diff

DIFF(7)               BSD Miscellaneous Information Manual               DIFF(7)

NAME
     diff — Comparing and Merging Files

Comparing and Merging Files
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     file is a newer version of the other file.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 byte and line numbers where two
     files differ.  cmp can also show all the bytes that differ between the two
     files, side by side.  A way to compare two files character by character is
     the Emacs command M-x compare-windows.  See Section.Dq Other Window , 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 Paul Eggert, Mike Haertel, David Hayes, Richard
     Stallman, and Len Tower. Wayne Davison designed and implemented the unified
     output format.  The basic algorithm is described by Eugene W. Myers in “An
     O(ND) Difference Algorithm and its Variations”, 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 by
     E. Ukkonen in “Algorithms for Approximate String Matching”, Information and
     Control Vol. 64, 1985, pp. 100--118. Unless the [--minimal] option is used,
     diff uses a heuristic by Paul Eggert that limits the cost to O(N^1.5 log N)
     at the price of producing suboptimal output for large inputs with many
     differences.  Related algorithms are surveyed by Alfred V. Aho in section
     6.3 of “Algorithms for Finding Patterns in Strings”, Handbook of
     Theoretical Computer Science (Jan Van Leeuwen, ed.), Vol. A, Algorithms and
     Complexity, Elsevier/MIT Press, 1990, pp. 255--300.

     GNU diff3 was written by Randy Smith. GNU sdiff was written by Thomas Lord.
     GNU cmp was written by Torbjörn Granlund and David MacKenzie.

     GNU patch was written mainly by Larry Wall and Paul Eggert; several GNU
     enhancements were contributed 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 bytes that can be either identical or different.  cmp reports
     the differences between two files byte by byte, instead of line by line. As
     a result, it is often 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, or whether one file is a prefix of the other.

     To illustrate the effect that considering changes byte by byte 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 byte 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.

   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 [-d] or [--minimal] option (see Section “diff
     Performance”).

   Suppressing Differences in Blank and Tab Spacing
     The [-E] or [--ignore-tab-expansion] option ignores the distinction between
     tabs and spaces on input. A tab is considered to be equivalent to the
     number of spaces to the next tab stop (see Section “Tabs”).

     The [-b] or [--ignore-space-change] option is stronger. It ignores white
     space at line end, and considers all other sequences of one or more white
     space characters within a line to be equivalent. With this option, 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] or [--ignore-all-space] option is stronger still. It ignores
     differences even if one line has white space where the other line 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 this option, 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 Whose Lines Are All Blank
     The [-B] or [--ignore-blank-lines] option ignores changes that consist
     entirely of blank lines. With this option, 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

     Normally this option affects only lines that are completely empty, but if
     you also specify the [-b] or [--ignore-space-change] option, or the [-w] or
     [--ignore-all-space] option, lines are also affected if they look empty but
     contain white space.  In other words, [-B] is equivalent to -I '^$' by
     default, but it is equivalent to [-I '^[[:space:]]*$'] if [-b] or [-w] is
     also specified.

   Suppressing Differences Whose Lines All Match a Regular Expression
     To ignore insertions and deletions of lines that match a grep -style
     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 '^[[:digit:]]' 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.

   Suppressing Case Differences
     GNU diff can treat lower case letters as equivalent to their upper case
     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.

   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] or [--brief] option selects
     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 and line number where the first
     difference occurs, or reports that one file is a prefix of the other. You
     can use the [-s], [--quiet], or [--silent] option to suppress that
     information, so that cmp produces no output and reports whether the files
     differ using only its exit status (see Section “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 Section “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 byte 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 report only whether files differ (but not how).
     Use the [-q] or [--brief] option for this.

     Normally, differing binary files count as trouble because the resulting
     diff output does not capture all the differences. This trouble causes diff
     to exit with status 2. However, this trouble cannot occur with the [-a] or
     [--text] option, or with the [-q] or [--brief] option, as these options
     both cause diff to generate a form of output that represents differences as
     requested.

     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.

     The [--strip-trailing-cr] causes diff to treat input lines that end in
     carriage return followed by newline as if they end in plain newline. This
     can be useful when comparing text that is imperfectly imported from many
     personal computer operating systems. This option affects how lines are
     read, which in turn affects how they are compared and output.

     If you want to compare two files byte by byte, you can use the cmp program
     with the [-l] or [--verbose] option to show the values of each differing
     byte in the two files. With GNU cmp, you can also use the [-b] or
     [--print-bytes] option to show the ASCII representation of those bytes.See
     Section “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 bytes 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]
     option.

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.

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

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

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

     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.

     Example of Context Format

     Here is the output of diff -c lao tzu (see Section “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 2002-02-21 23:30:39.942229878 -0800
           --- tzu 2002-02-21 23:30:50.442260588 -0800
           ***************
           *** 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!

     Example of Context Format with Less Context

     Here is the output of diff -C 1 lao tzu (see Section “Sample diff Input”,
     for the complete contents of the two files). Notice that at most one
     context line is reported here.

           *** lao 2002-02-21 23:30:39.942229878 -0800
           --- tzu 2002-02-21 23:30:50.442260588 -0800
           ***************
           *** 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!

     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

     The time stamp normally looks like 2002-02-21 23:30:39.942229878 -0800 to
     indicate the date, time with fractional seconds, and time zone in
     ftp://ftp.isi.edu/in-notes/rfc2822.txt (The fractional seconds are omitted
     on hosts that do not support fractional time stamps.) However, a
     traditional time stamp like Thu Feb 21 23:30:39 2002 is used if the LC_TIME
     locale category is either C or POSIX.

     You can change the header's content with the [--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-numbers ****
             from-file-line
             from-file-line...
           --- to-file-line-numbers ----
             to-file-line
             to-file-line...

     If a hunk contains two or more lines, its line numbers look like start,
     end.  Otherwise only its end line number appears. An empty hunk is
     considered to end at the line that precedes the hunk.

     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.

     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 three lines of context.

     Example of Unified Format

     Here is the output of the command diff -u lao tzu (see Section “Sample diff
     Input”, for the complete contents of the two files):

           --- lao 2002-02-21 23:30:39.942229878 -0800
           +++ tzu 2002-02-21 23:30:50.442260588 -0800
           @@ -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!

     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

     The time stamp looks like 2002-02-21 23:30:39.942229878 -0800 to indicate
     the date, time with fractional seconds, and time zone. The fractional
     seconds are omitted on hosts that do not support fractional time stamps.

     You can change the header's content with the [--label= label] option;
     seeSee Section “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-line-numbers to-file-line-numbers @@
            line-from-either-file
            line-from-either-file...

     If a hunk contains just one line, only its start line number appears.
     Otherwise its line numbers look like start, count.  An empty hunk is
     considered to start at the line that follows the hunk.

     If a hunk and its context contain two or more lines, its line numbers look
     like start, count.  Otherwise only its end line number appears. An empty
     hunk is considered to end at the line that precedes the hunk.

     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 print column:

     +       A line was added here to the first file.

     -       A line was removed here from the first file.

     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 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
     grep -style regular expression regexp to be the beginning of a section of
     the file. Here are suggested regular expressions for some common languages:

     ^[[:alpha:]$_]
             C, C++, Prolog

     ^(      Lisp

     ^@node  Texinfo

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

     The [-F] or [--show-function-line] option finds the nearest unchanged line
     that precedes each hunk of differences and matches the given regular
     expression. Then it adds 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, this option leaves the output for that hunk unchanged. If that
     line is more than 40 characters long, it outputs 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 Section “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] or [--show-c-function] option is equivalent to [-F
     '^[[:alpha:]$_]'] if the unified format is specified, otherwise [-c -F
     '^[[:alpha:]$_]'] (see Section “Specified Headings”).  GNU diff provides
     this option 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 [--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 [--label] option does not affect the
     file names in the pr header when the [-l] or [--paginate] option is used
     (see Section “Pagination”).

     Here are the first two lines of the output from diff -C 2 --label=original
     --label=modified 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 Section “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 Section “Incomplete Lines”.  However, when an
     output line represents two differing lines, one might be incomplete while
     the other is not. In this case, the output line is complete, but its the
     gutter is marked \ if the first line is incomplete, / if the second line
     is.

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

     You can use the sdiff command to interactively merge side by side
     differences.See Section “Interactive Merging”, for more information on
     merging files.

     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, the output is wider than
     usual: normally 130 print columns, which can fit onto a traditional printer
     line. You can set the width of the output with the [-W columns] or
     [--width= columns] option. The output is split into two halves of equal
     width, 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.

     Example of Side by Side Format

     Here is the output of the command diff -y -W 72 lao tzu (see Section
     “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!

   Showing Differences Without Context
     The “normal” diff output format shows each hunk of differences without any
     surrounding context.  Sometimes such output is the clearest way to see how
     lines have changed, without the clutter of nearby unchanged lines (although
     you can get similar results with the context or unified formats by using 0
     lines of context). However, this format is no longer widely used for
     sending out patches; for that purpose, the context format (see Section
     “Context Format”) and the unified format (see Section “Unified Format”) are
     superior. Normal format is the default for compatibility with older
     versions of diff and the POSIX standard. Use the [--normal] option to
     select this output format explicitly.

     Example of Normal Format

     Here is the output of the command diff lao tzu (see Section “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!

     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:

     la r    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.

     fc t    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.

     rd l    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.

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

     ed(Scripts)

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

     Like the normal format (see Section “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.

     Example ed(Script)

     Here is the output of diff -e lao tzu (see Section “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

     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.

     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 Section
     “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 Section “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.

     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 changed group
             format if it is specified; otherwise it is a format that outputs
             the line group as-is.

     --changed-group-format= format
             These line groups are hunks containing lines from both files. The
             default changed group format is the concatenation of the old and
             new group formats.

     --unchanged-group-format= format
             These line groups contain lines common to both files. The default
             unchanged group format is a format that outputs the line group as-
             is.

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

     %<      stands for the lines from the first file, including the trailing
             newline.  Each line is formatted according to the old line format
             (see Section “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 series of zero or more flags; an integer
             specifying the minimum field width; and a period followed by an
             optional integer specifying the minimum number of digits. The flags
             are - for left-justification, ' for separating the digit into
             groups as specified by the LC_NUMERIC locale category, and 0 for
             padding with zeros instead of spaces. 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 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-character change
     indicator to the left of the text. The first character 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 contents of the line, not counting its trailing
             newline (if any). This format ignores whether the line is
             incomplete;See Section “Incomplete Lines”.

     %L      stands for 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.

             where F is a printf conversion specification, stands for the line
             number formatted with F.  For example, %.5dn prints the line number
             using the printf format %.5d.  See Section.Dq 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 normal diff format. You can tailor this command to get fine control over
     diff 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

     Example of If-then-else Format

     Here is the output of diff -DTWO lao tzu (see Section “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 */

     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 [-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 like the diff [-D name] option, except
     it operates on a file and a diff to produce a merged file;See Section
     “patch Options”.

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 Section “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 Section
     “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 Section
     “Side by Side”.  The if-then-else line format preserves a line's
     incompleteness with %L, and discards the newline with %l ;See Section “Line
     Formats”.  Finally, with the ed and forward ed output formats (see Section
     “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
           .

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 as specified by the LC_COLLATE locale category. 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
     [--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 Section
     “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. Trailing white space and empty lines are
     ignored in the pattern file.

     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.

     If two directories differ only in that file names are lower case in one
     directory and upper case in the upper, diff normally reports many
     differences because it compares file names in a case sensitive way. With
     the [--ignore-file-name-case] option, diff ignores case differences in file
     names, so that for example the contents of the file Tao in one directory
     are compared to the contents of the file TAO in the other. The
     [--no-ignore-file-name-case] option cancels the effect of the
     [--ignore-file-name-case] option, reverting to the default behavior.

     If an [-x pattern] or [--exclude= pattern] option, or an [-X file] or
     [--exclude-from= file] option, is specified while the
     [--ignore-file-name-case] option is in effect, case is ignored when
     excluding file names matching the specified patterns.

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 Tab Stop 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 tab stop, 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. To use this form of output with patch, you must
     give patch the [-l] or [--ignore-white-space] option (see Section “Changed
     White Space”, for more information).  diff normally assumes that tab stops
     are set every 8 print columns, but this can be altered by the [--tabsize=
     columns] option.

     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 tab stops 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] or [--paginate] option does this by sending the diff output
     through the pr program. Here is what the page header might look like for
     diff -lc lao tzu:

           2002-02-22 14:20                 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.

     One way to improve diff performance is to use hard or symbolic links to
     files instead of copies. This improves performance because diff normally
     does not need to read two hard or symbolic links to the same file, since
     their contents must be identical. For example, suppose you copy a large
     directory hierarchy, make a few changes to the copy, and then often use
     diff -r to compare the original to the copy. If the original files are
     read-only, you can greatly improve performance by creating the copy using
     hard or symbolic links (e.g., with GNU cp -lR or cp -sR).  Before editing a
     file in the copy for the first time, you should break the link and replace
     it with a regular copy.

     You can also affect the performance of GNU diff by giving it options that
     change 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 [--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.

     Suppose a run of changed lines includes a sequence of lines at one end and
     there is an identical sequence of lines just outside the other end. The
     diff command is free to choose which identical sequence is included in the
     hunk.  In this case, diff normally shifts the hunk's boundaries when this
     merges adjacent hunks, or shifts a hunk's lines towards the end of the
     file. Merging hunks can make the output look nicer in some cases.

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 Section.Dq Invoking diff3 , for more information on how to run diff3.

   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 Section “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

   Example of diff3(Normal) Format
     Here is the output of the command diff3 lao tzu tao (see Section “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

   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 Section
     “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 Section
     “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 Section “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.

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

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

     Here is the output of the command diff3 with each of these three options
     (see Section “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 Section
     “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 [--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 --label X --label Y --label
     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] is assumed.

     For example, the command diff3 -m lao tzu tao (see Section “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 Section “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 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 Section “Side by Side”).  Use [-o file] or
     [--output= file] to specify where to put the merged text.See Section
     “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.Dq emerge , for more information.

   Specifying diff(Options) to sdiff
     The following sdiff options have the same meaning as for diff.  See
     Section.Dq diff Options , for the use of these options.

           -a -b -d -i -t -v
           -B -E -I regexp

           --expand-tabs
           --ignore-blank-lines  --ignore-case
           --ignore-matching-lines=regexp  --ignore-space-change
           --ignore-tab-expansion
           --left-column  --minimal  --speed-large-files
           --strip-trailing-cr  --suppress-common-lines
           --tabsize=columns  --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 Section “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.

     ed      Like eb, except precede each version with a header that shows what
             file and lines the version came from.

     el

     e1      Edit a copy of the left version, then copy the result to the
             output.

     er

     e2      Edit a copy of the right version, then copy the result to the
             output.

     l

     1       Copy the left version to the output.

     q       Quit.

     r

     2       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 a mail message containing a difference listing directly to patch.

     patch detects and warns about common problems like forward patches. It
     saves any patches that it could not apply. It can also maintain a
     patchlevel.h file to ensure 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
     Section “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,
     putting reject hunks (if any) into f.rej.

     See Section.Dq Invoking patch , for detailed information on the options to
     patch.

   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.

   Revision Control
     If a nonexistent input file is under a revision control system supported by
     patch, patch normally asks the user whether to get (or check out) the file
     from the revision control system. Patch currently supports RCS, ClearCase
     and SCCS. Under RCS and SCCS, patch also asks when the input file is read-
     only and matches the default version in the revision control system.

     The [-g num] or [--get= num] option affects access to files under supported
     revision control systems. If num is positive, patch gets the file without
     asking the user; if zero, patch neither asks the user nor gets the file;
     and if negative, patch asks the user before getting the file. The default
     value of num is given by the value of the PATCH_GET environment variable if
     it is set; if not, the default value is zero if patch is conforming to
     POSIX, negative otherwise.See Section “patch and POSIX”.

     The choice of revision control system is unaffected by the VERSION_CONTROL
     environment variable (see Section “Backup Names”).

   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 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. If a
     context diff contains trailing carriage return on each line, patch
     automatically ignores the carriage return. If a context diff has been
     encapsulated by prepending - to lines beginning with - as per
     ftp://ftp.isi.edu/in-notes/rfc934.txt, patch automatically unencapsulates
     the input.

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

     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 blank characters (i.e. spaces and tabs) loosely so that
     any nonempty sequence of blanks in the patch file matches any nonempty
     sequence of blanks in the input files. Non-blank 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.

     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; there is no point to setting it 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 Section “Reject Names”, 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.

     If the [--verbose] option is given, then 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.

     A patch against an empty file applies to a nonexistent file, and vice
     versa.See Section “Creating and Removing”.

     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.

     Predicting what patch(will) do

     It may not be obvious in advance what patch will do with a complicated or
     poorly formatted patch. If you are concerned that the input might cause
     patch to modify the wrong files, you can use the [--dry-run] option, which
     causes patch to print the results of applying patches without actually
     changing any files.  You can then inspect the diagnostics generated by the
     dry run to see whether patch will modify the files that you expect. If the
     patch does not do what you want, you can modify the patch (or the other
     options to patch) and try another dry run. Once you are satisfied with the
     proposed patch you can apply it by invoking patch as before, but this time
     without the [--dry-run] option.

   Creating and Removing Files
     Sometimes when comparing two directories, a file may exist in one directory
     but not the other. If you give diff the [-N] or [--new-file] option, or if
     you supply an old or new file that is named /dev/null or is empty and is
     dated the Epoch (1970-01-01 00:00:00 UTC), diff outputs a patch that adds
     or deletes the contents of this file. When given such a patch, patch
     normally creates a new file or removes the old file. However, when
     conforming to POSIX (see Section “patch and POSIX”), patch does not remove
     the old file, but leaves it empty. The [-E] or [--remove-empty-files]
     option causes patch to remove output files that are empty after applying a
     patch, even if the patch does not appear to be one that removed the file.

     If the patch appears to create a file that already exists, patch asks for
     confirmation before applying the patch.

   Updating Time Stamps on Patched Files
     When patch updates a file, it normally sets the file's last-modified time
     stamp to the current time of day. If you are using patch to track a
     software distribution, this can cause make to incorrectly conclude that a
     patched file is out of date. For example, if syntax.c depends on syntax.y,
     and patch updates syntax.c and then syntax.y, then syntax.c will normally
     appear to be out of date with respect to syntax.y even though its contents
     are actually up to date.

     The [-Z] or [--set-utc] option causes patch to set a patched file's
     modification and access times to the time stamps given in context diff
     headers. If the context diff headers do not specify a time zone, they are
     assumed to use Coordinated Universal Time (UTC, often known as GMT).

     The [-T] or [--set-time] option acts like [-Z] or [--set-utc], except that
     it assumes that the context diff headers' time stamps use local time
     instead of UTC. This option is not recommended, because patches using local
     time cannot easily be used by people in other time zones, and because local
     time stamps are ambiguous when local clocks move backwards during daylight-
     saving time adjustments. If the context diff headers specify a time zone,
     this option is equivalent to [-Z] or [--set-utc].

     patch normally refrains from setting a file's time stamps if the file's
     original last-modified time stamp does not match the time given in the diff
     header, of if the file's contents do not exactly match the patch. However,
     if the [-f] or [--force] option is given, the file's time stamps are set
     regardless.

     Due to the limitations of the current diff format, patch cannot update the
     times of files whose contents have not changed. Also, if you set file time
     stamps to values other than the current time of day, you should also remove
     (e.g., with make clean) all files that depend on the patched files, so that
     later invocations of make do not get confused by the patched files' times.

   Multiple Patches in a File
     If the patch file contains more than one patch, and if you do not specify
     an input file on the command line, patch tries to apply each patch 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
     Section “Making Patches”, for more on that topic).

     patch uses the following rules to intuit a file name from the leading text
     before a patch. First, patch takes an ordered list of candidate file names
     as follows:

     If the header is that of a context diff, patch takes the old and new
         file names in the header. A name is ignored if it does not have enough
         slashes to satisfy the [-p num] or [--strip= num] option. The name
         /dev/null is also ignored.

     If there is an Index: line in the leading garbage and if either the old
         and new names are both absent or if patch is conforming to POSIX, patch
         takes the name in the Index: line.

     For the purpose of the following rules, the candidate file names are
         considered to be in the order (old, new, index), regardless of the
         order that they appear in the header.

     Then patch selects a file name from the candidate list as follows:

     If some of the named files exist, patch selects the first name if
         conforming to POSIX, and the best name otherwise.

     If patch is not ignoring RCS, ClearCase, and SCCS (see Section
         “Revision Control”), and no named files exist but an RCS, ClearCase, or
         SCCS master is found, patch selects the first named file with an RCS,
         ClearCase, or SCCS master.

     If no named files exist, no RCS, ClearCase, or SCCS master was found,
         some names are given, patch is not conforming to POSIX, and the patch
         appears to create a file, patch selects the best name requiring the
         creation of the fewest directories.

     If no file name results from the above heuristics, you are asked for
         the name of the file to patch, and patch selects that name.

     To determine the best of a nonempty list of file names, patch first takes
     all the names with the fewest path name components; of those, it then takes
     all the names with the shortest basename; of those, it then takes all the
     shortest names; finally, it takes the first remaining name.

     See Section.Dq patch and POSIX , to see whether patch is conforming to
     POSIX.

   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 mail reading program, you can
     patch a file in the /usr/src/emacs directory directly from a message
     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. A sequence of one or more adjacent
     slashes is counted as a single slash. By default, patch strips off all
     leading directories, leaving just the base file names.

     For example, suppose the file name in the patch file is
     /gnu/src/emacs/etc/NEWS.  Using [-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.

     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.

   Backup Files
     Normally, patch creates a backup file if the patch does not exactly match
     the original input file, because in that case the original data might not
     be recovered if you undo the patch with patch -R (see Section “Reversed
     Patches”).  However, when conforming to POSIX, patch does not create backup
     files by default.See Section “patch and POSIX”.

     The [-b] or [--backup] option causes patch to make a backup file regardless
     of whether the patch matches the original input. The [--backup-if-mismatch]
     option causes patch to create backup files for mismatches files; this is
     the default when not conforming to POSIX. The [--no-backup-if-mismatch]
     option causes patch to not create backup files, even for mismatched
     patches; this is the default when conforming to POSIX.

     When backing up a file that does not exist, an empty, unreadable backup
     file is created as a placeholder to represent the nonexistent file.

   Backup File Names
     Normally, patch renames an original input file into a backup file by
     appending to its name the extension .orig, or ~ if using .orig would make
     the backup file name too long. The [-z backup-suffix] or [--suffix=
     backup-suffix] option causes patch to use 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 PATCH_VERSION_CONTROL
     environment variable, which the [-V] option overrides. If
     PATCH_VERSION_CONTROL is not set, the VERSION_CONTROL environment variable
     is used instead. Please note that these options and variables control
     backup file names; they do not affect the choice of revision control system
     (see Section “Revision Control”).

     The values of these environment variables and the argument to the [-V]
     option are like the GNU Emacs version-control variable (see Section “Backup
     Names”, 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.

     You can also tell patch to prepend a prefix, such as a directory name, to
     produce backup file names.  The [-B prefix] or [--prefix= prefix] option
     makes backup files by prepending prefix to them. The [-Y prefix] or
     [--basename-prefix= prefix] prepends prefix to the last file name component
     of backup file names instead; for example, [-Y ~] causes the backup name
     for dir/file.c to be dir/~file.c.  If you use either of these prefix
     options, the suffix-based options are ignored.

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

     Options that affect the names of backup files do not affect whether backups
     are made. For example, if you specify the [--no-backup-if-mismatch] option,
     none of the options described in this section have any affect, because no
     backups are made.

   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 # if using .rej would make the backup file name too long).

     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.

   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 produce more or fewer messages, to have it not ask for
     keyboard input, and to affect the way that file names are quoted in
     messages.

     patch exits with status 0 if all hunks are applied successfully, 1 if some
     hunks cannot be applied, and 2 if there is more serious trouble. 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.

     Controlling the Verbosity of patch

     You can cause patch to produce more messages by using the [--verbose]
     option. For example, when you give this option, 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.

     Inhibiting Keyboard Input

     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
     causes patch to do 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(Quoting) Style

     When patch outputs a file name in a diagnostic message, it can format the
     name in any of several ways. This can be useful to output file names
     unambiguously, even if they contain punctuation or special characters like
     newlines. The [--quoting-style= word] option controls how names are output.
     The word should be one of the following:

     literal
             Output names as-is.

     shell   Quote names for the shell if they contain shell metacharacters or
             would cause ambiguous output.

     shell-always
             Quote names for the shell, even if they would normally not require
             quoting.

     c       Quote names as for a C language string.

     escape  Quote as with c except omit the surrounding double-quote
             characters.

     You can specify the default value of the [--quoting-style] option with the
     environment variable QUOTING_STYLE.  If that environment variable is not
     set, the default value is shell, but this default may change in a future
     version of patch.

   patch(and) the POSIX Standard
     If you specify the [--posix] option, or set the POSIXLY_CORRECT environment
     variable, patch conforms more strictly to the POSIX standard, as follows:

     Take the first existing file from the list (old, new, index) when
         intuiting file names from diff headers.See Section “Multiple Patches”.

     Do not remove files that are removed by a diff.See Section “Creating
         and Removing”.

     Do not ask whether to get files from RCS, ClearCase, or SCCS.See
         Section “Revision Control”.

     Require that all options precede the files in the command line.

     Do not backup files, even when there is a mismatch.See Section
         “Backups”.

   GNU patch(and) Traditional patch
     The current version of GNU patch normally follows the POSIX standard.See
     Section “patch and POSIX”, for the few exceptions to this general rule.

     Unfortunately, POSIX redefined the behavior of patch in several important
     ways. You should be aware of the following differences if you must
     interoperate with traditional patch, or with GNU patch version 2.1 and
     earlier.

     In traditional patch, the [-p] option's operand was optional, and a
         bare [-p] was equivalent to [-p0].  The [-p] option now requires an
         operand, and [-p 0] is now equivalent to [-p0].  For maximum
         compatibility, use options like [-p0] and [-p1].

         Also, traditional patch simply counted slashes when stripping path
         prefixes; patch now counts pathname components. That is, a sequence of
         one or more adjacent slashes now counts as a single slash. For maximum
         portability, avoid sending patches containing // in file names.

     In traditional patch, backups were enabled by default. This behavior is
         now enabled with the [-b] or [--backup] option.

         Conversely, in POSIX patch, backups are never made, even when there is
         a mismatch. In GNU patch, this behavior is enabled with the
         [--no-backup-if-mismatch] option, or by conforming to POSIX.

         The [-b suffix] option of traditional patch is equivalent to the -b -z
         suffix options of GNU patch.

     Traditional patch used a complicated (and incompletely documented)
         method to intuit the name of the file to be patched from the patch
         header. This method did not conform to POSIX, and had a few gotchas.
         Now patch uses a different, equally complicated (but better documented)
         method that is optionally POSIX-conforming; we hope it has fewer
         gotchas. The two methods are compatible if the file names in the
         context diff header and the Index: line are all identical after prefix-
         stripping. Your patch is normally compatible if each header's file
         names all contain the same number of slashes.

     When traditional patch asked the user a question, it sent the question
         to standard error and looked for an answer from the first file in the
         following list that was a terminal: standard error, standard output,
         /dev/tty, and standard input. Now patch sends questions to standard
         output and gets answers from /dev/tty.  Defaults for some answers have
         been changed so that patch never goes into an infinite loop when using
         default answers.

     Traditional patch exited with a status value that counted the number of
         bad hunks, or with status 1 if there was real trouble. Now patch exits
         with status 1 if some hunks failed, or with 2 if there was real
         trouble.

     Limit yourself to the following options when sending instructions meant
         to be executed by anyone running GNU patch, traditional patch, or a
         patch that conforms to POSIX. Spaces are significant in the following
         list, and operands are required.

               -c
               -d dir
               -D define
               -e
               -l
               -n
               -N
               -o outfile
               -pnum
               -R
               -r rejectfile

Tips for Making and Using Patches
     Use some common sense when making and using patches. For example, when
     sending bug fixes to a program's maintainer, send several small patches,
     one per independent subject, instead of one large, harder-to-digest patch
     that covers all the subjects.

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

   Tips for Patch Producers
     To create a patch that changes an older version of a package into a newer
     version, first make a copy of the older and newer versions in adjacent
     subdirectories.  It is common to do that by unpacking tar archives of the
     two versions.

     To generate the patch, use the command diff -Naur old new where old and new
     identify the old and new directories. The names old and new should not
     contain any slashes. The [-N] option lets the patch create and remove
     files; [-a] lets the patch update non-text files; [-u] generates useful
     time stamps and enough context; and [-r] lets the patch update
     subdirectories. Here is an example command, using Bourne shell syntax:

           diff -Naur gcc-3.0.3 gcc-3.0.4

     Tell your recipients how to apply the patches. This should include which
     working directory to use, and which patch options to use; the option -p1 is
     recommended. Test your procedure by pretending to be a recipient and
     applying your patches to a copy of the original files.

     See Section.Dq Avoiding Common Mistakes , for how to avoid common mistakes
     when generating a patch.

   Tips for Patch Consumers
     A patch producer should tell recipients how to apply the patches, so the
     first rule of thumb for a patch consumer is to follow the instructions
     supplied with the patch.

     GNU diff can analyze files with arbitrarily long lines and files that end
     in incomplete lines. However, older versions of patch cannot patch such
     files. If you are having trouble applying such patches, try upgrading to a
     recent version of GNU patch.

   Avoiding Common Mistakes
     When producing a patch for multiple files, apply diff to directories whose
     names do not have slashes. This reduces confusion when the patch consumer
     specifies the [-p number] option, since this option can have surprising
     results when the old and new file names have different numbers of slashes.
     For example, do not send a patch with a header that looks like this:

           diff -Naur v2.0.29/prog/README prog/README
           --- v2.0.29/prog/README 2002-03-10 23:30:39.942229878 -0800
           +++ prog/README 2002-03-17 20:49:32.442260588 -0800

     because the two file names have different numbers of slashes, and different
     versions of patch interpret the file names differently. To avoid confusion,
     send output that looks like this instead:

           diff -Naur v2.0.29/prog/README v2.0.30/prog/README
           --- v2.0.29/prog/README 2002-03-10 23:30:39.942229878 -0800
           +++ v2.0.30/prog/README 2002-03-17 20:49:32.442260588 -0800

     Make sure you have specified the file names correctly, either in a context
     diff header or with an Index: line. Take care to not send out reversed
     patches, since these make people wonder whether they have already applied
     the patch.

     Avoid sending patches that compare backup file names like README.orig or
     README~, since this might confuse patch into patching a backup file instead
     of the real file. Instead, send patches that compare the same base file
     names in different directories, e.g.  old/README and new/README.

     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.

   Generating Smaller Patches
     The simplest way to generate a patch is to use diff -Naur (see Section
     “Tips for Patch Producers”), but 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), exclude them from the patch by giving diff the [-x
     pattern] option (see Section “Comparing Directories”).  If you want your
     patch to modify a derived file because your recipients lack tools to build
     it, make sure that the patch for the derived file follows any patches for
     files that it depends on, so that the recipients' time stamps will not
     confuse make.

     Now you can create the patch using diff -Naur.  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.

     You can also shrink the patch size by using fewer lines of context, but
     bear in mind that patch typically needs at least two lines for proper
     operation when patches do not exactly match the input files.

Invoking cmp
     The cmp command compares two files, and if they differ, tells the first
     byte and line number where they differ or reports that one file is a prefix
     of the other.  Bytes and lines are numbered starting with 1. The arguments
     of cmp are as follows:

           cmp options... from-file [to-file [from-skip [to-skip]]]

     The file name - is always the standard input.  cmp also uses the standard
     input if one file name is omitted. The from-skip and to-skip operands
     specify how many bytes to ignore at the start of each file; they are
     equivalent to the [--ignore-initial= from-skip: to-skip] option.

     By default, cmp outputs nothing if the two files have the same contents. If
     one file is a prefix of the other, cmp prints to standard error a message
     of the following form:

           cmp: EOF on shorter-file

     Otherwise, cmp prints to standard output a message of the following form:

           from-file to-file differ: char byte-number, line line-number

     The message formats can differ outside the POSIX locale. Also, POSIX allows
     the EOF message to be followed by a blank and some additional information.

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

   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: [-bl] is equivalent to [-b -l].

     -b

     --print-bytes
             Print the differing bytes. Display control bytes as a ^ followed by
             a letter of the alphabet and precede bytes that have the high bit
             set with M- (which stands for “meta”).

     --help  Output a summary of usage and then exit.

     -i skip

     --ignore-initial= skip
             Ignore any differences in the first skip bytes of the input files.
             Treat files with fewer than skip bytes as if they are empty. If
             skip is of the form [from-skip: to-skip], skip the first from-skip
             bytes of the first input file and the first to-skip bytes of the
             second.

     -l

     --verbose
             Output the (decimal) byte numbers and (octal) values of all
             differing bytes, instead of the default standard output.

     -n count

     --bytes= count
             Compare at most count input bytes.

     -s

     --quiet

     --silent
             Do not print anything; only return an exit status indicating
             whether the files differ.

     -v

     --version
             Output version information and then exit.

     In the above table, operands that are byte counts are normally decimal, but
     may be preceded by 0 for octal and 0x for hexadecimal.

     A byte count can be followed by a suffix to specify a multiple of that
     count; in this case an omitted integer is understood to be 1. A bare size
     letter, or one followed by iB, specifies a multiple using powers of 1024. A
     size letter followed by B specifies powers of 1000 instead. For example,
     [-n 4M] and [-n 4MiB] are equivalent to [-n 4194304], whereas [-n 4MB] is
     equivalent to [-n 4000000].  This notation is upward compatible with the
     http://www.bipm.fr/enus/3_SI/si-prefixes.html for decimal multiples and
     with the http://physics.nist.gov/cuu/Units/binary.html

     The following suffixes are defined. Large sizes like 1Y may be rejected by
     your computer due to limitations of its arithmetic.

     kB      kilobyte: 10^3 = 1000.

     k

     K

     KiB     kibibyte: 2^10 = 1024.  K is special: the SI prefix is k and the
             IEC 60027-2 prefix is Ki, but tradition and POSIX use k to mean
             KiB.

     MB      megabyte: 10^6 = 1,000,000.

     M

     MiB     mebibyte: 2^20 = 1,048,576.

     GB      gigabyte: 10^9 = 1,000,000,000.

     G

     GiB     gibibyte: 2^30 = 1,073,741,824.

     TB      terabyte: 10^12 = 1,000,000,000,000.

     T

     TiB     tebibyte: 2^40 = 1,099,511,627,776.

     PB      petabyte: 10^15 = 1,000,000,000,000,000.

     P

     PiB     pebibyte: 2^50 = 1,125,899,906,842,624.

     EB      exabyte: 10^18 = 1,000,000,000,000,000,000.

     E

     EiB     exbibyte: 2^60 = 1,152,921,504,606,846,976.

     ZB      zettabyte: 10^21 = 1,000,000,000,000,000,000,000

     Z

     ZiB     2^70 = 1,180,591,620,717,411,303,424. ( Zi is a GNU extension to
             IEC 60027-2.)

     YB      yottabyte: 10^24 = 1,000,000,000,000,000,000,000,000.

     Y

     YiB     2^80 = 1,208,925,819,614,629,174,706,176. ( Yi is a GNU extension
             to IEC 60027-2.)

Invoking diff
     The format for running the diff command is:

           diff options... files...

     In the simplest case, two file names from-file and to-file are given, and
     diff compares the contents of 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 one file is a directory and the other is not, diff compares the file in
     the directory whose name is that of the non-directory.  The non-directory
     file must not be -.

     If two file names are given and both 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.

     If the [--from-file= file] option is given, the number of file names is
     arbitrary, and file is compared to each named file. Similarly, if the
     [--to-file= file] option is given, each named file is compared to file.

     diff options begin with -, so normally file names 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. Normally, differing binary
     files count as trouble, but this can be altered by using the [-a] or
     [--text] option, or the [-q] or [--brief] option.

   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.

     -a

     --text  Treat all files as text and compare them line-by-line, even if they
             do not seem to be text.See Section “Binary”.

     -b

     --ignore-space-change
             Ignore changes in amount of white space.See Section “White Space”.

     -B

     --ignore-blank-lines
             Ignore changes that just insert or delete blank lines.See Section
             “Blank Lines”.

     --binary
             Read and write data in binary mode.See Section “Binary”.

     -c      Use the context output format, showing three lines of context.See
             Section “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 Section “Context
             Format”.  For proper operation, patch typically needs at least two
             lines of context.

             On older systems, diff supports an obsolete option [- lines] that
             has effect when combined with [-c] or [-p].  POSIX 1003.1-2001 (see
             Section “Standards conformance”) does not allow this; use [-C
             lines] instead.

     --changed-group-format= format
             Use format to output a line group containing differing lines from
             both files in if-then-else format.See Section “Line Group Formats”.

     -d

     --minimal
             Change the algorithm perhaps find a smaller set of changes. This
             makes diff slower (sometimes much slower).See Section “diff
             Performance”.

     -D name

     --ifdef= name
             Make merged #ifdef format output, conditional on the preprocessor
             macro name.  See Section.Dq If-then-else .

     -e

     --ed    Make output that is a valid ed script.See Section “ed Scripts”.

     -E

     --ignore-tab-expansion
             Ignore changes due to tab expansion.See Section “White Space”.

     -f

     --forward-ed
             Make output that looks vaguely like an ed script but has changes in
             the order they appear in the file.See Section “Forward ed”.

     -F regexp

     --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
             Section.Dq Specified Headings .

     --from-file= file
             Compare file to each operand; file may be a directory.

     --help  Output a summary of usage and then exit.

     --horizon-lines= lines
             Do not discard the last lines lines of the common prefix and the
             first lines lines of the common suffix.See Section “diff
             Performance”.

     -i

     --ignore-case
             Ignore changes in case; consider upper- and lower-case letters
             equivalent.See Section “Case Folding”.

     -I regexp

     --ignore-matching-lines= regexp
             Ignore changes that just insert or delete lines that match regexp.
             See Section.Dq Specified Lines .

     --ignore-file-name-case
             Ignore case when comparing file names during recursive
             comparison.See Section “Comparing Directories”.

     -l

     --paginate
             Pass the output through pr to paginate it.See Section “Pagination”.

     --label= label
             Use label instead of the file name in the context format (see
             Section “Context Format”) and unified format (see Section “Unified
             Format”) headers.See Section “RCS”.

     --left-column
             Print only the left column of two common lines in side by side
             format.See Section “Side by Side Format”.

     --line-format= format
             Use format to output all input lines in if-then-else format.See
             Section “Line Formats”.

     -n

     --rcs   Output RCS-format diffs; like [-f] except that each command
             specifies the number of lines affected.See Section “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 Section
             “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 Section “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 Section “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 Section “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 Section “Line Formats”.

     -p

     --show-c-function
             Show which C function each change is in.See Section “C Function
             Headings”.

     -q

     --brief
             Report only whether the files differ, not the details of the
             differences.See Section “Brief”.

     -r

     --recursive
             When comparing directories, recursively compare any subdirectories
             found.See Section “Comparing Directories”.

     -s

     --report-identical-files
             Report when two files are the same.See Section “Comparing
             Directories”.

     -S file

     --starting-file= file
             When comparing directories, start with the file file.  This is used
             for resuming an aborted comparison.See Section “Comparing
             Directories”.

     --speed-large-files
             Use heuristics to speed handling of large files that have numerous
             scattered small changes.See Section “diff Performance”.

     --strip-trailing-cr
             Strip any trailing carriage return at the end of an input line.See
             Section “Binary”.

     --suppress-common-lines
             Do not print common lines in side by side format.See Section “Side
             by Side Format”.

     -t

     --expand-tabs
             Expand tabs to spaces in the output, to preserve the alignment of
             tabs in the input files.See Section “Tabs”.

     -T

     --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 Section “Tabs”.

     --tabsize= columns
             Assume that tab stops are set every columns (default 8) print
             columns.See Section “Tabs”.

     --to-file= file
             Compare each operand to file ; file may be a directory.

     -u      Use the unified output format, showing three lines of context.See
             Section “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 Section “Line Group Formats”.

     --unchanged-line-format= format
             Use format to output a line common to both files in if-then-else
             format.See Section “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 Section “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 Section “Unified
             Format”.  For proper operation, patch typically needs at least two
             lines of context.

             On older systems, diff supports an obsolete option [- lines] that
             has effect when combined with [-u].  POSIX 1003.1-2001 (see Section
             “Standards conformance”) does not allow this; use [-U lines]
             instead.

     -v

     --version
             Output version information and then exit.

     -w

     --ignore-all-space
             Ignore white space when comparing lines.See Section “White Space”.

     -W columns

     --width= columns
             Output at most columns (default 130) print columns per line in side
             by side format.See Section “Side by Side Format”.

     -x pattern

     --exclude= pattern
             When comparing directories, ignore files and subdirectories whose
             basenames match pattern.  See Section.Dq Comparing Directories .

     -X file

     --exclude-from= file
             When comparing directories, ignore files and subdirectories whose
             basenames match any pattern contained in file.  See Section.Dq
             Comparing Directories .

     -y

     --side-by-side
             Use the side by side output format.See Section “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.

   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

     --text  Treat all files as text and compare them line-by-line, even if they
             do not appear to be text.See Section “Binary”.

     -A

     --show-all
             Incorporate all unmerged changes from older to yours into mine,
             surrounding conflicts with bracket lines.See Section “Marking
             Conflicts”.

     --diff-program= program
             Use the compatible comparison program program to compare files
             instead of diff.

     -e

     --ed    Generate an ed script that incorporates all the changes from older
             to yours into mine.  See Section.Dq Which Changes .

     -E

     --show-overlap
             Like [-e], except bracket lines from overlapping changes' first and
             third files.See Section “Marking Conflicts”.  With [-E], an
             overlapping change looks like this:

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

     --help  Output a summary of usage and then exit.

     -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
             Section.Dq Saving the Changed File .

     --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 --label X --label Y --label 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 Section.Dq 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 Section “Bypassing ed”.

     --strip-trailing-cr
             Strip any trailing carriage return at the end of an input line.See
             Section “Binary”.

     -T

     --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 Section “Tabs”.

     -v

     --version
             Output version information and then exit.

     -x

     --overlap-only
             Like [-e], except output only the overlapping changes.See Section
             “Which Changes”.

     -X      Like [-E], except output only the overlapping changes. In other
             words, like [-x], except bracket changes as in [-E].  See
             Section.Dq Marking Conflicts .

     -3

     --easy-only
             Like [-e], except output only the nonoverlapping changes.See
             Section “Which Changes”.

Invoking patch
     Normally patch is invoked like this:

           patch <patchfile

     The full format for invoking patch is:

           patch options... [origfile [patchfile]]

     You can also specify where to read the patch from with the [-i patchfile]
     or [--input= patchfile] option. If you do not specify patchfile, or if
     patchfile is -, patch reads the patch (that is, the diff output) from the
     standard input.

     If you do not specify an input file on the command line, patch tries to
     intuit from the leading text (any text in the patch that comes before the
     diff output) which file to edit.See Section “Multiple Patches”.

     By default, patch replaces the original input file with the patched
     version, possibly after renaming the original file into a backup file (see
     Section “Backup Names”, for a description of how patch names backup files).
     You can also specify where to put the output with the [-o file] or
     [--output= file] option; however, do not use this option if file is one of
     the input files.

   Options to patch
     Here is a summary of all of the options that GNU patch accepts.See Section
     “patch and Tradition”, for which of these options are safe to use in older
     versions of patch.

     Multiple single-letter options that do not take an argument can be combined
     into a single command line argument with only one dash.

     -b

     --backup
             Back up the original contents of each file, even if backups would
             normally not be made.See Section “Backups”.

     -B prefix

     --prefix= prefix
             Prepend prefix to backup file names.See Section “Backup Names”.

     --backup-if-mismatch
             Back up the original contents of each file if the patch does not
             exactly match the file. This is the default behavior when not
             conforming to POSIX.See Section “Backups”.

     --binary
             Read and write all files in binary mode, except for standard output
             and /dev/tty.  This option has no effect on POSIX-conforming
             systems like GNU/Linux. On systems where this option makes a
             difference, the patch should be generated by diff -a --binary.  See
             Section.Dq Binary .

     -c

     --context
             Interpret the patch file as a context diff.See Section “patch
             Input”.

     -d directory

     --directory= directory
             Make directory directory the current directory for interpreting
             both file names in the patch file, and file names given as
             arguments to other options.See Section “patch Directories”.

     -D name

     --ifdef= name
             Make merged if-then-else output using name.  See Section.Dq If-
             then-else .

     --dry-run
             Print the results of applying the patches without actually changing
             any files.See Section “Dry Runs”.

     -e

     --ed    Interpret the patch file as an ed script.See Section “patch Input”.

     -E

     --remove-empty-files
             Remove output files that are empty after the patches have been
             applied.See Section “Creating and Removing”.

     -f

     --force
             Assume that the user knows exactly what he or she is doing, and do
             not ask any questions.See Section “patch Messages”.

     -F lines

     --fuzz= lines
             Set the maximum fuzz factor to lines.  See Section.Dq Inexact .

     -g num

     --get= num
             If num is positive, get input files from a revision control system
             as necessary; if zero, do not get the files; if negative, ask the
             user whether to get the files.See Section “Revision Control”.

     --help  Output a summary of usage and then exit.

     -i patchfile

     --input= patchfile
             Read the patch from patchfile rather than from standard input.See
             Section “patch Options”.

     -l

     --ignore-white-space
             Let any sequence of blanks (spaces or tabs) in the patch file match
             any sequence of blanks in the input file.See Section “Changed White
             Space”.

     -n

     --normal
             Interpret the patch file as a normal diff.See Section “patch
             Input”.

     -N

     --forward
             Ignore patches that patch thinks are reversed or already applied.
             See also [-R].  See Section.Dq Reversed Patches .

     --no-backup-if-mismatch
             Do not back up the original contents of files. This is the default
             behavior when conforming to POSIX.See Section “Backups”.

     -o file

     --output= file
             Use file as the output file name.See Section “patch Options”.

     -p number

     --strip= number
             Set the file name strip count to number.  See Section.Dq patch
             Directories .

     --posix
             Conform to POSIX, as if the POSIXLY_CORRECT environment variable
             had been set.See Section “patch and POSIX”.

     --quoting-style= word
             Use style word to quote names in diagnostics, as if the
             QUOTING_STYLE environment variable had been set to word.  See
             Section.Dq patch Quoting Style .

     -r reject-file

     --reject-file= reject-file
             Use reject-file as the reject file name.See Section “Reject Names”.

     -R

     --reverse
             Assume that this patch was created with the old and new files
             swapped.See Section “Reversed Patches”.

     -s

     --quiet

     --silent
             Work silently unless an error occurs.See Section “patch Messages”.

     -t

     --batch
             Do not ask any questions.See Section “patch Messages”.

     -T

     --set-time
             Set the modification and access times of patched files from time
             stamps given in context diff headers, assuming that the context
             diff headers use local time.See Section “Patching Time Stamps”.

     -u

     --unified
             Interpret the patch file as a unified diff.See Section “patch
             Input”.

     -v

     --version
             Output version information and then exit.

     -V backup-style

     --version=control= backup-style
             Select the naming convention for backup file names.See Section
             “Backup Names”.

     --verbose
             Print more diagnostics than usual.See Section “patch Messages”.

     -x number

     --debug= number
             Set internal debugging flags. Of interest only to patch patchers.

     -Y prefix

     --basename-prefix= prefix
             Prepend prefix to base names of backup files.See Section “Backup
             Names”.

     -z suffix

     --suffix= suffix
             Use suffix as the backup extension instead of .orig or ~.  See
             Section.Dq Backup Names .

     -Z

     --set-utc
             Set the modification and access times of patched files from time
             stamps given in context diff headers, assuming that the context
             diff headers use UTC.See Section “Patching Time Stamps”.

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.

     sdiff without [-o] (or [--output]) produces a side-by-side difference. This
     usage is obsolete; use the [-y] or [--side-by-side] option of diff instead.

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

   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

     --text  Treat all files as text and compare them line-by-line, even if they
             do not appear to be text.See Section “Binary”.

     -b

     --ignore-space-change
             Ignore changes in amount of white space.See Section “White Space”.

     -B

     --ignore-blank-lines
             Ignore changes that just insert or delete blank lines.See Section
             “Blank Lines”.

     -d

     --minimal
             Change the algorithm to perhaps find a smaller set of changes. This
             makes sdiff slower (sometimes much slower).See Section “diff
             Performance”.

     --diff-program= program
             Use the compatible comparison program program to compare files
             instead of diff.

     -E

     --ignore-tab-expansion
             Ignore changes due to tab expansion.See Section “White Space”.

     --help  Output a summary of usage and then exit.

     -i

     --ignore-case
             Ignore changes in case; consider upper- and lower-case to be the
             same.See Section “Case Folding”.

     -I regexp

     --ignore-matching-lines= regexp
             Ignore changes that just insert or delete lines that match regexp.
             See Section.Dq Specified Lines .

     -l

     --left-column
             Print only the left column of two common lines.See Section “Side by
             Side Format”.

     -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 Section “Side by Side Format”.

     --speed-large-files
             Use heuristics to speed handling of large files that have numerous
             scattered small changes.See Section “diff Performance”.

     --strip-trailing-cr
             Strip any trailing carriage return at the end of an input line.See
             Section “Binary”.

     -t

     --expand-tabs
             Expand tabs to spaces in the output, to preserve the alignment of
             tabs in the input files.See Section “Tabs”.

     --tabsize= columns
             Assume that tab stops are set every columns (default 8) print
             columns.See Section “Tabs”.

     -v

     --version
             Output version information and then exit.

     -w columns

     --width= columns
             Output at most columns (default 130) print columns per line.See
             Section “Side by Side Format”.  Note that for historical reasons,
             this option is [-W] in diff, [-w] in sdiff.

     -W

     --ignore-all-space
             Ignore white space when comparing lines.See Section “White Space”.
             Note that for historical reasons, this option is [-w] in diff, [-W]
             in sdiff.

Standards conformance
     In a few cases, the GNU utilities' default behavior is incompatible with
     the POSIX standard. To suppress these incompatibilities, define the
     POSIXLY_CORRECT environment variable. Unless you are checking for POSIX
     conformance, you probably do not need to define POSIXLY_CORRECT.

     Normally options and operands can appear in any order, and programs act as
     if all the options appear before any operands. For example, diff lao tzu -C
     2 acts like diff -C 2 lao tzu, since 2 is an option-argument of [-C].
     However, if the POSIXLY_CORRECT environment variable is set, options must
     appear before operands, unless otherwise specified for a particular
     command.

     Newer versions of POSIX are occasionally incompatible with older versions.
     For example, older versions of POSIX allowed the command diff -c -10 to
     have the same meaning as diff -C 10, but POSIX 1003.1-2001 diff no longer
     allows digit-string options like [-10].

     The GNU utilities normally conform to the version of POSIX that is standard
     for your system. To cause them to conform to a different version of POSIX,
     define the _POSIX2_VERSION environment variable to a value of the form
     yyyymm specifying the year and month the standard was adopted. Two values
     are currently supported for _POSIX2_VERSION: 199209 stands for POSIX
     1003.2-1992, and 200112 stands for POSIX 1003.1-2001. For example, if you
     are running older software that assumes an older version of POSIX and uses
     diff -c -10, you can work around the compatibility problems by setting
     _POSIX2_VERSION=199209 in your environment.

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 gvc@gnu.org to coordinate
     with other volunteers.

   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 Multibyte and Varying-Width Characters

     diff, diff3 and sdiff treat each line of input as a string of unibyte
     characters. This can mishandle multibyte characters in some cases. For
     example, when asked to ignore spaces, diff does not properly ignore a
     multibyte space character.

     Also, diff currently assumes that each byte is one column wide, and this
     assumption is incorrect in some locales, e.g., locales that use UTF-8
     encoding. This causes problems with the [-y] or [--side-by-side] option of
     diff.

     These problems need to be fixed without unduly affecting the performance of
     the utilities in unibyte environments.

     The IBM GNU/Linux Technology Center Internationalization Team has proposed
     http://oss.software.ibm.com/developer/opensource/linux/patches/i18n/diffutils-2.7.2-i18n-0.1.patch.gz
     Unfortunately, these patches are incomplete and are to an older version of
     diff, so more work needs to be done in this area.

     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 directory
     subtree into the file.

     There should be a way to specify that a file has been removed 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. There should be a way to tell patch that
     a file's time stamp has changed, even if its contents have not changed.

     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 as if they were the pointed-to files,
     except that a recursive diff reports an error if it detects infinite loops
     of symbolic links (e.g., symbolic links to ..).  diff 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.

     Outputting Diffs in Time Stamp Order

     Applying patch to a multiple-file diff can result in files whose time
     stamps are out of order.  GNU patch has options to restore the time stamps
     of the updated files (see Section “Patching Time Stamps”), but sometimes it
     is useful to generate a patch that works even if the recipient does not
     have GNU patch, or does not use these options. One way to do this would be
     to implement a diff option to output diffs in time stamp order.

     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.

     A partial substitute is to filter one or both files before comparing, e.g.:

           sed 's/foo/bar/g' file1 | diff - file2

     However, this outputs the filtered text, not the original.

     Improving Performance

     When comparing two large directory structures, one of which was originally
     copied from the other with time stamps preserved (e.g., with cp -pR), it
     would greatly improve performance if an option told diff to assume that two
     files with the same size and time stamps have the same content.See Section
     “diff Performance”.

   Reporting Bugs
     If you think you have found a bug in GNU cmp, diff, diff3, or sdiff, please
     report it by electronic mail to the
     http://mail.gnu.org/mailman/listinfo/bug-gnu-utils bug-gnu-utils@gnu.org.
     Please send bug reports for GNU patch to bug-patch@gnu.org.  Send as
     precise a description of the problem as you can, including the output of
     the [--version] option and sample input files that produce the bug, if
     applicable. If you have a nontrivial fix for the bug, please send it as
     well. If you have a patch, please send it too. It may simplify the
     maintainer's job if the patch is relative to a recent test release, which
     you can find in the directory ftp://alpha.gnu.org/gnu/diffutils/

Copying This Manual
   GNU Free Documentation License
           Copyright © 2000,2001,2002 Free Software Foundation, Inc. 59 Temple
           Place, Suite 330, Boston, MA 02111-1307, USA

           Everyone is permitted to copy and distribute verbatim copies of this
           license document, but changing it is not allowed.

     1.   PREAMBLE

          The purpose of this License is to make a manual, textbook, or other
          functional and useful document free in the sense of freedom: to assure
          everyone the effective freedom to copy and redistribute it, with or
          without modifying it, either commercially or noncommercially.
          Secondarily, this License preserves for the author and publisher a way
          to get credit for their work, while not being considered responsible
          for modifications made by others.

          This License is a kind of “copyleft”, which means that derivative
          works of the document must themselves be free in the same sense. It
          complements the GNU General Public License, which is a copyleft
          license designed for free software.

          We have designed this License in order to use it for manuals for free
          software, because free software needs free documentation: a free
          program should come with manuals providing the same freedoms that the
          software does. But this License is not limited to software manuals; it
          can be used for any textual work, regardless of subject matter or
          whether it is published as a printed book. We recommend this License
          principally for works whose purpose is instruction or reference.

     2.   APPLICABILITY AND DEFINITIONS

          This License applies to any manual or other work, in any medium, that
          contains a notice placed by the copyright holder saying it can be
          distributed under the terms of this License. Such a notice grants a
          world-wide, royalty-free license, unlimited in duration, to use that
          work under the conditions stated herein. The “Document”, below, refers
          to any such manual or work. Any member of the public is a licensee,
          and is addressed as “you”. You accept the license if you copy, modify
          or distribute the work in a way requiring permission under copyright
          law.

          A “Modified Version” of the Document means any work containing the
          Document or a portion of it, either copied verbatim, or with
          modifications and/or translated into another language.

          A “Secondary Section” is a named appendix or a front-matter section of
          the Document that deals exclusively with the relationship of the
          publishers or authors of the Document to the Document's overall
          subject (or to related matters) and contains nothing that could fall
          directly within that overall subject.  (Thus, if the Document is in
          part a textbook of mathematics, a Secondary Section may not explain
          any mathematics.) The relationship could be a matter of historical
          connection with the subject or with related matters, or of legal,
          commercial, philosophical, ethical or political position regarding
          them.

          The “Invariant Sections” are certain Secondary Sections whose titles
          are designated, as being those of Invariant Sections, in the notice
          that says that the Document is released under this License. If a
          section does not fit the above definition of Secondary then it is not
          allowed to be designated as Invariant. The Document may contain zero
          Invariant Sections. If the Document does not identify any Invariant
          Sections then there are none.

          The “Cover Texts” are certain short passages of text that are listed,
          as Front-Cover Texts or Back-Cover Texts, in the notice that says that
          the Document is released under this License. A Front-Cover Text may be
          at most 5 words, and a Back-Cover Text may be at most 25 words.

          A “Transparent” copy of the Document means a machine-readable copy,
          represented in a format whose specification is available to the
          general public, that is suitable for revising the document
          straightforwardly with generic text editors or (for images composed of
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          drawing editor, and that is suitable for input to text formatters or
          for automatic translation to a variety of formats suitable for input
          to text formatters. A copy made in an otherwise Transparent file
          format whose markup, or absence of markup, has been arranged to thwart
          or discourage subsequent modification by readers is not Transparent.
          An image format is not Transparent if used for any substantial amount
          of text. A copy that is not “Transparent” is called “Opaque”.

          Examples of suitable formats for Transparent copies include plain
          ascii without markup, Texinfo input format, LaTeX input format, SGML
          or XML using a publicly available DTD, and standard-conforming simple
          HTML, PostScript or PDF designed for human modification. Examples of
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          include proprietary formats that can be read and edited only by
          proprietary word processors, SGML or XML for which the DTD and/or
          processing tools are not generally available, and the machine-
          generated HTML, PostScript or PDF produced by some word processors for
          output purposes only.

          The “Title Page” means, for a printed book, the title page itself,
          plus such following pages as are needed to hold, legibly, the material
          this License requires to appear in the title page. For works in
          formats which do not have any title page as such, “Title Page” means
          the text near the most prominent appearance of the work's title,
          preceding the beginning of the body of the text.

          A section “Entitled XYZ” means a named subunit of the Document whose
          title either is precisely XYZ or contains XYZ in parentheses following
          text that translates XYZ in another language. (Here XYZ stands for a
          specific section name mentioned below, such as “Acknowledgements”,
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          of such a section when you modify the Document means that it remains a
          section “Entitled XYZ” according to this definition.

          The Document may include Warranty Disclaimers next to the notice which
          states that this License applies to the Document. These Warranty
          Disclaimers are considered to be included by reference in this
          License, but only as regards disclaiming warranties: any other
          implication that these Warranty Disclaimers may have is void and has
          no effect on the meaning of this License.

     3.   VERBATIM COPYING

          You may copy and distribute the Document in any medium, either
          commercially or noncommercially, provided that this License, the
          copyright notices, and the license notice saying this License applies
          to the Document are reproduced in all copies, and that you add no
          other conditions whatsoever to those of this License. You may not use
          technical measures to obstruct or control the reading or further
          copying of the copies you make or distribute. However, you may accept
          compensation in exchange for copies. If you distribute a large enough
          number of copies you must also follow the conditions in section 3.

          You may also lend copies, under the same conditions stated above, and
          you may publicly display copies.

     4.   COPYING IN QUANTITY

          If you publish printed copies (or copies in media that commonly have
          printed covers) of the Document, numbering more than 100, and the
          Document's license notice requires Cover Texts, you must enclose the
          copies in covers that carry, clearly and legibly, all these Cover
          Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
          the back cover. Both covers must also clearly and legibly identify you
          as the publisher of these copies. The front cover must present the
          full title with all words of the title equally prominent and visible.
          You may add other material on the covers in addition. Copying with
          changes limited to the covers, as long as they preserve the title of
          the Document and satisfy these conditions, can be treated as verbatim
          copying in other respects.

          If the required texts for either cover are too voluminous to fit
          legibly, you should put the first ones listed (as many as fit
          reasonably) on the actual cover, and continue the rest onto adjacent
          pages.

          If you publish or distribute Opaque copies of the Document numbering
          more than 100, you must either include a machine-readable Transparent
          copy along with each Opaque copy, or state in or with each Opaque copy
          a computer-network location from which the general network-using
          public has access to download using public-standard network protocols
          a complete Transparent copy of the Document, free of added material.
          If you use the latter option, you must take reasonably prudent steps,
          when you begin distribution of Opaque copies in quantity, to ensure
          that this Transparent copy will remain thus accessible at the stated
          location until at least one year after the last time you distribute an
          Opaque copy (directly or through your agents or retailers) of that
          edition to the public.

          It is requested, but not required, that you contact the authors of the
          Document well before redistributing any large number of copies, to
          give them a chance to provide you with an updated version of the
          Document.

     5.   MODIFICATIONS

          You may copy and distribute a Modified Version of the Document under
          the conditions of sections 2 and 3 above, provided that you release
          the Modified Version under precisely this License, with the Modified
          Version filling the role of the Document, thus licensing distribution
          and modification of the Modified Version to whoever possesses a copy
          of it. In addition, you must do these things in the Modified Version:

          1.   Use in the Title Page (and on the covers, if any) a title
               distinct from that of the Document, and from those of previous
               versions (which should, if there were any, be listed in the
               History section of the Document). You may use the same title as a
               previous version if the original publisher of that version gives
               permission.

          2.   List on the Title Page, as authors, one or more persons or
               entities responsible for authorship of the modifications in the
               Modified Version, together with at least five of the principal
               authors of the Document (all of its principal authors, if it has
               fewer than five), unless they release you from this requirement.

          3.   State on the Title page the name of the publisher of the Modified
               Version, as the publisher.

          4.   Preserve all the copyright notices of the Document.

          5.   Add an appropriate copyright notice for your modifications
               adjacent to the other copyright notices.

          6.   Include, immediately after the copyright notices, a license
               notice giving the public permission to use the Modified Version
               under the terms of this License, in the form shown in the
               Addendum below.

          7.   Preserve in that license notice the full lists of Invariant
               Sections and required Cover Texts given in the Document's license
               notice.

          8.   Include an unaltered copy of this License.

          9.   Preserve the section Entitled “History”, Preserve its Title, and
               add to it an item stating at least the title, year, new authors,
               and publisher of the Modified Version as given on the Title Page.
               If there is no section Entitled “History” in the Document, create
               one stating the title, year, authors, and publisher of the
               Document as given on its Title Page, then add an item describing
               the Modified Version as stated in the previous sentence.

          10.  Preserve the network location, if any, given in the Document for
               public access to a Transparent copy of the Document, and likewise
               the network locations given in the Document for previous versions
               it was based on. These may be placed in the “History” section.
               You may omit a network location for a work that was published at
               least four years before the Document itself, or if the original
               publisher of the version it refers to gives permission.

          11.  For any section Entitled “Acknowledgements” or “Dedications”,
               Preserve the Title of the section, and preserve in the section
               all the substance and tone of each of the contributor
               acknowledgements and/or dedications given therein.

          12.  Preserve all the Invariant Sections of the Document, unaltered in
               their text and in their titles. Section numbers or the equivalent
               are not considered part of the section titles.

          13.  Delete any section Entitled “Endorsements”. Such a section may
               not be included in the Modified Version.

          14.  Do not retitle any existing section to be Entitled “Endorsements”
               or to conflict in title with any Invariant Section.

          15.  Preserve any Warranty Disclaimers.

          If the Modified Version includes new front-matter sections or
          appendices that qualify as Secondary Sections and contain no material
          copied from the Document, you may at your option designate some or all
          of these sections as invariant.  To do this, add their titles to the
          list of Invariant Sections in the Modified Version's license notice.
          These titles must be distinct from any other section titles.

          You may add a section Entitled “Endorsements”, provided it contains
          nothing but endorsements of your Modified Version by various
          parties---for example, statements of peer review or that the text has
          been approved by an organization as the authoritative definition of a
          standard.

          You may add a passage of up to five words as a Front-Cover Text, and a
          passage of up to 25 words as a Back-Cover Text, to the end of the list
          of Cover Texts in the Modified Version. Only one passage of Front-
          Cover Text and one of Back-Cover Text may be added by (or through
          arrangements made by) any one entity. If the Document already includes
          a cover text for the same cover, previously added by you or by
          arrangement made by the same entity you are acting on behalf of, you
          may not add another; but you may replace the old one, on explicit
          permission from the previous publisher that added the old one.

          The author(s) and publisher(s) of the Document do not by this License
          give permission to use their names for publicity for or to assert or
          imply endorsement of any Modified Version.

     6.   COMBINING DOCUMENTS

          You may combine the Document with other documents released under this
          License, under the terms defined in section 4 above for modified
          versions, provided that you include in the combination all of the
          Invariant Sections of all of the original documents, unmodified, and
          list them all as Invariant Sections of your combined work in its
          license notice, and that you preserve all their Warranty Disclaimers.

          The combined work need only contain one copy of this License, and
          multiple identical Invariant Sections may be replaced with a single
          copy. If there are multiple Invariant Sections with the same name but
          different contents, make the title of each such section unique by
          adding at the end of it, in parentheses, the name of the original
          author or publisher of that section if known, or else a unique number.
          Make the same adjustment to the section titles in the list of
          Invariant Sections in the license notice of the combined work.

          In the combination, you must combine any sections Entitled “History”
          in the various original documents, forming one section Entitled
          “History”; likewise combine any sections Entitled “Acknowledgements”,
          and any sections Entitled “Dedications”. You must delete all sections
          Entitled “Endorsements.”

     7.   COLLECTIONS OF DOCUMENTS

          You may make a collection consisting of the Document and other
          documents released under this License, and replace the individual
          copies of this License in the various documents with a single copy
          that is included in the collection, provided that you follow the rules
          of this License for verbatim copying of each of the documents in all
          other respects.

          You may extract a single document from such a collection, and
          distribute it individually under this License, provided you insert a
          copy of this License into the extracted document, and follow this
          License in all other respects regarding verbatim copying of that
          document.

     8.   AGGREGATION WITH INDEPENDENT WORKS

          A compilation of the Document or its derivatives with other separate
          and independent documents or works, in or on a volume of a storage or
          distribution medium, is called an “aggregate” if the copyright
          resulting from the compilation is not used to limit the legal rights
          of the compilation's users beyond what the individual works permit.
          When the Document is included in an aggregate, this License does not
          apply to the other works in the aggregate which are not themselves
          derivative works of the Document.

          If the Cover Text requirement of section 3 is applicable to these
          copies of the Document, then if the Document is less than one half of
          the entire aggregate, the Document's Cover Texts may be placed on
          covers that bracket the Document within the aggregate, or the
          electronic equivalent of covers if the Document is in electronic form.
          Otherwise they must appear on printed covers that bracket the whole
          aggregate.

     9.   TRANSLATION

          Translation is considered a kind of modification, so you may
          distribute translations of the Document under the terms of section 4.
          Replacing Invariant Sections with translations requires special
          permission from their copyright holders, but you may include
          translations of some or all Invariant Sections in addition to the
          original versions of these Invariant Sections. You may include a
          translation of this License, and all the license notices in the
          Document, and any Warranty Disclaimers, provided that you also include
          the original English version of this License and the original versions
          of those notices and disclaimers. In case of a disagreement between
          the translation and the original version of this License or a notice
          or disclaimer, the original version will prevail.

          If a section in the Document is Entitled “Acknowledgements”,
          “Dedications”, or “History”, the requirement (section 4) to Preserve
          its Title (section 1) will typically require changing the actual
          title.

     10.  TERMINATION

          You may not copy, modify, sublicense, or distribute the Document
          except as expressly provided for under this License. Any other attempt
          to copy, modify, sublicense or distribute the Document is void, and
          will automatically terminate your rights under this License. However,
          parties who have received copies, or rights, from you under this
          License will not have their licenses terminated so long as such
          parties remain in full compliance.

     11.  FUTURE REVISIONS OF THIS LICENSE

          The Free Software Foundation may publish new, revised versions of the
          GNU Free Documentation License from time to time. Such new versions
          will be similar in spirit to the present version, but may differ in
          detail to address new problems or concerns. See
          http://www.gnu.org/copyleft/

          Each version of the License is given a distinguishing version number.
          If the Document specifies that a particular numbered version of this
          License “or any later version” applies to it, you have the option of
          following the terms and conditions either of that specified version or
          of any later version that has been published (not as a draft) by the
          Free Software Foundation. If the Document does not specify a version
          number of this License, you may choose any version ever published (not
          as a draft) by the Free Software Foundation.

     ADDENDUM: How to use this License for your documents

     To use this License in a document you have written, include a copy of the
     License in the document and put the following copyright and license notices
     just after the title page:

             Copyright (C)  year  your name.
             Permission is granted to copy, distribute and/or modify this document
             under the terms of the GNU Free Documentation License, Version 1.2
             or any later version published by the Free Software Foundation;
             with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
             Texts.  A copy of the license is included in the section entitled “GNU
             Free Documentation License”.


     If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
     replace the “with...Texts.” line with this:

               with the Invariant Sections being list their titles, with
               the Front-Cover Texts being list, and with the Back-Cover Texts
               being list.


     If you have Invariant Sections without Cover Texts, or some other
     combination of the three, merge those two alternatives to suit the
     situation.

     If your document contains nontrivial examples of program code, we recommend
     releasing these examples in parallel under your choice of free software
     license, such as the GNU General Public License, to permit their use in
     free software.

Translations of This Manual
     Nishio Futoshi of the GNUjdoc project has prepared a Japanese translation
     of this manual. Its most recent version can be found at
     http://openlab.ring.gr.jp/gnujdoc/cvsweb/cvsweb.cgi/gnujdoc/

Index
BSD                             October 28, 2021                             BSD