Man

Command Section
DIFF(7)            FreeBSD 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     one file is a newer version of the other file.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 Torbj"rn 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 [email protected] 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 [email protected].
     Please send bug reports for GNU patch to [email protected].  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 (C) 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
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          available 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 transparent image formats include PNG, XCF and JPG.
          Opaque formats 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", "Dedications", "Endorsements", or "History".) To
          "Preserve the Title" 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

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Command Section