Headings). GNU `diff` provides them for the sake of convenience.

### 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 Pagination).

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

    *** original
    --- modified

## 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 White Space).
`|`
  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 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 Interactive Merging, for more information on merging files.

- Controlling side by side output format.
- Sample side by side output.

## 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.

### An Example of Side by Side Format

Here is the output of the command `diff -y -W 72 lao tzu` (see Sample diff Input, 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!

## Making Edit Scripts

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

- Using `diff` to produce commands for `ed`.
- Making forward `ed` scripts.
- A special `diff` output format used by RCS.

### `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 Normal), 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`.

- A detailed description of `ed` format.
- A sample `ed` script.

#### 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.

#### Example `ed` Script

Here is the output of `diff -e lao tzu` (see Sample diff Input, 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

### 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.

### 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 Forward ed), 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 Sample diff Input, 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!

## 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.

- Formats for general if-then-else line groups.
- Formats for each line in a line group.
- A detailed description of if-then-else format.
- Sample if-then-else format output.

### 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 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.
F n
  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.

### 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 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 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

### 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 /* ! 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 patch Options.

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

Here is the output of `diff -DTWO lao tzu` (see Sample diff Input, 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 /* ! 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 */

# Comparing Directories

You can use `diff` to compare some or all of the files in two directory trees. When both file name arguments to `diff` are directories, it compares each file that is contained in both directories, examining file names in alphabetical order. Normally `diff` is silent about pairs of files that contain no differences, but if you use the `-s` or `--report-identical-files` option, it reports pairs of identical files. Normally `diff` reports subdirectories common to both directories without comparing subdirectories' files, but if you use the `-r` or `--recursive` option, it compares every corresponding pair of files in the directory trees, as many levels deep as they go.

For file names that are in only one of the directories, `diff` normally does not show the contents of the file that exists; it reports only that the file exists in that directory and not in the other. You can make `diff` act as though the file existed but was empty in the other directory, so that it outputs the entire contents of the file that actually exists. (It is output as either an insertion or a deletion, depending on whether it is in the first or the second directory given.) To do this, use the `-N` or `--new-file` option.

If the older directory contains one or more large files that are not in the newer directory, you can make the patch smaller by using the `-P` or `--unidirectional-new-file` option instead of `-N`. This option is like `-N` except that it only inserts the contents of files that appear in the second directory but not the first (that is, files that were added). At the top of the patch, write instructions for the user applying the patch to remove the files that were deleted before applying the patch. See Making Patches, for more discussion of making patches for distribution.

To ignore some files while comparing directories, use the `-x pattern` or `--exclude=pattern` option. This option ignores any files or subdirectories whose base names match the shell pattern pattern. Unlike in the shell, a period at the start of the base of a file name matches a wildcard at the start of a pattern. You should enclose pattern in quotes so that the shell does not expand it. For example, the option `-x '*.[ao]'` ignores any file whose name ends with `.a` or `.o`.

This option accumulates if you specify it more than once. For example, using the options `-x 'RCS' -x '*,v'` ignores any file or subdirectory whose base name is `RCS` or ends with `,v`.

If you need to give this option many times, you can instead put the patterns in a file, one pattern per line, and use the `-X file` or `--exclude-from=file` option.

If you have been comparing two directories and stopped partway through, later you might want to continue where you left off. You can do this by using the `-S file` or `--starting-file=file` option. This compares only the file file and all alphabetically later files in the topmost directory level.

# Making `diff` Output Prettier

`diff` provides several ways to adjust the appearance of its output. These adjustments can be applied to any output format.

- Preserving the alignment of tabstops.
- Page numbering and timestamping `diff` output.

## Preserving Tabstop Alignment

The lines of text in some of the `diff` output formats are preceded by one or two characters that indicate whether the text is inserted, deleted, or changed. The addition of those characters can cause tabs to move to the next tabstop, throwing off the alignment of columns in the line. GNU `diff` provides two ways to make tab-aligned columns line up correctly.

The first way is to have `diff` convert all tabs into the correct number of spaces before outputting them; select this method with the `-t` or `--expand-tabs` option. `diff` assumes that tabstops are set every 8 columns. To use this form of output with `patch`, you must give `patch` the `-l` or `--ignore-white-space` option (see Changed White Space, for more information).

The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line. This ensures that all following tab characters are in the same position relative to tabstops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the `-T` or `--initial-tab` option.

## Paginating `diff` Output

It can be convenient to have long output page-numbered and time-stamped. The `-l` and `--paginate` options do this by sending the `diff` output through the `pr` program. Here is what the page header might look like for `diff -lc lao tzu`:

    Mar 11 13:37 1991  diff -lc lao tzu Page 1

# `diff` Performance Tradeoffs

GNU `diff` runs quite efficiently; however, in some circumstances you can cause it to run faster or produce a more compact set of changes. There are two ways that you can affect the performance of GNU `diff` by changing the way it compares files.

Performance has more than one dimension. These options improve one aspect of performance at the cost of another, or they improve performance in some cases while hurting it in others.

The way that GNU `diff` determines which lines have changed always comes up with a near-minimal set of differences. Usually it is good enough for practical purposes. If the `diff` output is large, you might want `diff` to use a modified algorithm that sometimes produces a smaller set of differences. The `-d` or `--minimal` option does this; however, it can also cause `diff` to run more slowly than usual, so it is not the default behavior.

When the files you are comparing are large and have small groups of changes scattered throughout them, you can use the `-H` or `--speed-large-files` option to make a different modification to the algorithm that `diff` uses. If the input files have a constant small density of changes, this option speeds up the comparisons without changing the output. If not, `diff` might produce a larger set of differences; however, the output will still be correct.

Normally `diff` discards the prefix and suffix that is common to both files before it attempts to find a minimal set of differences. This makes `diff` run faster, but occasionally it may produce non-minimal output. The `--horizon-lines=lines` option prevents `diff` from discarding the last lines lines of the prefix and the first lines lines of the suffix. This gives `diff` further opportunities to find a minimal output.

# Comparing Three Files

Use the program `diff3` to compare three files and show any differences among them. (`diff3` can also merge files; see diff3 Merging).

The ``normal'' `diff3` output format shows each hunk of differences without surrounding context. Hunks are labeled depending on whether they are two-way or three-way, and lines are annotated by their location in the input files.

See Invoking diff3, for more information on how to run `diff3`.

- Sample `diff3` input for examples.
- A detailed description of normal output format.
- The format of normal output format.
- Sample output in the normal format.

## A Third Sample Input File

Here is a third sample file that will be used in examples to illustrate the output of `diff3` and how various options can change it. The first two files are the same that we used for `diff` (see Sample diff Input). This is the third sample file, called `tao`:

    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 result.
    The two are the same,
    But after they are produced,
      they have different names.
      -- The Way of Lao-Tzu, tr. Wing-tsit Chan

## Detailed Description of `diff3` Normal Format

Each hunk begins with a line marked `====`. Three-way hunks have plain `====` lines, and two-way hunks have `1`, `2`, or `3` appended to specify which of the three input files differ in that hunk. The hunks contain copies of two or three sets of input lines each preceded by one or two commands identifying where the lines came from.

Normally, two spaces precede each copy of an input line to distinguish it from the commands. But with the `-T` or `--initial-tab` option, `diff3` uses a tab instead of two spaces; this lines up tabs correctly. See Tabs, for more information.

Commands take the following forms:

file:la
  This hunk appears after line l of file file, and contains no lines in that file. To edit this file to yield the other files, one must append hunk lines taken from the other files. For example, `1:11a` means that the hunk follows line 11 in the first file and contains no lines from that file.
file:rc
  This hunk contains the lines in the range r of file file. The range r is a comma-separated pair of line numbers, or just one number if the range is a singleton. To edit this file to yield the other files, one must change the specified lines to be the lines taken from the other files. For example, `2:11,13c` means that the hunk contains lines 11 through 13 from the second file.

If the last line in a set of input lines is incomplete (see Incomplete Lines), it is distinguished on output from a full line by a following line that starts with `\`.

## `diff3` Hunks

Groups of lines that differ in two or three of the input files are called diff3 hunks, by analogy with `diff` hunks (see Hunks). If all three input files differ in a `diff3` hunk, the hunk is called a three-way hunk; if just two input files differ, it is a two-way hunk.

As with `diff`, several solutions are possible. When comparing the files `A`, `B`, and `C`, `diff3` normally finds `diff3` hunks by merging the two-way hunks output by the two commands `diff A B` and `diff A C`. This does not necessarily minimize the size of the output, but exceptions should be rare.

For example, suppose `F` contains the three lines `a`, `b`, `f`, `G` contains the lines `g`, `b`, `g`, and `H` contains the lines `a`, `b`, `h`. `diff3 F G H` might output the following:

    ====2
    1:1c
    3:1c
      a
    2:1c
      g
    ====
    1:3c
      f
    2:3c
      g
    3:3c
      h

because it found a two-way hunk containing `a` in the first and third files and `g` in the second file, then the single line `b` common to all three files, then a three-way hunk containing the last line of each file.

## An Example of `diff3` Normal Format

Here is the output of the command `diff3 lao tzu tao` (see Sample diff3 Input, for the complete contents of the files). Notice that it shows only the lines that are different among the three files.

    ====2
    1:1,2c
    3:1,2c
      The Way that can be told of is not the eternal Way;
      The name that can be named is not the eternal name.
    2:0a
    ====1
    1:4c
      The Named is the mother of all things.
    2:2,3c
    3:4,5c
      The named is the mother of all things.

    ====3
    1:8c
    2:7c
        so we may see their outcome.
    3:9c
        so we may see their result.
    ====
    1:11a
    2:11,13c
      They both may be called deep and profound.
      Deeper and more profound,
      The door of all subtleties!
    3:13,14c

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

# 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`.

- Selecting changes to incorporate.
- Marking conflicts.
- Generating merged output directly.
- How `diff3` merges incomplete lines.
- Emulating System V behavior.

## 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 Marking Conflicts).

Here is the output of the command `diff3` with each of these three options (see Sample diff3 Input, 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
    .

## 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 Sample diff3 Input), `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`.

## 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 Sample diff3 Input 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

## How `diff3` Merges Incomplete Lines

With `-m`, incomplete lines (see 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.

## 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`.

# Interactive Merging with `sdiff`

With `sdiff`, you can merge two files interactively based on a side-by-side `-y` format comparison (see Side by Side). Use `-o file` or `--output=file` to specify where to put the merged text. See Invoking sdiff, for more details on the options to `sdiff`.

Another way to merge files interactively is to use the Emacs Lisp package `emerge`. See Section emerge of

### The GNU Emacs Manual

, for more information.

- Summary of `sdiff` options.
- Merging two files interactively.

## Specifying `diff` Options to `sdiff`

The following `sdiff` options have the same meaning as for `diff`. See diff Options, for the use of these options.

    -a -b -d -i -t -v
    -B -H -I regexp
    --ignore-blank-lines  --ignore-case
    --ignore-matching-lines=regexp  --ignore-space-change
    --left-column  --minimal  --speed-large-files
    --suppress-common-lines  --expand-tabs
    --text  --version  --width=columns

For historical reasons, `sdiff` has alternate names for some options. The `-l` option is equivalent to the `--left-column` option, and similarly `-s` is equivalent to `--suppress-common-lines`. The meaning of the `sdiff` `-w` and `-W` options is interchanged from that of `diff`: with `sdiff`, `-w columns` is equivalent to `--width=columns`, and `-W` is equivalent to `--ignore-all-space`. `sdiff` without the `-o` option is equivalent to `diff` with the `-y` or `--side-by-side` option (see Side by Side).

## Merge Commands

Groups of common lines, with a blank gutter, are copied from the first file to the output. After each group of differing lines, `sdiff` prompts with `%` and pauses, waiting for one of the following commands. Follow each command with `RET`.

e
  Discard both versions. Invoke a text editor on an empty temporary file, then copy the resulting file to the output.
eb
  Concatenate the two versions, edit the result in a temporary file, then copy the edited result to the output.
el
  Edit a copy of the left version, then copy the result to the output.
er
  Edit a copy of the right version, then copy the result to the output.
l
  Copy the left version to the output.
q
  Quit.
r
  Copy the right version to the output.
s
  Silently copy common lines.
v
  Verbosely copy common lines. This is the default.

The text editor invoked is specified by the `EDITOR` environment variable if it is set. The default is system-dependent.

# Merging with `patch`

`patch` takes comparison output produced by `diff` and applies the differences to a copy of the original file, producing a patched version. With `patch`, you can distribute just the changes to a set of files instead of distributing the entire file set; your correspondents can apply `patch` to update their copy of the files with your changes. `patch` automatically determines the diff format, skips any leading or trailing headers, and uses the headers to determine which file to patch. This lets your correspondents feed an article or message containing a difference listing directly to `patch`.

`patch` detects and warns about common problems like forward patches. It saves the original version of the files it patches, and saves any patches that it could not apply. It can also maintain a `patchlevel.h` file to ensures that your correspondents apply diffs in the proper order.

`patch` accepts a series of diffs in its standard input, usually separated by headers that specify which file to patch. It applies `diff` hunks (see Hunks) one by one. If a hunk does not exactly match the original file, `patch` uses heuristics to try to patch the file as well as it can. If no approximate match can be found, `patch` rejects the hunk and skips to the next hunk. `patch` normally replaces each file f with its new version, saving the original file in `f.orig`, and putting reject hunks (if any) into `f.rej`.

See Invoking patch, for detailed information on the options to `patch`. See Backups, for more information on how `patch` names backup files. See Rejects, for more information on where `patch` puts reject hunks.

- Selecting the type of `patch` input.
- Dealing with imperfect patches.
- Removing empty files after patching.
- Handling multiple patches in a file specially.
- Messages and questions `patch` can produce.

## Selecting the `patch` Input Format

`patch` normally determines which `diff` format the patch file uses by examining its contents. For patch files that contain particularly confusing leading text, you might need to use one of the following options to force `patch` to interpret the patch file as a certain format of diff. The output formats listed here are the only ones that `patch` can understand.

-c
--context
  context diff.
-e
--ed
  `ed` script.
-n
--normal
  normal diff.
-u
--unified
  unified diff.

## Applying Imperfect Patches

`patch` tries to skip any leading text in the patch file, apply the diff, and then skip any trailing text. Thus you can feed a news article or mail message directly to `patch`, and it should work. If the entire diff is indented by a constant amount of white space, `patch` automatically ignores the indentation.

However, certain other types of imperfect input require user intervention.

- When tabs and spaces don't match exactly.
- Applying reversed patches correctly.
- Helping `patch` find close matches.

### Applying Patches with Changed White Space

Sometimes mailers, editors, or other programs change spaces into tabs, or vice versa. If this happens to a patch file or an input file, the files might look the same, but `patch` will not be able to match them properly. If this problem occurs, use the `-l` or `--ignore-white-space` option, which makes `patch` compare white space loosely so that any sequence of white space in the patch file matches any sequence of white space in the input files. Non-white-space characters must still match exactly. Each line of the context must still match a line in the input file.

### Applying Reversed Patches

Sometimes people run `diff` with the new file first instead of second. This creates a diff that is ``reversed''. To apply such patches, give `patch` the `-R` or `--reverse` option. `patch` then attempts to swap each hunk around before applying it. Rejects come out in the swapped format. The `-R` option does not work with `ed` scripts because there is too little information in them to reconstruct the reverse operation.

Often `patch` can guess that the patch is reversed. If the first hunk of a patch fails, `patch` reverses the hunk to see if it can apply it that way. If it can, `patch` asks you if you want to have the `-R` option set; if it can't, `patch` continues to apply the patch normally. This method cannot detect a reversed patch if it is a normal diff and the first command is an append (which should have been a delete) since appends always succeed, because a null context matches anywhere. But most patches add or change lines rather than delete them, so most reversed normal diffs begin with a delete, which fails, and `patch` notices.

If you apply a patch that you have already applied, `patch` thinks it is a reversed patch and offers to un-apply the patch. This could be construed as a feature. If you did this inadvertently and you don't want to un-apply the patch, just answer `n` to this offer and to the subsequent ``apply anyway'' question---or type `C-c` to kill the `patch` process.

### Helping `patch` Find Inexact Matches

For context diffs, and to a lesser extent normal diffs, `patch` can detect when the line numbers mentioned in the patch are incorrect, and it attempts to find the correct place to apply each hunk of the patch. As a first guess, it takes the line number mentioned in the hunk, plus or minus any offset used in applying the previous hunk. If that is not the correct place, `patch` scans both forward and backward for a set of lines matching the context given in the hunk.

First `patch` looks for a place where all lines of the context match. If it cannot find such a place, and it is reading a context or unified diff, and the maximum fuzz factor is set to 1 or more, then `patch` makes another scan, ignoring the first and last line of context. If that fails, and the maximum fuzz factor is set to 2 or more, it makes another scan, ignoring the first two and last two lines of context are ignored. It continues similarly if the maximum fuzz factor is larger.

The `-F lines` or `--fuzz=lines` option sets the maximum fuzz factor to lines. This option only applies to context and unified diffs; it ignores up to lines lines while looking for the place to install a hunk. Note that a larger fuzz factor increases the odds of making a faulty patch. The default fuzz factor is 2; it may not be set to more than the number of lines of context in the diff, ordinarily 3.

If `patch` cannot find a place to install a hunk of the patch, it writes the hunk out to a reject file (see Rejects, for information on how reject files are named). It writes out rejected hunks in context format no matter what form the input patch is in. If the input is a normal or `ed` diff, many of the contexts are simply null. The line numbers on the hunks in the reject file may be different from those in the patch file: they show the approximate location where `patch` thinks the failed hunks belong in the new file rather than in the old one.

As it completes each hunk, `patch` tells you whether the hunk succeeded or failed, and if it failed, on which line (in the new file) `patch` thinks the hunk should go. If this is different from the line number specified in the diff, it tells you the offset. A single large offset may indicate that `patch` installed a hunk in the wrong place. `patch` also tells you if it used a fuzz factor to make the match, in which case you should also be slightly suspicious.

`patch` cannot tell if the line numbers are off in an `ed` script, and can only detect wrong line numbers in a normal diff when it finds a change or delete command. It may have the same problem with a context diff using a fuzz factor equal to or greater than the number of lines of context shown in the diff (typically 3). In these cases, you should probably look at a context diff between your original and patched input files to see if the changes make sense. Compiling without errors is a pretty good indication that the patch worked, but not a guarantee.

`patch` usually produces the correct results, even when it must make many guesses. However, the results are guaranteed only when the patch is applied to an exact copy of the file that the patch was generated from.

## Removing Empty Files

Sometimes when comparing two directories, the first directory contains a file that the second directory does not. If you give `diff` the `-N` or `--new-file` option, it outputs a diff that deletes the contents of this file. By default, `patch` leaves an empty file after applying such a diff. The `-E` or `--remove-empty-files` option to `patch` deletes output files that are empty after applying the diff.

## Multiple Patches in a File

If the patch file contains more than one patch, `patch` tries to apply each of them as if they came from separate patch files. This means that it determines the name of the file to patch for each patch, and that it examines the leading text before each patch for file names and prerequisite revision level (see Making Patches, for more on that topic).

For the second and subsequent patches in the patch file, you can give options and another original file name by separating their argument lists with a `+`. However, the argument list for a second or subsequent patch may not specify a new patch file, since that does not make sense.

For example, to tell `patch` to strip the first three slashes from the name of the first patch in the patch file and none from subsequent patches, and to use `code.c` as the first input file, you can use:

    patch -p3 code.c + -p0 < patchfile

The `-S` or `--skip` option ignores the current patch from the patch file, but continue looking for the next patch in the file. Thus, to ignore the first and third patches in the patch file, you can use:

    patch -S + + -S + < patch file

## Messages and Questions from `patch`

`patch` can produce a variety of messages, especially if it has trouble decoding its input. In a few situations where it's not sure how to proceed, `patch` normally prompts you for more information from the keyboard. There are options to suppress printing non-fatal messages and stopping for keyboard input.

The message `Hmm...` indicates that `patch` is reading text in the patch file, attempting to determine whether there is a patch in that text, and if so, what kind of patch it is.

You can inhibit all terminal output from `patch`, unless an error occurs, by using the `-s`, `--quiet`, or `--silent` option.

There are two ways you can prevent `patch` from asking you any questions. The `-f` or `--force` option assumes that you know what you are doing. It assumes the following:

- skip patches that do not contain file names in their headers;
- patch files even though they have the wrong version for the `Prereq:` line in the patch;
- assume that patches are not reversed even if they look like they are.

The `-t` or `--batch` option is similar to `-f`, in that it suppresses questions, but it makes somewhat different assumptions:

- skip patches that do not contain file names in their headers (the same as `-f`);
- skip patches for which the file has the wrong version for the `Prereq:` line in the patch;
- assume that patches are reversed if they look like they are.

`patch` exits with a non-zero status if it creates any reject files. When applying a set of patches in a loop, you should check the exit status, so you don't apply a later patch to a partially patched file.

# Tips for Making Patch Distributions

Here are some things you should keep in mind if you are going to distribute patches for updating a software package.

Make sure you have specified the file names correctly, either in a context diff header or with an `Index:` line. If you are patching files in a subdirectory, be sure to tell the patch user to specify a `-p` or `--strip` option as needed. Take care to not send out reversed patches, since these make people wonder whether they have already applied the patch.

To save people from partially applying a patch before other patches that should have gone before it, you can make the first patch in the patch file update a file with a name like `patchlevel.h` or `version.c`, which contains a patch level or version number. If the input file contains the wrong version number, `patch` will complain immediately.

An even clearer way to prevent this problem is to put a `Prereq:` line before the patch. If the leading text in the patch file contains a line that starts with `Prereq:`, `patch` takes the next word from that line (normally a version number) and checks whether the next input file contains that word, preceded and followed by either white space or a newline. If not, `patch` prompts you for confirmation before proceeding. This makes it difficult to accidentally apply patches in the wrong order.

Since `patch` does not handle incomplete lines properly, make sure that all the source files in your program end with a newline whenever you release a version.

To create a patch that changes an older version of a package into a newer version, first make a copy of the older version in a scratch directory. Typically you do that by unpacking a `tar` or `shar` archive of the older version.

You might be able to reduce the size of the patch by renaming or removing some files before making the patch. If the older version of the package contains any files that the newer version does not, or if any files have been renamed between the two versions, make a list of `rm` and `mv` commands for the user to execute in the old version directory before applying the patch. Then run those commands yourself in the scratch directory.

If there are any files that you don't need to include in the patch because they can easily be rebuilt from other files (for example, `TAGS` and output from `yacc` and `makeinfo`), replace the versions in the scratch directory with the newer versions, using `rm` and `ln` or `cp`.

Now you can create the patch. The de-facto standard `diff` format for patch distributions is context format with two lines of context, produced by giving `diff` the `-C 2` option. Do not use less than two lines of context, because `patch` typically needs at least two lines for proper operation. Give `diff` the `-P` option in case the newer version of the package contains any files that the older one does not. Make sure to specify the scratch directory first and the newer directory second.

Add to the top of the patch a note telling the user any `rm` and `mv` commands to run before applying the patch. Then you can remove the scratch directory.

# Invoking `cmp`

The `cmp` command compares two files, and if they differ, tells the first byte and line number where they differ. Its arguments are as follows:

    cmp options··· from-file [to-file]

The file name `-` is always the standard input. `cmp` also uses the standard input if one file name is omitted.

An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.

- Summary of options to `cmp`.

## Options to `cmp`

Below is a summary of all of the options that GNU `cmp` accepts. Most options have two equivalent names, one of which is a single letter preceded by `-`, and the other of which is a long name preceded by `--`. Multiple single letter options (unless they take an argument) can be combined into a single command line word: `-cl` is equivalent to `-c -l`.

-c
  Print the differing characters. Display control characters as a `^` followed by a letter of the alphabet and precede characters that have the high bit set with `M-` (which stands for ``meta'').
--ignore-initial=bytes
  Ignore any differences in the the first bytes bytes of the input files. Treat files with fewer than bytes bytes as if they are empty.
-l
  Print the (decimal) offsets and (octal) values of all differing bytes.
--print-chars
  Print the differing characters. Display control characters as a `^` followed by a letter of the alphabet and precede characters that have the high bit set with `M-` (which stands for ``meta'').
--quiet
-s
--silent
  Do not print anything; only return an exit status indicating whether the files differ.
--verbose
  Print the (decimal) offsets and (octal) values of all differing bytes.
-v
--version
  Output the version number of `cmp`.

# Invoking `diff`

The format for running the `diff` command is:

    diff options··· from-file to-file

In the simplest case, `diff` compares the contents of the two files from-file and to-file. A file name of `-` stands for text read from the standard input. As a special case, `diff - -` compares a copy of standard input to itself.

If from-file is a directory and to-file is not, `diff` compares the file in from-file whose file name is that of to-file, and vice versa. The non-directory file must not be `-`.

If both from-file and to-file are directories, `diff` compares corresponding files in both directories, in alphabetical order; this comparison is not recursive unless the `-r` or `--recursive` option is given. `diff` never compares the actual contents of a directory as if it were a file. The file that is fully specified may not be standard input, because standard input is nameless and the notion of ``file with the same name'' does not apply.

`diff` options begin with `-`, so normally from-file and to-file may not begin with `-`. However, `--` as an argument by itself treats the remaining arguments as file names even if they begin with `-`.

An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.

- Summary of options to `diff`.

## Options to `diff`

Below is a summary of all of the options that GNU `diff` accepts. Most options have two equivalent names, one of which is a single letter preceded by `-`, and the other of which is a long name preceded by `--`. Multiple single letter options (unless they take an argument) can be combined into a single command line word: `-ac` is equivalent to `-a -c`. Long named options can be abbreviated to any unique prefix of their name. Brackets ([ and ]) indicate that an option takes an optional argument.

-lines
  Show lines (an integer) lines of context. This option does not specify an output format by itself; it has no effect unless it is combined with `-c` (see Context Format) or `-u` (see Unified Format). This option is obsolete. For proper operation, `patch` typically needs at least two lines of context.
-a
  Treat all files as text and compare them line-by-line, even if they do not seem to be text. See Binary.
-b
  Ignore changes in amount of white space. See White Space.
-B
  Ignore changes that just insert or delete blank lines. See Blank Lines.
--binary
  Read and write data in binary mode. See Binary.
--brief
  Report only whether the files differ, not the details of the differences. See Brief.
-c
  Use the context output format. See Context Format.
-C lines
--context[=lines]
  Use the context output format, showing lines (an integer) lines of context, or three if lines is not given. See Context Format. For proper operation, `patch` typically needs at least two lines of context.
--changed-group-format=format
  Use format to output a line group containing differing lines from both files in if-then-else format. See Line Group Formats.
-d
  Change the algorithm perhaps find a smaller set of changes. This makes `diff` slower (sometimes much slower). See diff Performance.
-D name
  Make merged `#ifdef` format output, conditional on the preprocessor macro name. See If-then-else.
-e
--ed
  Make output that is a valid `ed` script. See ed Scripts.
--exclude=pattern
  When comparing directories, ignore files and subdirectories whose basenames match pattern. See Comparing Directories.
--exclude-from=file
  When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file. See Comparing Directories.
--expand-tabs
  Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See Tabs.
-f
  Make output that looks vaguely like an `ed` script but has changes in the order they appear in the file. See Forward ed.
-F regexp
  In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp. See Specified Headings.
--forward-ed
  Make output that looks vaguely like an `ed` script but has changes in the order they appear in the file. See Forward ed.
-h
  This option currently has no effect; it is present for Unix compatibility.
-H
  Use heuristics to speed handling of large files that have numerous scattered small changes. See diff Performance.
--horizon-lines=lines
  Do not discard the last lines lines of the common prefix and the first lines lines of the common suffix. See diff Performance.
-i
  Ignore changes in case; consider upper- and lower-case letters equivalent. See Case Folding.
-I regexp
  Ignore changes that just insert or delete lines that match regexp. See Specified Folding.
--ifdef=name
  Make merged if-then-else output using name. See If-then-else.
--ignore-all-space
  Ignore white space when comparing lines. See White Space.
--ignore-blank-lines
  Ignore changes that just insert or delete blank lines. See Blank Lines.
--ignore-case
  Ignore changes in case; consider upper- and lower-case to be the same. See Case Folding.
--ignore-matching-lines=regexp
  Ignore changes that just insert or delete lines that match regexp. See Specified Folding.
--ignore-space-change
  Ignore changes in amount of white space. See White Space.
--initial-tab
  Output a tab rather than a space before the text of a line in normal or context format. This causes the alignment of tabs in the line to look normal. See Tabs.
-l
  Pass the output through `pr` to paginate it. See Pagination.
-L label
  Use label instead of the file name in the context format (see Context Format) and unified format (see Unified Format) headers. See RCS.
--label=label
  Use label instead of the file name in the context format (see Context Format) and unified format (see Unified Format) headers.
--left-column
  Print only the left column of two common lines in side by side format. See Side by Side Format.
--line-format=format
  Use format to output all input lines in if-then-else format. See Line Formats.
--minimal
  Change the algorithm to perhaps find a smaller set of changes. This makes `diff` slower (sometimes much slower). See diff Performance.
-n
  Output RCS-format diffs; like `-f` except that each command specifies the number of lines affected. See RCS.
-N
--new-file
  In directory comparison, if a file is found in only one directory, treat it as present but empty in the other directory. See Comparing Directories.
--new-group-format=format
  Use format to output a group of lines taken from just the second file in if-then-else format. See Line Group Formats.
--new-line-format=format
  Use format to output a line taken from just the second file in if-then-else format. See Line Formats.
--old-group-format=format
  Use format to output a group of lines taken from just the first file in if-then-else format. See Line Group Formats.
--old-line-format=format
  Use format to output a line taken from just the first file in if-then-else format. See Line Formats.
-p
  Show which C function each change is in. See C Function Headings.
-P
  When comparing directories, if a file appears only in the second directory of the two, treat it as present but empty in the other. See Comparing Directories.
--paginate
  Pass the output through `pr` to paginate it. See Pagination.
-q
  Report only whether the files differ, not the details of the differences. See Brief.
-r
  When comparing directories, recursively compare any subdirectories found. See Comparing Directories.
--rcs
  Output RCS-format diffs; like `-f` except that each command specifies the number of lines affected. See RCS.
--recursive
  When comparing directories, recursively compare any subdirectories found. See Comparing Directories.
--report-identical-files
  Report when two files are the same. See Comparing Directories.
-s
  Report when two files are the same. See Comparing Directories.
-S file
  When comparing directories, start with the file file. This is used for resuming an aborted comparison. See Comparing Directories.
--sdiff-merge-assist
  Print extra information to help `sdiff`. `sdiff` uses this option when it runs `diff`. This option is not intended for users to use directly.
--show-c-function
  Show which C function each change is in. See C Function Headings.
--show-function-line=regexp
  In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp. See Specified Headings.
--side-by-side
  Use the side by side output format. See Side by Side Format.
--speed-large-files
  Use heuristics to speed handling of large files that have numerous scattered small changes. See diff Performance.
--starting-file=file
  When comparing directories, start with the file file. This is used for resuming an aborted comparison. See Comparing Directories.
--suppress-common-lines
  Do not print common lines in side by side format. See Side by Side Format.
-t
  Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See Tabs.
-T
  Output a tab rather than a space before the text of a line in normal or context format. This causes the alignment of tabs in the line to look normal. See Tabs.
--text
  Treat all files as text and compare them line-by-line, even if they do not appear to be text. See Binary.
-u
  Use the unified output format. See Unified Format.
--unchanged-group-format=format
  Use format to output a group of common lines taken from both files in if-then-else format. See Line Group Formats.
--unchanged-line-format=format
  Use format to output a line common to both files in if-then-else format. See Line Formats.
--unidirectional-new-file
  When comparing directories, if a file appears only in the second directory of the two, treat it as present but empty in the other. See Comparing Directories.
-U lines
--unified[=lines]
  Use the unified output format, showing lines (an integer) lines of context, or three if lines is not given. See Unified Format. For proper operation, `patch` typically needs at least two lines of context.
-v
--version
  Output the version number of `diff`.
-w
  Ignore white space when comparing lines. See White Space.
-W columns
--width=columns
  Use an output width of columns in side by side format. See Side by Side Format.
-x pattern
  When comparing directories, ignore files and subdirectories whose basenames match pattern. See Comparing Directories.
-X file
  When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file. See Comparing Directories.
-y
  Use the side by side output format. See Side by Side Format.

# Invoking `diff3`

The `diff3` command compares three files and outputs descriptions of their differences. Its arguments are as follows:

    diff3 options··· mine older yours

The files to compare are mine, older, and yours. At most one of these three file names may be `-`, which tells `diff3` to read the standard input for that file.

An exit status of 0 means `diff3` was successful, 1 means some conflicts were found, and 2 means trouble.

- Summary of options to `diff3`.

## Options to `diff3`

Below is a summary of all of the options that GNU `diff3` accepts. Multiple single letter options (unless they take an argument) can be combined into a single command line argument.

-a
  Treat all files as text and compare them line-by-line, even if they do not appear to be text. See Binary.
-A
  Incorporate all changes from older to yours into mine, surrounding all conflicts with bracket lines. See Marking Conflicts.
-e
  Generate an `ed` script that incorporates all the changes from older to yours into mine. See Which Changes.
-E
  Like `-e`, except bracket lines from overlapping changes' first and third files. See Marking Conflicts. With `-e`, an overlapping change looks like this:

      <<<<<<< mine
      lines from mine
      =======
      lines from yours
      >>>>>>> yours

--ed
  Generate an `ed` script that incorporates all the changes from older to yours into mine. See Which Changes.
--easy-only
  Like `-e`, except output only the nonoverlapping changes. See Which Changes.
-i
  Generate `w` and `q` commands at the end of the `ed` script for System V compatibility. This option must be combined with one of the `-AeExX3` options, and may not be combined with `-m`. See Saving the Changed File.
--initial-tab
  Output a tab rather than two spaces before the text of a line in normal format. This causes the alignment of tabs in the line to look normal. See Tabs.
-L label
--label=label
  Use the label label for the brackets output by the `-A`, `-E` and `-X` options. This option may be given up to three times, one for each input file. The default labels are the names of the input files. Thus `diff3 -L X -L Y -L Z -m A B C` acts like `diff3 -m 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`. See Marking Conflicts.
-m
--merge
  Apply the edit script to the first file and send the result to standard output. Unlike piping the output from `diff3` to `ed`, this works even for binary files and incomplete lines. `-A` is assumed if no edit script option is specified. See Bypassing ed.
--overlap-only
  Like `-e`, except output only the overlapping changes. See Which Changes.
--show-all
  Incorporate all unmerged changes from older to yours into mine, surrounding all overlapping changes with bracket lines. See Marking Conflicts.
--show-overlap
  Like `-e`, except bracket lines from overlapping changes' first and third files. See Marking Conflicts.
-T
  Output a tab rather than two spaces before the text of a line in normal format. This causes the alignment of tabs in the line to look normal. See Tabs.
--text
  Treat all files as text and compare them line-by-line, even if they do not appear to be text. See Binary.
-v
--version
  Output the version number of `diff3`.
-x
  Like `-e`, except output only the overlapping changes. See Which Changes.
-X
  Like `-E`, except output only the overlapping changes. In other words, like `-x`, except bracket changes as in `-E`. See Marking Conflicts.
-3
  Like `-e`, except output only the nonoverlapping changes. See Which Changes.

# Invoking `patch`

Normally `patch` is invoked like this:

    patch <patchfile

The full format for invoking `patch` is:

    patch options··· [origfile [patchfile]] [+ options··· [origfile]]···

If you do not specify patchfile, or if patchfile is `-`, `patch` reads the patch (that is, the `diff` output) from the standard input.

You can specify one or more of the original files as orig arguments; each one and options for interpreting it is separated from the others with a `+`. See Multiple Patches, for more information.

If you do not specify an input file on the command line, `patch` tries to figure out from the leading text (any text in the patch that comes before the `diff` output) which file to edit. In the header of a context or unified diff, `patch` looks in lines beginning with `***`, `---`, or `+++`; among those, it chooses the shortest name of an existing file. Otherwise, if there is an `Index:` line in the leading text, `patch` tries to use the file name from that line. If `patch` cannot figure out the name of an existing file from the leading text, it prompts you for the name of the file to patch.

If the input file does not exist or is read-only, and a suitable RCS or SCCS file exists, `patch` attempts to check out or get the file before proceeding.

By default, `patch` replaces the original input file with the patched version, after renaming the original file into a backup file (see Backups, for a description of how `patch` names backup files). You can also specify where to put the output with the `-o output-file` or `--output=output-file` option.

- Changing directory and stripping directories.
- Backup file names.
- Reject file names.
- Summary table of options to `patch`.

## Applying Patches in Other Directories

The `-d directory` or `--directory=directory` option to `patch` makes directory directory the current directory for interpreting both file names in the patch file, and file names given as arguments to other options (such as `-B` and `-o`). For example, while in a news reading program, you can patch a file in the `/usr/src/emacs` directory directly from the article containing the patch like this:

    | patch -d /usr/src/emacs

Sometimes the file names given in a patch contain leading directories, but you keep your files in a directory different from the one given in the patch. In those cases, you can use the `-p[number]` or `--strip[=number]` option to set the file name strip count to number. The strip count tells `patch` how many slashes, along with the directory names between them, to strip from the front of file names. `-p` with no number given is equivalent to `-p0`. By default, `patch` strips off all leading directories, leaving just the base file names, except that when a file name given in the patch is a relative file name and all of its leading directories already exist, `patch` does not strip off the leading directory. (A relative file name is one that does not start with a slash.)

`patch` looks for each file (after any slashes have been stripped) in the current directory, or if you used the `-d directory` option, in that directory.

For example, suppose the file name in the patch file is `/gnu/src/emacs/etc/NEWS`. Using `-p` or `-p0` gives the entire file name unmodified, `-p1` gives `gnu/src/emacs/etc/NEWS` (no leading slash), `-p4` gives `etc/NEWS`, and not specifying `-p` at all gives `NEWS`.

## Backup File Names

Normally, `patch` renames an original input file into a backup file by appending to its name the extension `.orig`, or `~` on systems that do not support long file names. The `-b backup-suffix` or `--suffix=backup-suffix` option uses backup-suffix as the backup extension instead.

Alternately, you can specify the extension for backup files with the `SIMPLE_BACKUP_SUFFIX` environment variable, which the options override.

`patch` can also create numbered backup files the way GNU Emacs does. With this method, instead of having a single backup of each file, `patch` makes a new backup file name each time it patches a file. For example, the backups of a file named `sink` would be called, successively, `sink.~1~`, `sink.~2~`, `sink.~3~`, etc.

The `-V backup-style` or `--version-control=backup-style` option takes as an argument a method for creating backup file names. You can alternately control the type of backups that `patch` makes with the `VERSION_CONTROL` environment variable, which the `-V` option overrides. The value of the `VERSION_CONTROL` environment variable and the argument to the `-V` option are like the GNU Emacs `version-control` variable (see The GNU Emacs Manual emacs, for more information on backup versions in Emacs). They also recognize synonyms that are more descriptive. The valid values are listed below; unique abbreviations are acceptable.

t
numbered
  Always make numbered backups.
nil
existing
  Make numbered backups of files that already have them, simple backups of the others. This is the default.
never
simple
  Always make simple backups.

Alternately, you can tell `patch` to prepend a prefix, such as a directory name, to produce backup file names. The `-B backup-prefix` or `--prefix=backup-prefix` option makes backup files by prepending backup-prefix to them. If you use this option, `patch` ignores any `-b` option that you give.

If the backup file already exists, `patch` creates a new backup file name by changing the first lowercase letter in the last component of the file name into uppercase. If there are no more lowercase letters in the name, it removes the first character from the name. It repeats this process until it comes up with a backup file name that does not already exist.

If you specify the output file with the `-o` option, that file is the one that is backed up, not the input file.

## Reject File Names

The names for reject files (files containing patches that `patch` could not find a place to apply) are normally the name of the output file with `.rej` appended (or `#` on systems that do not support long file names).

Alternatively, you can tell `patch` to place all of the rejected patches in a single file. The `-r reject-file` or `--reject-file=reject-file` option uses reject-file as the reject file name.

## Options to `patch`

Here is a summary of all of the options that `patch` accepts. Older versions of `patch` do not accept long-named options or the `-t`, `-E`, or `-V` options.

Multiple single-letter options that do not take an argument can be combined into a single command line argument (with only one dash). Brackets ([ and ]) indicate that an option takes an optional argument.

-b backup-suffix
  Use backup-suffix as the backup extension instead of `.orig` or `~`. See Backups.
-B backup-prefix
  Use backup-prefix as a prefix to the backup file name. If this option is specified, any `-b` option is ignored. See Backups.
--batch
  Do not ask any questions. See patch Messages.
-c
--context
  Interpret the patch file as a context diff. See patch Input.
-d directory
--directory=directory
  Makes directory directory the current directory for interpreting both file names in the patch file, and file names given as arguments to other options. See patch Directories.
-D name
  Make merged if-then-else output using format. See If-then-else.
--debug=number
  Set internal debugging flags. Of interest only to `patch` patchers.
-e
--ed
  Interpret the patch file as an `ed` script. See patch Input.
-E
  Remove output files that are empty after the patches have been applied. See Empty Files.
-f
  Assume that the user knows exactly what he or she is doing, and do not ask any questions. See patch Messages.
-F lines
  Set the maximum fuzz factor to lines. See Inexact.
--force
  Assume that the user knows exactly what he or she is doing, and do not ask any questions. See patch Messages.
--forward
  Ignore patches that `patch` thinks are reversed or already applied. See also `-R`. See Reversed Patches.
--fuzz=lines
  Set the maximum fuzz factor to lines. See Inexact.
--help
  Print a summary of the options that `patch` recognizes, then exit.
--ifdef=name
  Make merged if-then-else output using format. See If-then-else.
--ignore-white-space
-l
  Let any sequence of white space in the patch file match any sequence of white space in the input file. See Changed White Space.
-n
--normal
  Interpret the patch file as a normal diff. See patch Input.
-N
  Ignore patches that `patch` thinks are reversed or already applied. See also `-R`. See Reversed Patches.
-o output-file
--output=output-file
  Use output-file as the output file name. See patch Options.
-p[number]
  Set the file name strip count to number. See patch Directories.
--prefix=backup-prefix
  Use backup-prefix as a prefix to the backup file name. If this option is specified, any `-b` option is ignored. See Backups.
--quiet
  Work silently unless an error occurs. See patch Messages.
-r reject-file
  Use reject-file as the reject file name. See Rejects.
-R
  Assume that this patch was created with the old and new files swapped. See Reversed Patches.
--reject-file=reject-file
  Use reject-file as the reject file name. See Rejects.
--remove-empty-files
  Remove output files that are empty after the patches have been applied. See Empty Files.
--reverse
  Assume that this patch was created with the old and new files swapped. See Reversed Patches.
-s
  Work silently unless an error occurs. See patch Messages.
-S
  Ignore this patch from the patch file, but continue looking for the next patch in the file. See Multiple Patches.
--silent
  Work silently unless an error occurs. See patch Messages.
--skip
  Ignore this patch from the patch file, but continue looking for the next patch in the file. See Multiple Patches.
--strip[=number]
  Set the file name strip count to number. See patch Directories.
--suffix=backup-suffix
  Use backup-suffix as the backup extension instead of `.orig` or `~`. See Backups.
-t
  Do not ask any questions. See patch Messages.
-u
--unified
  Interpret the patch file as a unified diff. See patch Input.
-v
  Output the revision header and patch level of `patch`.
-V backup-style
  Select the kind of backups to make. See Backups.
--version
  Output the revision header and patch level of `patch`, then exit.
--version=control=backup-style
  Select the kind of backups to make. See Backups.
-x number
  Set internal debugging flags. Of interest only to `patch` patchers.

# Invoking `sdiff`

The `sdiff` command merges two files and interactively outputs the results. Its arguments are as follows:

    sdiff -o outfile options··· from-file to-file

This merges from-file with to-file, with output to outfile. If from-file is a directory and to-file is not, `sdiff` compares the file in from-file whose file name is that of to-file, and vice versa. from-file and to-file may not both be directories.

`sdiff` options begin with `-`, so normally from-file and to-file may not begin with `-`. However, `--` as an argument by itself treats the remaining arguments as file names even if they begin with `-`. You may not use `-` as an input file.

An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.

`sdiff` without `-o` (or `--output`) produces a side-by-side difference. This usage is obsolete; use `diff --side-by-side` instead.

- Summary of options to `diff`.

## Options to `sdiff`

Below is a summary of all of the options that GNU `sdiff` accepts. Each option has two equivalent names, one of which is a single letter preceded by `-`, and the other of which is a long name preceded by `--`. Multiple single letter options (unless they take an argument) can be combined into a single command line argument. Long named options can be abbreviated to any unique prefix of their name.

-a
  Treat all files as text and compare them line-by-line, even if they do not appear to be text. See Binary.
-b
  Ignore changes in amount of white space. See White Space.
-B
  Ignore changes that just insert or delete blank lines. See Blank Lines.
-d
  Change the algorithm to perhaps find a smaller set of changes. This makes `sdiff` slower (sometimes much slower). See diff Performance.
-H
  Use heuristics to speed handling of large files that have numerous scattered small changes. See diff Performance.
--expand-tabs
  Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See Tabs.
-i
  Ignore changes in case; consider upper- and lower-case to be the same. See Case Folding.
-I regexp
  Ignore changes that just insert or delete lines that match regexp. See Specified Folding.
--ignore-all-space
  Ignore white space when comparing lines. See White Space.
--ignore-blank-lines
  Ignore changes that just insert or delete blank lines. See Blank Lines.
--ignore-case
  Ignore changes in case; consider upper- and lower-case to be the same. See Case Folding.
--ignore-matching-lines=regexp
  Ignore changes that just insert or delete lines that match regexp. See Specified Folding.
--ignore-space-change
  Ignore changes in amount of white space. See White Space.
-l
--left-column
  Print only the left column of two common lines. See Side by Side Format.
--minimal
  Change the algorithm to perhaps find a smaller set of changes. This makes `sdiff` slower (sometimes much slower). See diff Performance.
-o file
--output=file
  Put merged output into file. This option is required for merging.
-s
--suppress-common-lines
  Do not print common lines. See Side by Side Format.
--speed-large-files
  Use heuristics to speed handling of large files that have numerous scattered small changes. See diff Performance.
-t
  Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See Tabs.
--text
  Treat all files as text and compare them line-by-line, even if they do not appear to be text. See Binary.
-v
--version
  Output the version number of `sdiff`.
-w columns
--width=columns
  Use an output width of columns. See Side by Side Format. Note that for historical reasons, this option is `-W` in `diff`, `-w` in `sdiff`.
-W
  Ignore horizontal white space when comparing lines. See White Space. Note that for historical reasons, this option is `-w` in `diff`, `-W` in `sdiff`.

# Incomplete Lines

When an input file ends in a non-newline character, its last line is called an incomplete line because its last character is not a newline. All other lines are called full lines and end in a newline character. Incomplete lines do not match full lines unless differences in white space are ignored (see White Space).

An incomplete line is normally distinguished on output from a full line by a following line that starts with `\`. However, the RCS format (see RCS) outputs the incomplete line as-is, without any trailing newline or following line. The side by side format normally represents incomplete lines as-is, but in some cases uses a `\` or `/` gutter marker; See Side by Side. The if-then-else line format preserves a line's incompleteness with `%L`, and discards the newline with `%l`; See Line Formats. Finally, with the `ed` and forward `ed` output formats (see Output Formats) `diff` cannot represent an incomplete line, so it pretends there was a newline and reports an error.

For example, suppose `F` and `G` are one-byte files that contain just `f` and `g`, respectively. Then `diff F G` outputs

    1c1
    < f
    \ No newline at end of file
    ---
    > g
    \ No newline at end of file

(The exact message may differ in non-English locales.) `diff -n F G` outputs the following without a trailing newline:

    d1 1
    a1 1
    g

`diff -e F G` reports two errors and outputs the following:

    1c
    g
    .

# Future Projects

Here are some ideas for improving GNU `diff` and `patch`. The GNU project has identified some improvements as potential programming projects for volunteers. You can also help by reporting any bugs that you find.

If you are a programmer and would like to contribute something to the GNU project, please consider volunteering for one of these projects. If you are seriously contemplating work, please write to `gnu@prep.ai.mit.edu` to coordinate with other volunteers.

- Suggested projects for improvements.
- Reporting bugs.

## Suggested Projects for Improving GNU `diff` and `patch`

One should be able to use GNU `diff` to generate a patch from any pair of directory trees, and given the patch and a copy of one such tree, use `patch` to generate a faithful copy of the other. Unfortunately, some changes to directory trees cannot be expressed using current patch formats; also, `patch` does not handle some of the existing formats. These shortcomings motivate the following suggested projects.

- Handling changes to the directory structure.
- Handling symbolic links, device special files, etc.
- Handling file names that contain unusual characters.
- Patching non-text files.
- Handling files that do not fit in memory.
- Ignoring certain changes while showing others.

### Handling Changes to the Directory Structure

`diff` and `patch` do not handle some changes to directory structure. For example, suppose one directory tree contains a directory named `D` with some subsidiary files, and another contains a file with the same name `D`. `diff -r` does not output enough information for `patch` to transform the the directory subtree into the file.

There should be a way to specify that a file has been deleted without having to include its entire contents in the patch file. There should also be a way to tell `patch` that a file was renamed, even if there is no way for `diff` to generate such information.

These problems can be fixed by extending the `diff` output format to represent changes in directory structure, and extending `patch` to understand these extensions.

### Files that are Neither Directories Nor Regular Files

Some files are neither directories nor regular files: they are unusual files like symbolic links, device special files, named pipes, and sockets. Currently, `diff` treats symbolic links like regular files; it treats other special files like regular files if they are specified at the top level, but simply reports their presence when comparing directories. This means that `patch` cannot represent changes to such files. For example, if you change which file a symbolic link points to, `diff` outputs the difference between the two files, instead of the change to the symbolic link.

`diff` should optionally report changes to special files specially, and `patch` should be extended to understand these extensions.

### File Names that Contain Unusual Characters

When a file name contains an unusual character like a newline or white space, `diff -r` generates a patch that `patch` cannot parse. The problem is with format of `diff` output, not just with `patch`, because with odd enough file names one can cause `diff` to generate a patch that is syntactically correct but patches the wrong files. The format of `diff` output should be extended to handle all possible file names.

### Arbitrary Limits

GNU `diff` can analyze files with arbitrarily long lines and files that end in incomplete lines. However, `patch` cannot patch such files. The `patch` internal limits on line lengths should be removed, and `patch` should be extended to parse `diff` reports of incomplete lines.

### Handling Files that Do Not Fit in Memory

`diff` operates by reading both files into memory. This method fails if the files are too large, and `diff` should have a fallback.

One way to do this is to scan the files sequentially to compute hash codes of the lines and put the lines in equivalence classes based only on hash code. Then compare the files normally. This does produce some false matches.

Then scan the two files sequentially again, checking each match to see whether it is real. When a match is not real, mark both the ``matching'' lines as changed. Then build an edit script as usual.

The output routines would have to be changed to scan the files sequentially looking for the text to print.

### Ignoring Certain Changes

It would be nice to have a feature for specifying two strings, one in from-file and one in to-file, which should be considered to match. Thus, if the two strings are `foo` and `bar`, then if two lines differ only in that `foo` in file 1 corresponds to `bar` in file 2, the lines are treated as identical.

It is not clear how general this feature can or should be, or what syntax should be used for it.

## Reporting Bugs

If you think you have found a bug in GNU `cmp`, `diff`, `diff3`, `sdiff`, or `patch`, please report it by electronic mail to `bug-gnu-utils@prep.ai.mit.edu`. Send as precise a description of the problem as you can, including sample input files that produce the bug, if applicable.

Because Larry Wall has not released a new version of `patch` since mid 1988 and the GNU version of `patch` has been changed since then, please send bug reports for `patch` by electronic mail to both `bug-gnu-utils@prep.ai.mit.edu` and `lwall@netlabs.com`.

# Concept Index

- `cmp` invocation
  - Invoking `cmp`
- `cmp` options
  - Options to `cmp`
- `diff3` hunks
  - `diff3` Hunks
- `diff3` invocation
  - Invoking `diff3`
- `diff3` options
  - Options to `diff3`
- `diff3` sample input
  - A Third Sample Input File
- `diff` invocation
  - Invoking `diff`
- `diff` options
  - Options to `diff`
- `diff` sample input
  - Two Sample Input Files
- `ed` script output format
  - `ed` Scripts
- `ifdef` output format
  - Merging Files with If-then-else
- `patch` input format
  - Selecting the `patch` Input Format
- `patch` invocation
  - Invoking `patch`
- `patch` messages and questions
  - Messages and Questions from `patch`
- `patch` options
  - Options to `patch`
- `sdiff` invocation
  - Invoking `sdiff`
- `sdiff` options
  - Options to `sdiff`
- `sdiff` output format
  - Specifying `diff` Options to `sdiff`
- `!` output format
  - Showing Differences in Their Context
- `<<<<<<<` for marking conflicts
  - Marking Conflicts
- `<` output format
  - Showing Differences Without Context
- `+-` output format
  - Unified Format
- aligning tabstops
  - Preserving Tabstop Alignment
- alternate file names
  - Showing Alternate File Names
- backup file names
  - Backup File Names
- binary file diff
  - Binary Files and Forcing Text Comparisons
- binary file patching
  - Arbitrary Limits
- blank and tab difference suppression
  - Suppressing Differences in Blank and Tab Spacing
- blank line difference suppression
  - Suppressing Differences in Blank Lines
- brief difference reports
  - Summarizing Which Files Differ
- bug reports
  - Reporting Bugs
- C function headings
  - Showing C Function Headings
- C if-then-else output format
  - Merging Files with If-then-else
- case difference suppression
  - Suppressing Case Differences
- columnar output
  - Showing Differences Side by Side
- comparing three files
  - Comparing Three Files
- conflict
  - Merging From a Common Ancestor
- conflict marking
  - Marking Conflicts
- context output format
  - Showing Differences in Their Context
- diagnostics from `patch`
  - Messages and Questions from `patch`
- diff merging
  - Interactive Merging with `sdiff`
- directories and patch
  - Applying Patches in Other Directories
- directory structure changes
  - Handling Changes to the Directory Structure
- empty files, removing
  - Removing Empty Files
- file name alternates
  - Showing Alternate File Names
- file names with unusual characters
  - File Names that Contain Unusual Characters
- format of `diff3` output
  - Comparing Three Files
- format of `diff` output
  - `diff` Output Formats
- formats for if-then-else line groups
  - Line Group Formats
- forward `ed` script output format
  - Forward `ed` Scripts
- full lines
  - Incomplete Lines
- function headings, C
  - Showing C Function Headings
- fuzz factor when patching
  - Helping `patch` Find Inexact Matches
- headings
  - Showing Which Sections Differences Are in
- hunks
  - Hunks
- hunks for `diff3`
  - `diff3` Hunks
- if-then-else output format
  - Merging Files with If-then-else
- imperfect patch application
  - Applying Imperfect Patches
- incomplete line merging
  - How `diff3` Merges Incomplete Lines
- incomplete lines
  - Incomplete Lines
- inexact patches
  - Helping `patch` Find Inexact Matches
- interactive merging
  - Interactive Merging with `sdiff`
- introduction
  - What Comparison Means
- invoking `cmp`
  - Invoking `cmp`
- invoking `diff3`
  - Invoking `diff3`
- invoking `diff`
  - Invoking `diff`
- invoking `patch`
  - Invoking `patch`
- invoking `sdiff`
  - Invoking `sdiff`
- large files
  - Handling Files that Do Not Fit in Memory
- line formats
  - Line Formats
- line group formats
  - Line Group Formats
- merge commands
  - Merge Commands
- merged `diff3` format
  - Generating the Merged Output Directly
- merged output format
  - Merging Files with If-then-else
- merging from a common ancestor
  - Merging From a Common Ancestor
- merging interactively
  - Merge Commands
- messages from `patch`
  - Messages and Questions from `patch`
- multiple patches
  - Multiple Patches in a File
- newline treatment by `diff`
  - Incomplete Lines
- normal output format
  - Showing Differences Without Context
- options for `cmp`
  - Options to `cmp`
- options for `diff3`
  - Options to `diff3`
- options for `diff`
  - Options to `diff`
- options for `patch`
  - Options to `patch`
- options for `sdiff`
  - Options to `sdiff`
- output formats
  - `diff` Output Formats
- overlap
  - Merging From a Common Ancestor
- overlapping change, selection of
  - Selecting Which Changes to Incorporate
- overview of `diff` and `patch`
  - Overview
- paginating `diff` output
  - Paginating `diff` Output
- patch making tips
  - Tips for Making Patch Distributions
- patching directories
  - Applying Patches in Other Directories
- performance of `diff`
  - `diff` Performance Tradeoffs
- projects for directories
  - Suggested Projects for Improving GNU `diff` and `patch`
- RCS script output format
  - RCS Scripts
- regular expression matching headings
  - Showing Lines That Match Regular Expressions
- regular expression suppression
  - Suppressing Lines Matching a Regular Expression
- reject file names
  - Reject File Names
- removing empty files
  - Removing Empty Files
- reporting bugs
  - Reporting Bugs
- reversed patches
  - Applying Reversed Patches
- sample input for `diff3`
  - A Third Sample Input File
- sample input for `diff`
  - Two Sample Input Files
- script output formats
  - Making Edit Scripts
- section headings
  - Showing Which Sections Differences Are in
- side by side
  - Showing Differences Side by Side
- side by side format
  - Controlling Side by Side Format
- special files
  - Files that are Neither Directories Nor Regular Files
- specified headings
  - Showing Lines That Match Regular Expressions
- summarizing which files differ
  - Summarizing Which Files Differ
- System V `diff3` compatibility
  - Saving the Changed File
- tab and blank difference suppression
  - Suppressing Differences in Blank and Tab Spacing
- tabstop alignment
  - Preserving Tabstop Alignment
- text versus binary diff
  - Binary Files and Forcing Text Comparisons
- tips for patch making
  - Tips for Making Patch Distributions
- two-column output
  - Showing Differences Side by Side
- unified output format
  - Unified Format
- unmerged change
  - Selecting Which Changes to Incorporate
- white space in patches
  - Applying Patches with Changed White Space