\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename diff.info
@settitle Comparing and Merging Files
@setchapternewpage odd
@c %**end of header

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

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

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

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

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

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

@titlepage
@title Comparing and Merging Files
@subtitle @code{diff}, @code{diff3}, @code{sdiff}, @code{cmp}, and @code{patch}
@subtitle Edition 1.1, for @code{diff} 2.1 and @code{patch} 2.0.12g8
@subtitle January 1993
@author by David MacKenzie, Paul Eggert, and Richard Stallman

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1992 Free Software Foundation, Inc.

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

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

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

@node Top, , , (dir)

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

This is Edition 1.0, for @code{diff} 2.1 and @code{patch} 2.0.12g8.
@end ifinfo

@menu
* Overview::		Preliminary information.

* Comparison::		What file comparison means.
* Output Formats::	Formats for difference reports.
* Comparing Directories::	Comparing files and directories.
* Adjusting Output::	Making @code{diff} output prettier.
* diff Performance::	Making @code{diff} smarter or faster.
* Comparing Three Files:: Formats for three-way difference reports.

* diff3 Merging::	Merging from a common ancestor.
* Interactive Merging::	Interactive merging with @code{sdiff}.
* Merging with patch::	Using @code{patch} to change old files into new ones.
* Making Patches::	Tips for making patch distributions.

* Invoking cmp::	How to run @code{cmp} and a summary of its options.
* Invoking diff::	How to run @code{diff} and a summary of its options.
* Invoking diff3::	How to run @code{diff3} and a summary of its options.
* Invoking patch::	How to run @code{patch} and a summary of its options.
* Invoking sdiff::	How to run @code{sdiff} and a summary of its options.

* Incomplete Lines::	Lines that lack trailing newlines.
* Projects::		If you think you've found a bug or other shortcoming.

* Concept Index::	Index of concepts.
@end menu

@node Overview, Comparison, , Top
@unnumbered Overview
@cindex overview of @code{diff} and @code{patch}

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 @code{diff} command to show differences between two
files, or each corresponding file in two directories.  @code{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 @dfn{diff} or @dfn{patch}.  For files that are identical,
@code{diff} normally produces no output; for binary (non-text) files,
@code{diff} normally reports only that they are different.

You can use the @code{cmp} command to show the offsets and line numbers
where two files differ.  @code{cmp} can also show all the characters
that differ between the two files, side by side.  Another way to compare
two files character by character is the Emacs command @kbd{M-x
compare-windows}.  @xref{Other Window, , Other Window, emacs, The GNU
Emacs Manual}, for more information on that command.

You can use the @code{diff3} command to show differences among three
files.  When two people have made independent changes to a common
original, @code{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 @code{sdiff} command to merge two files interactively.

You can use the set of differences produced by @code{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 @code{diff} output, you can use the
@code{patch} program to update, or @dfn{patch}, a copy of the file.  If you
think of @code{diff} as subtracting one file from another to produce
their difference, you can think of @code{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 @code{diff} was written by Mike Haertel, David Hayes, Richard
Stallman, Len Tower, and Paul Eggert.  Wayne Davison designed and
implemented the unified output format.  The basic algorithm is described
in ``An O(ND) Difference Algorithm and its Variations'', Eugene Myers,
@cite{Algorithmica} Vol. 1 No. 2, 1986, p. 251; and in ``A File
Comparison Program'', W. Miller and E. Myers, @cite{Software Practice
and Experience} Vol. 15 No. 11, 1985, p. 1025.
@c From: "Gene Myers" <gene@cs.arizona.edu>
@c They are about the same basic algorithm; the Algorithmica
@c paper gives a rigorous treatment and the sub-algorithm for
@c delivering scripts and should be the primary reference, but
@c both should be mentioned.
 
GNU @code{diff3} was written by Randy Smith.  GNU @code{sdiff} was
written by Thomas Lord.  GNU @code{cmp} was written by Torbjorn Granlund
and David MacKenzie.

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

@node Comparison, Output Formats, Overview, Top
@chapter What Comparison Means
@cindex introduction

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.  @code{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 @code{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 whitespace between words or
lines.  @code{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 whitespace and alphabetic case.

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

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

@code{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.

@menu
* Hunks::		Groups of differing lines.
* Whitespace::		Suppressing differences in whitespace.
* Blank Lines::		Suppressing differences in blank lines.
* Case Folding::	Suppressing differences in alphabetic case.
* Specified Folding::	Suppressing differences that match regular expressions.
* Brief::		Summarizing which files are different.
* Binary::		Comparing binary files or forcing text comparisons.
@end menu

@node Hunks, Whitespace, , Comparison
@section Hunks
@cindex hunks

When comparing two files, @code{diff} finds sequences of lines common to
both files, interspersed with groups of differing lines called
@dfn{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.  @code{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 @file{F} contains the three lines
@samp{a}, @samp{b}, @samp{c}, and the file @file{G} contains the same
three lines in reverse order @samp{c}, @samp{b}, @samp{a}.  If
@code{diff} finds the line @samp{c} as common, then the command
@samp{diff F G} produces this output:

@example
1,2d0
< a
< b
3a2,3
> b
> a
@end example

@noindent
But if @code{diff} notices the common line @samp{b} instead, it produces
this output:

@example
1c1
< a
---
> c
3c3
< c
---
> a
@end example

@noindent
It is also possible to find @samp{a} as the common line.  @code{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
@samp{--minimal} option (@pxref{diff Performance}).

@node Whitespace, Blank Lines, Hunks, Comparison
@section Suppressing Differences in Blank and Tab Spacing
@cindex blank and tab difference suppression
@cindex tab and blank difference suppression

The @samp{-b} and @samp{--ignore-space-change} options ignore blanks and
tabs at line end, and to consider all other sequences of one or more
blank and tab characters to be equivalent.  With these options,
@code{diff} considers the following two lines to be equivalent, where
@samp{$} denotes the line end:

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

The @samp{-w} and @samp{--ignore-all-space} options are stronger than
@samp{-b}.  They ignore difference even if one file has whitespace where
the other file has none, and they ignore all whitespace characters, not
just blanks and tabs.  (The @dfn{whitespace} characters include
backspace, tab, vertical tab, formfeed, carriage return, space, and
no-break space.)  With these options, @code{diff} considers the
following two lines to be equivalent, where @samp{$} denotes the line
end and @samp{^M} denotes a carriage return:

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

@node Blank Lines, Case Folding, Whitespace, Comparison
@section Suppressing Differences in Blank Lines
@cindex blank line difference suppression

The @samp{-B} and @samp{--ignore-blank-lines} options ignore insertions
or deletions of blank lines.  These options normally affect only lines
that are completely empty; they do not affect lines that look empty but
contain space or tab characters.  With these options, for example, a
file containing
@example
1.  A point is that which has no part.

2.  A line is breadthless length.
-- Euclid, The Elements, I
@end example
@noindent
is considered identical to a file containing
@example
1.  A point is that which has no part.
2.  A line is breadthless length.


-- Euclid, The Elements, I
@end example

@node Case Folding, Specified Folding, Blank Lines, Comparison
@section Suppressing Case Differences
@cindex case difference suppression

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

@node Specified Folding, Brief, Case Folding, Comparison
@section Suppressing Lines Matching a Regular Expression
@cindex regular expression suppression

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

However, @samp{-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, @code{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 @samp{-I} option.  @code{diff} tries to match each
line against each regular expression, starting with the last one given.

@node Brief, Binary, Specified Folding, Comparison
@section Summarizing Which Files Differ
@cindex summarizing which files differ
@cindex brief difference reports

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, @code{diff} simply reports whether files differ.  The @samp{-q}
and @samp{--brief} options select this output format.

This format is especially useful when comparing the contents of two
directories.  It is also much faster than doing the normal line by line
comparisons, because @code{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
@code{cmp}.  For files that are identical, @code{cmp} produces no
output.  When the files differ, by default, @code{cmp} outputs the byte
offset and line number where the first difference occurs.  You can use
the @samp{-s} option to suppress that information, so that @code{cmp}
produces no output and reports whether the files differ using only its
exit status (@pxref{Invoking cmp}).

@c Fix this.
Unlike @code{diff}, @code{cmp} cannot compare directories; it can only
compare two files.

@node Binary, , Brief, Comparison
@section Binary Files and Forcing Text Comparisons
@cindex binary file diff
@cindex text versus binary diff

If @code{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 (@pxref{Brief}), and
reports only that the binary files are different.  This is because line
by line comparisons are usually not meaningful for binary files.

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

Sometimes you might want to force @code{diff} to consider files to be
text.  For example, you might be comparing text files that contain
null characters; @code{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 @code{diff} to consider all
files to be text files, and compare them line by line, by using the
@samp{-a} or @samp{--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 @code{diff} output will consist of hunks
showing differences between long lines of whatever characters the files
contain.

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

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

If @code{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.  @code{diff3} uses the same test as
@code{diff} to decide whether a file is binary.  As with @code{diff}, if
the input files contain a few non-text characters but otherwise are like
text files, you can force @code{diff3} to consider all files to be text
files and compare them line by line by using the @samp{-a} or
@samp{--text} options.

@node Output Formats, Comparing Directories, Comparison, Top
@chapter @code{diff} Output Formats
@cindex output formats
@cindex format of @code{diff} output

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

@menu
* Sample diff Input::	Sample @code{diff} input files for examples.
* Normal::		Showing differences without surrounding text.
* Context::		Showing differences with the surrounding text.
* Side by Side::        Showing differences in two columns.
* Scripts::		Generating scripts for other programs.
* If-then-else::	Merging files with if-then-else.
@end menu

@node Sample diff Input, Normal, , Output Formats
@section Two Sample Input Files
@cindex @code{diff} sample input
@cindex sample input for @code{diff}

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

This is the file @file{lao}:

@example
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.
@end example

This is the file @file{tzu}:

@example
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!
@end example

In this example, the first hunk contains just the first two lines of
@file{lao}, the second hunk contains the fourth line of @file{lao}
opposing the second and third lines of @file{tzu}, and the last hunk
contains just the last three lines of @file{tzu}.

@node Normal, Context, Sample diff Input, Output Formats
@section Showing Differences Without Context
@cindex normal output format
@cindex @samp{<} output format

The ``normal'' @code{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 (@pxref{Context Format}) and the unified format
(@pxref{Unified Format}) are superior.  Normal format is the default for
compatibility with older versions of @code{diff} and the POSIX standard.

@menu
* Detailed Normal::	A detailed description of normal output format.
* Example Normal::	Sample output in the normal format.
@end menu

@node Detailed Normal, Example Normal, , Normal
@subsection 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:

@example
@var{change-command}
< @var{from-file-line}
< @var{from-file-line}@dots{}
---
> @var{to-file-line}
> @var{to-file-line}@dots{}
@end example

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:

@table @samp
@item @var{l}a@var{r}
Add the lines in range @var{r} of the second file after line @var{l} of
the first file.  For example, @samp{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.

@item @var{f}c@var{t}
Replace the lines in range @var{f} of the first file with lines in range
@var{t} of the second file.  This is like a combined add and delete, but
more compact.  For example, @samp{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.

@item @var{r}d@var{l}
Delete the lines in range @var{r} from the first file; line @var{l} is where
they would have appeared in the second file had they not been deleted.
For example, @samp{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.
@end table

@node Example Normal, , Detailed Normal, Normal
@subsection An Example of Normal Format

Here is the output of the command @samp{diff lao tzu}
(@pxref{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.

@example
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!
@end example

@node Context, Side by Side, Normal, Output Formats
@section Showing Differences in Their Context
@cindex context output format
@cindex @samp{!} output format

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 @dfn{context}.

GNU @code{diff} provides two output formats that show context around the
differing lines: @dfn{context format} and @dfn{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 @code{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.  @code{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, @code{patch} can adjust
the line numbers accordingly and still apply the diff correctly.
@xref{Imperfect}, for more information on using @code{patch} to apply
imperfect diffs.

@menu
* Context Format::	An output format that shows surrounding lines.
* Unified Format::	A more compact output format that shows context.
* Sections::		Showing which sections of the files differences are in.
* Alternate Names::	Showing alternate file names in context headers.
@end menu

@node Context Format, Unified Format, , Context
@subsection 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 @samp{-C @var{lines}},
@samp{--context@r{[}=@var{lines}@r{]}}, or @samp{-c} option.  The
argument @var{lines} that some of these options take is the number of
lines of context to show.  If you do not specify @var{lines}, it
defaults to three.

@menu
* Detailed Context::	A detailed description of the context output format.
* Example Context::	Sample output in context format.
* Less Context::	Another sample with less context.
@end menu

@node Detailed Context, Example Context, , Context Format
@subsubsection Detailed Description of Context Format

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

@example
*** @var{from-file} @var{from-file-modification-time}
--- @var{to-file} @var{to-file-modification time}
@end example

@noindent
You can change the header's content with the @samp{-L @var{label}} or
@samp{--label=@var{label}} option; see @ref{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:

@example
***************
*** @var{from-file-line-range} ****
  @var{from-file-line}
  @var{from-file-line}@dots{}
--- @var{to-file-line-range} ----
  @var{to-file-line}
  @var{to-file-line}@dots{}
@end example

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:

@table @samp
@item !
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
@samp{!} in the part of this hunk for the other file.

@item +
An ``inserted'' line in the second file that corresponds to nothing in
the first file.

@item -
A ``deleted'' line in the first file that corresponds to nothing in the
second file.
@end table

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

@node Example Context, Less Context, Detailed Context, Context Format
@subsubsection An Example of Context Format

Here is the output of @samp{diff -c lao tzu} (@pxref{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.

@example
*** lao	Sat Jan 26 23:30:39 1991
--- tzu	Sat Jan 26 23:30:50 1991
***************
*** 1,7 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
  The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,
--- 1,6 ----
  The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
! 
  Therefore let there always be non-being,
    so we may see their subtlety,
  And let there always be being,
***************
*** 9,11 ****
--- 8,13 ----
  The two are the same,
  But after they are produced,
    they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!
@end example

@node Less Context, , Example Context, Context Format
@subsubsection An Example of Context Format with Less Context

Here is the output of @samp{diff --context=1 lao tzu} (@pxref{Sample
diff Input}, for the complete contents of the two files).  Notice that
at most one context line is reported here.

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

@node Unified Format, Sections, Context Format, Context
@subsection Unified Format
@cindex unified output format
@cindex @samp{+-} output 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 @samp{-U @var{lines}},
@samp{--unified@r{[}=@var{lines}@r{]}}, or @samp{-u}
option.  The argument @var{lines} is the number of lines of context to
show.  When it is not given, it defaults to three.

At present, only GNU @code{diff} can produce this format and only GNU
@code{patch} can automatically apply diffs in this format.

@menu
* Detailed Unified::	A detailed description of unified format.
* Example Unified::	Sample output in unified format.
@end menu

@node Detailed Unified, Example Unified, , Unified Format
@subsubsection Detailed Description of Unified Format

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

@example
--- @var{from-file} @var{from-file-modification-time}
+++ @var{to-file} @var{to-file-modification-time}
@end example

@noindent
You can change the header's content with the @samp{-L @var{label}} or
@samp{--label=@var{label}} option; see @xref{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:

@example
@@@@ @var{from-file-range} @var{to-file-range} @@@@
 @var{line-from-either-file}
 @var{line-from-either-file}@dots{}
@end example

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

@table @samp
@item +
A line was added here to the first file.

@item -
A line was removed here from the first file.
@end table

@node Example Unified, , Detailed Unified, Unified Format
@subsubsection An Example of Unified Format

Here is the output of the command @samp{diff -u lao tzu}
(@pxref{Sample diff Input}, for the complete contents of the two files):

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

@node Sections, Alternate Names, Unified Format, Context
@subsection Showing Which Sections Differences Are in
@cindex headings
@cindex section headings

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 @code{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.

@menu
* Specified Headings::	Showing headings that match regular expressions.
* C Function Headings::	Showing headings of C functions.
@end menu

@node Specified Headings, C Function Headings, , Sections
@subsubsection Showing Lines That Match Regular Expressions
@cindex specified headings
@cindex regular expression matching headings

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

@c Please add to this list, e.g. Fortran, Pascal.
@table @samp
@item ^[A-Za-z_]
C, C++, Prolog
@item ^(
Lisp
@item ^@@\(chapter\|appendix\|unnumbered\|chapheading\)
Texinfo
@end table

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

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

@node C Function Headings, , Specified Headings, Sections
@subsubsection Showing C Function Headings
@cindex C function headings
@cindex function headings, C

To show in which functions differences occur for C and similar
languages, you can use the @samp{-p} or @samp{--show-c-function} option.
This option automatically selects the context output format
(@pxref{Context Format}), with the default number of lines of context.
You can override that number with @samp{-C @var{lines}} later in the
command line.  You can override both the format and the number with
@samp{-U @var{lines}} later in the command line.

The @samp{-p} and @samp{--show-c-function} options are equivalent to
@samp{-c -F'^[_a-zA-Z$]'} (@pxref{Specified Headings}).  GNU @code{diff}
provides them for the sake of convenience.

@node Alternate Names, , Sections, Context
@subsection Showing Alternate File Names
@cindex alternate file names
@cindex file name alternates

If you are comparing two files that have meaningless or uninformative
names, you might want @code{diff} to show alternate names in the header
of the context and unified output formats.  To do this, use the @samp{-L
@var{label}} or @samp{--label=@var{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, @code{diff} reports an error.  The @samp{-L} option does not
affect the file names in the @code{pr} header when the @samp{-l} or
@samp{--paginate} option is used (@pxref{Pagination}).

Here are the first two lines of the output from @samp{diff -C2
-Loriginal -Lmodified lao tzu}:

@example
*** original
--- modified
@end example

@node Side by Side, Scripts, Context, Output Formats
@section Showing Differences Side by Side
@cindex side by side
@cindex two-column output
@cindex columnar output

@code{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:

@table @asis
@item 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
@samp{--ignore} options (@pxref{Whitespace}).

@item @samp{|}
The corresponding lines differ, and they are either both complete
or both incomplete.

@item @samp{<}
The files differ and only the first file contains the line.

@item @samp{>}
The files differ and only the second file contains the line.

@item @samp{(}
Only the first file contains the line, but the difference is ignored.

@item @samp{)}
Only the second file contains the line, but the difference is ignored.

@item @samp{\}
The corresponding lines differ, and only the first line is incomplete.

@item @samp{/}
The corresponding lines differ, and only the second line is incomplete.
@end table

Normally, an output line is incomplete if and only if the lines that it
contains are incomplete; @xref{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 @samp{\} if the first line is incomplete,
@samp{/} 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 @code{sdiff} command to interactively merge side by side
differences.  @xref{Interactive Merging}, for more information on merging files.

@menu
* Side by Side Format::		Controlling side by side output format.
* Example Side by Side::	Sample side by side output.
@end menu

@node Side by Side Format, Example Side by Side, , Side by Side
@section Controlling Side by Side Format
@cindex side by side format

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

The @samp{--left-column} option prints only the left column of two
common lines.  The @samp{--suppress-common-lines} option suppresses
common lines entirely.

@node Example Side by Side, , Side by Side Format, Side by Side
@subsection An Example of Side by Side Format

Here is the output of the command @samp{diff -y -W 72 lao tzu}
(@pxref{Sample diff Input}, for the complete contents of the two files).

@example
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!
@end example

@node Scripts, If-then-else, Side by Side, Output Formats
@section Making Edit Scripts
@cindex script output formats

Several output modes produce command scripts for editing @var{from-file}
to produce @var{to-file}.

@menu
* ed Scripts::		Using @code{diff} to produce commands for @code{ed}.
* Forward ed::		Making forward @code{ed} scripts.
* RCS::			A special @code{diff} output format used by RCS.
@end menu

@node ed Scripts, Forward ed, , Scripts
@subsection @code{ed} Scripts
@cindex @code{ed} script output format

@code{diff} can produce commands that direct the @code{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 @code{patch}, it is almost obsolete.  Use the
@samp{-e} or @samp{--ed} option to select this output format.

Like the normal format (@pxref{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 @file{d} contains the output of @samp{diff -e old new}, then
the command @samp{(cat d && echo w) | ed - old} edits @file{old} to make
it a copy of @file{new}.  More generally, if @file{d1}, @file{d2},
@dots{}, @file{dN} contain the outputs of @samp{diff -e old new1},
@samp{diff -e new1 new2}, @dots{}, @samp{diff -e newN-1 newN},
respectively, then the command @samp{(cat d1 d2 @dots{} dN && echo w) |
ed - old} edits @file{old} to make it a copy of @file{newN}.

@menu
* Detailed ed::		A detailed description of @code{ed} format.
* Example ed::		A sample @code{ed} script.
@end menu

@node Detailed ed, Example ed, , ed Scripts
@subsubsection Detailed Description of @code{ed} Format

The @code{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
@code{ed} interprets line numbers in succeeding commands.  @code{ed}
format hunks look like this:

@example
@var{change-command}
@var{to-file-line}
@var{to-file-line}@dots{}
.
@end example

Because @code{ed} uses a single period on a line to indicate the end of
input, GNU @code{diff} protects lines of changes that contain a single
period on a line by writing two periods instead, then writing a
subsequent @code{ed} command to change the two periods into one.  The
@code{ed} format cannot represent an incomplete line, so if the second
file ends in a changed incomplete line, @code{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:

@table @samp
@item @var{l}a
Add text from the second file after line @var{l} in the first file.  For
example, @samp{8a} means to add the following lines after line 8 of file
1.

@item @var{r}c
Replace the lines in range @var{r} in the first file with the following
lines.  Like a combined add and delete, but more compact.  For example,
@samp{5,7c} means change lines 5--7 of file 1 to read as the text file
2.

@item @var{r}d
Delete the lines in range @var{r} from the first file.  For example,
@samp{5,7d} means delete lines 5--7 of file 1.
@end table

@node Example ed, , Detailed ed, ed Scripts
@subsubsection Example @code{ed} Script

Here is the output of @samp{diff -e lao tzu} (@pxref{Sample
diff Input}, for the complete contents of the two files):

@example
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
@end example

@node Forward ed, RCS, ed Scripts, Scripts
@subsection Forward @code{ed} Scripts
@cindex forward @code{ed} script output format

@code{diff} can produce output that is like an @code{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
@code{ed} format, forward @code{ed} format cannot represent incomplete
lines.

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

@node RCS, , Forward ed, Scripts
@subsection RCS Scripts
@cindex RCS script output format

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 @samp{-n} or
@samp{--rcs} option to select this output format.  It is like the
forward @code{ed} format (@pxref{Forward ed}), but it can represent
arbitrary changes to the contents of a file because it avoids the
forward @code{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 @samp{a} and @samp{d}
commands are used instead of @samp{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 @samp{diff -n lao tzu} (@pxref{Sample
diff Input}, for the complete contents of the two files):

@example
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!
@end example

@node If-then-else, , Scripts, Output Formats
@section Merging Files with If-then-else
@cindex merged output format
@cindex if-then-else output format
@cindex C if-then-else output format
@cindex @code{ifdef} output format

You can use @code{diff} to merge two files of C source code.  The output
of @code{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 @code{#ifdef @var{name}} or
@code{#ifndef @var{name}}, @code{#else}, and @code{#endif}.  When
compiling the output, you select which version to use by either defining
or leaving undefined the macro @var{name}.

To merge two files, use @code{diff} with the @samp{-D @var{name}} or
@samp{--ifdef=@var{name}} option.  The argument @var{name} is the C
preprocessor identifier to use in the @code{#ifdef} and @code{#ifndef}
directives.

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

@example
    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);
@end example

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

@menu
* Line Group Formats::		Formats for general if-then-else line groups.
* Line Formats::		Formats for each line in a line group.
* Detailed If-then-else::	A detailed description of if-then-else format.
* Example If-then-else::	Sample if-then-else format output.
@end menu

@node Line Group Formats, Line Formats, , If-then-else
@subsection Line Group Formats
@cindex line group formats
@cindex formats for if-then-else line groups

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 @file{old}
and @file{new}, and outputs a merged file in which old regions are
surrounded by @samp{\begin@{em@}}-@samp{\end@{em@}} lines, and new
regions are surrounded by @samp{\begin@{bf@}}-@samp{\end@{bf@}} lines.

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

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.

@example
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
@end example

To specify a line group format, use @code{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 @var{format}, because it
typically contains shell metacharacters.

@table @samp
@item --old-group-format=@var{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 a format that outputs the line group as-is.

@item --new-group-format=@var{format}
These line groups are hunks containing only lines from the second
file.  The default new group format is same as the the changed group
format if it is specified; otherwise it is a format that outputs the
line group as-is.

@item --changed-group-format=@var{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.

@item --unchanged-group-format=@var{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.
@end table

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

@table @samp
@item %<
stands for the lines from the first file, including the trailing newline.
Each line is formatted according to the old line format (@pxref{Line Formats}).

@item %>
stands for the lines from the second file, including the trailing newline.
Each line is formatted according to the new line format.

@item %=
stands for the lines common to both files, including the trailing newline.
Each line is formatted according to the unchanged line format.

@item %0
stands for a null character.

@item %%
stands for @samp{%}.

@end table

@node Line Formats, Detailed If-then-else, Line Group Formats, If-then-else
@subsection Line Formats
@cindex line formats

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

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

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

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

@table @samp
@item --old-line-format=@var{format}
formats lines just from the first file.

@item --new-line-format=@var{format}
formats lines just from the second file.

@item --unchanged-line-format=@var{format}
formats lines common to both files.
@end table

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

@table @samp
@item %l
stands for the the contents of the line, not counting its trailing
newline (if any).  This format ignores whether the line is incomplete;
@xref{Incomplete Lines}.

@item %L
stands for the the contents of the line, including its trailing newline
(if any).  If a line is incomplete, this format preserves its
incompleteness.

@item %0
stands for a null character.

@item %%
stands for @samp{%}.

@end table

The default line format is @samp{%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 @samp{%l} or @samp{%L} in a line
format is just after a tab stop (e.g. by preceding @samp{%l} or
@samp{%L} with a tab character), or you should use the @samp{-t} or
@samp{--expand-tabs} option.

@node Detailed If-then-else, Example If-then-else, Line Formats, If-then-else
@subsection Detailed Description of If-then-else Format

For lines common to both files, @code{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,
@code{diff} uses the old line group format; if the hunk contains only
lines from the second file, @code{diff} uses the new group format;
otherwise, @code{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 @samp{--ifdef=@var{name}} is equivalent to
the following sequence of options using shell syntax:

@example
--old-group-format='#ifndef @var{name}
%<#endif /* not @var{name} */
' \
--new-group-format='#ifdef @var{name}
%>#endif /* @var{name} */
' \
--unchanged-group-format='%=' \
--changed-group-format='#ifndef @var{name}
%<#else /* @var{name} */
%>#endif /* @var{name} */
'
@end example

You should carefully check the @code{diff} output for proper nesting.
For example, when using the the @samp{-D @var{name}} or
@samp{--ifdef=@var{name}} option, you should check that if the
differing lines contain any of the C preprocessor directives
@samp{#ifdef}, @samp{#ifndef}, @samp{#else}, @samp{#elif}, or
@samp{#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 @code{patch} @samp{-D @var{name}} option behaves just like
the @code{diff} @samp{-D @var{name}} option, except it operates on
a file and a diff to produce a merged file; @xref{patch Options}.

@node Example If-then-else, , Detailed If-then-else, If-then-else
@subsection An Example of If-then-else Format

Here is the output of @samp{diff -DTWO lao tzu} (@pxref{Sample
diff Input}, for the complete contents of the two files):

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

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

@node Comparing Directories, Adjusting Output, Output Formats, Top
@chapter Comparing Directories

You can use @code{diff} to compare some or all of the files in two
directory trees.  When both file name arguments to @code{diff} are
directories, it compares each file that is contained in both
directories, examining file names in alphabetical order.  Normally
@code{diff} is silent about pairs of files that contain no differences,
but if you use the @samp{-s} or @samp{--report-identical-files} option,
it reports pairs of identical files.  Normally @code{diff} reports
subdirectories common to both directories without comparing
subdirectories' files, but if you use the @samp{-r} or
@samp{--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, @code{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 @code{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 @samp{-N} or @samp{--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
@samp{-P} or @samp{--unidirectional-new-file} option instead of @samp{-N}.
This option is like @samp{-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.  @xref{Making Patches}, for more discussion of
making patches for distribution.

To ignore some files while comparing directories, use the @samp{-x
@var{pattern}} or @samp{--exclude=@var{pattern}} option.  This option
ignores any files or subdirectories whose base names match the shell
pattern @var{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 @var{pattern} in quotes so that the shell does not
expand it.  For example, the option @samp{-x '*.[ao]'} ignores any file
whose name ends with @samp{.a} or @samp{.o}.

This option accumulates if you specify it more than once.  For example,
using the options @samp{-x 'RCS' -x '*,v'} ignores any file or
subdirectory whose base name is @samp{RCS} or ends with @samp{,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 @samp{-X
@var{file}} or @samp{--exclude-from=@var{file}} option.

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

@node Adjusting Output, diff Performance, Comparing Directories, Top
@chapter Making @code{diff} Output Prettier

@code{diff} provides several ways to adjust the appearance of its output.
These adjustments can be applied to any output format.

@menu
* Tabs::		Preserving the alignment of tabstops.
* Pagination::		Page numbering and timestamping @code{diff} output.
@end menu

@node Tabs, Pagination, , Adjusting Output
@section Preserving Tabstop Alignment
@cindex tabstop alignment
@cindex aligning tabstops

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

The first way is to have @code{diff} convert all tabs into the correct
number of spaces before outputting them; select this method with the
@samp{-t} or @samp{--expand-tabs} option.  @code{diff} assumes that
tabstops are set every 8 columns.  To use this form of output with
@code{patch}, you must give @code{patch} the @samp{-l} or
@samp{--ignore-whitespace} option (@pxref{Changed Whitespace}, for more
information).

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

@node Pagination, , Tabs, Adjusting Output
@section Paginating @code{diff} Output
@cindex paginating @code{diff} output

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

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

@node diff Performance, Comparing Three Files, Adjusting Output, Top
@chapter @code{diff} Performance Tradeoffs
@cindex performance of @code{diff}

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

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

The way that GNU @code{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 @code{diff} output is large, you
might want @code{diff} to use a modified algorithm that sometimes
produces a smaller set of differences.  The @samp{-d} or
@samp{--minimal} option does this; however, it can also cause
@code{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 @samp{-H} or
@samp{--speed-large-files} option to make a different modification to
the algorithm that @code{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, @code{diff} might produce a larger set of
differences; however, the output will still be correct.

@node Comparing Three Files, diff3 Merging, diff Performance, Top
@chapter Comparing Three Files
@cindex comparing three files
@cindex format of @code{diff3} output

Use the program @code{diff3} to compare three files and show any
differences among them.  (@code{diff3} can also merge files; see
@ref{diff3 Merging}).

The ``normal'' @code{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.

@xref{Invoking diff3}, for more information on how to run @code{diff3}.

@menu
* Sample diff3 Input::		Sample @code{diff3} input for examples.
* Detailed diff3 Normal::	A detailed description of normal output format.
* diff3 Hunks::			The format of normal output format.
* Example diff3 Normal::	Sample output in the normal format.
@end menu

@node Sample diff3 Input, Detailed diff3 Normal, , Comparing Three Files
@section A Third Sample Input File
@cindex @code{diff3} sample input
@cindex sample input for @code{diff3}

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

@example
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
@end example

@node Detailed diff3 Normal, diff3 Hunks, Sample diff3 Input, Comparing Three Files
@section Detailed Description of @code{diff3} Normal Format

Each hunk begins with a line marked @samp{====}.  Three-way hunks have
plain @samp{====} lines, and two-way hunks have @samp{1}, @samp{2}, or
@samp{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.  Two spaces precede each copy of an input line to
distinguish it from the commands.  Commands take the following forms:

@table @samp
@item @var{file}:@var{l}a
This hunk appears after line @var{l} of file @var{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, @samp{1:11a} means that the hunk follows line 11 in the first
file and contains no lines from that file.

@item @var{file}:@var{r}c
This hunk contains the lines in the range @var{r} of file @var{file}.
The range @var{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, @samp{2:11,13c} means that the hunk
contains lines 11 through 13 from the second file.
@end table

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

@node diff3 Hunks, Example diff3 Normal, Detailed diff3 Normal, Comparing Three Files
@section @code{diff3} Hunks
@cindex hunks for @code{diff3}
@cindex @code{diff3} hunks

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

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

For example, suppose @file{F} contains the three lines @samp{a},
@samp{b}, @samp{f}, @file{G} contains the lines @samp{g}, @samp{b},
@samp{g}, and @file{H} contains the lines @samp{a}, @samp{b},
@samp{h}.  @samp{diff3 F G H} might output the following:

@example
====2
1:1c
3:1c
  a
2:1c
  g
====
1:3c
  f
2:3c
  g
3:3c
  h
@end example

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

@node Example diff3 Normal, , diff3 Hunks, Comparing Three Files
@section An Example of @code{diff3} Normal Format

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

@example
====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
@end example

@node diff3 Merging, Interactive Merging, Comparing Three Files, Top
@chapter Merging From a Common Ancestor
@cindex merging from a common ancestor

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

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

@code{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:

@example
diff3 @var{mine} @var{older} @var{yours}
@end example

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

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

@code{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 @samp{<<<<<<<} and @samp{>>>>>>>} lines.

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

@menu
* Which Changes::		Selecting changes to incorporate.
* Marking Conflicts::		Marking conflicts.
* Bypassing ed::		Generating merged output directly.
* Merging Incomplete Lines::	How @code{diff3} merges incomplete lines.
* Saving the Changed File::	Emulating System V behavior.
@end menu

@node Which Changes, Marking Conflicts, , diff3 Merging
@section Selecting Which Changes to Incorporate
@cindex overlapping change, selection of
@cindex unmerged change

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

The @samp{-e}, @samp{-3} and @samp{-x} options select only
@dfn{unmerged changes}, i.e. changes where @var{mine} and @var{yours}
differ; they ignore changes from @var{older} to @var{yours} where
@var{mine} and @var{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 @samp{-A} or @samp{--show-all} option
(@pxref{Marking Conflicts}).

Here is the output of the command @code{diff3} with each of these three
options (@pxref{Sample diff3 Input}, for the complete contents of the files).
Notice that @samp{-e} outputs the union of the disjoint sets of changes
output by @samp{-3} and @samp{-x}.

Output of @samp{diff3 -e lao tzu tao}:
@example
11a

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

Output of @samp{diff3 -3 lao tzu tao}:
@example
8c
  so we may see their result.
.
@end example

Output of @samp{diff3 -x lao tzu tao}:
@example
11a

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

@node Marking Conflicts, Bypassing ed, Which Changes, diff3 Merging
@section Marking Conflicts
@cindex conflict marking
@cindex @samp{<<<<<<<} for marking conflicts

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

@example
<<<<<<< @var{A}
@r{lines from @var{A}}
=======
@r{lines from @var{B}}
>>>>>>> @var{B}
@end example

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

@example
<<<<<<< @var{A}
@r{lines from @var{A}}
||||||| @var{B}
@r{lines from @var{B}}
=======
@r{lines from @var{C}}
>>>>>>> @var{C}
@end example

The @samp{-A} or @samp{--show-all} option acts like the @samp{-e}
option, except that it brackets conflicts, and it outputs all changes
from @var{older} to @var{yours}, not just the unmerged changes.  Thus,
given the sample input files (@pxref{Sample diff3 Input}), @samp{diff3
-A lao tzu tao} puts brackets around the conflict where only @file{tzu}
differs:

@example
<<<<<<< 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
@end example

And it outputs the three-way conflict as follows:

@example
<<<<<<< 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
@end example

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

@example
<<<<<<< lao
=======

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

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

@node Bypassing ed, Merging Incomplete Lines, Marking Conflicts, diff3 Merging
@section Generating the Merged Output Directly
@cindex merged @code{diff3} format

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

For example, the command @samp{diff3 -m lao tzu tao}
(@pxref{Sample diff3 Input} for a copy of the input files) would output
the following:

@example
<<<<<<< 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
@end example

@node Merging Incomplete Lines, Saving the Changed File, Bypassing ed, diff3 Merging
@section How @code{diff3} Merges Incomplete Lines
@cindex incomplete line merging

With @samp{-m}, incomplete lines (@pxref{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 @samp{|||||||}, @samp{=======} or @samp{>>>>>>>}
brackets appear somewhere other than the start of a line because
they are appended to the incomplete line.

Without @samp{-m}, if an @code{ed} script option is specified and an
incomplete line is found, @code{diff3} generates a warning and acts as
if a newline had been present.

@node Saving the Changed File, , Merging Incomplete Lines, diff3 Merging
@section Saving the Changed File
@cindex System V @code{diff3} compatibility

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

The @samp{-i} option requires one of the @code{ed} script options
@samp{-AeExX3}, and is incompatible with the merged output option
@samp{-m}.

@node Interactive Merging, Merging with patch, diff3 Merging, Top
@chapter Interactive Merging with @code{sdiff}
@cindex diff merging
@cindex interactive merging

With @code{sdiff}, you can merge two files interactively based on a
side-by-side @samp{-y} format comparison (@pxref{Side by Side}).  Use
@samp{-o @var{file}} or @samp{--output=@var{file}} to specify where to
put the merged text.  @xref{Invoking sdiff}, for more details on the
options to @code{sdiff}.

Another way to merge files interactively is to use the Emacs Lisp
package @code{emerge}.  @xref{emerge, , emerge, emacs, The GNU Emacs
Manual}, for more information.

@menu
* sdiff Option Summary::Summary of @code{sdiff} options.
* Merge Commands::	Merging two files interactively.
@end menu

@node sdiff Option Summary, Merge Commands, , Interactive Merging
@section Specifying @code{diff} Options to @code{sdiff}
@cindex @code{sdiff} output format

The following @code{sdiff} options have the same meaning as for
@code{diff}.  @xref{diff Options}, for the use of these options.

@example
-a -b -d -i -t -v
-B -H -I @var{regexp}

--ignore-blank-lines  --ignore-case
--ignore-matching-lines=@var{regexp}  --ignore-space-change
--left-column  --minimal  --speed-large-files
--suppress-common-lines  --expand-tabs
--text  --version  --width=@var{columns}
@end example

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

@node Merge Commands, , sdiff Option Summary, Interactive Merging
@section Merge Commands
@cindex merge commands
@cindex merging interactively

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

@table @samp
@item e
Discard both versions.
Invoke a text editor on an empty temporary file,
then copy the resulting file to the output.

@item eb
Concatenate the two versions, edit the result in a temporary file,
then copy the edited result to the output.

@item el
Edit a copy of the left version, then copy the result to the output.

@item er
Edit a copy of the right version, then copy the result to the output.

@item l
Copy the left version to the output.

@item q
Quit.

@item r
Copy the right version to the output.

@item s
Silently copy common lines.

@item v
Verbosely copy common lines.  This is the default.
@end table

The text editor invoked is specified by the @code{EDITOR} environment
variable if it is set.  The default is system-dependent.

@node Merging with patch, Making Patches, Interactive Merging, Top
@chapter Merging with @code{patch}

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

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

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

@xref{Invoking patch}, for detailed information on the options to
@code{patch}.  @xref{Backups}, for more information on how
@code{patch} names backup files.  @xref{Rejects}, for more information
on where @code{patch} puts reject hunks.

@menu
* patch Input::		Selecting the type of @code{patch} input.
* Imperfect::		Dealing with imperfect patches.
* Empty Files::		Removing empty files after patching.
* Multiple Patches::	Handling multiple patches in a file specially.
* patch Messages::	Messages and questions @code{patch} can produce.
@end menu

@node patch Input, Imperfect, , Merging with patch
@section Selecting the @code{patch} Input Format
@cindex @code{patch} input format

@code{patch} normally determines which @code{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 @code{patch} to interpret the patch file as a
certain format of diff.  The output formats listed here are the only
ones that @code{patch} can understand.

@table @samp
@item -c
@itemx --context
context diff.

@item -e
@itemx --ed
@code{ed} script.

@item -n
@itemx --normal
normal diff.

@item -u
@itemx --unified
unified diff.
@end table

@node Imperfect, Empty Files, patch Input, Merging with patch
@section Applying Imperfect Patches
@cindex imperfect patch application

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

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

@menu
* Changed Whitespace::	When tabs and spaces don't match exactly.
* Reversed Patches::	Applying reversed patches correctly.
* Inexact::		Helping @code{patch} find close matches.
@end menu

@node Changed Whitespace, Reversed Patches, , Imperfect
@subsection Applying Patches with Changed Whitespace
@cindex whitespace in patches

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 @code{patch} will not be able to match
them properly.  If this problem occurs, use the @samp{-l} or
@samp{--ignore-whitespace} option, which makes @code{patch} compare
whitespace loosely so that any sequence of whitespace in the patch file
matches any sequence of whitespace in the input files.  Non-whitespace
characters must still match exactly.  Each line of the context must
still match a line in the input file.

@node Reversed Patches, Inexact, Changed Whitespace, Imperfect
@subsection Applying Reversed Patches
@cindex reversed patches

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

Often @code{patch} can guess that the patch is reversed.  If the first
hunk of a patch fails, @code{patch} reverses the hunk to see if it can
apply it that way.  If it can, @code{patch} asks you if you want to have
the @samp{-R} option set; if it can't, @code{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
@code{patch} notices.

If you apply a patch that you have already applied, @code{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 @samp{n} to this offer and to
the subsequent ``apply anyway'' question---or type @kbd{C-c} to kill the
@code{patch} process.

@node Inexact, , Reversed Patches, Imperfect
@subsection Helping @code{patch} Find Inexact Matches
@cindex inexact patches
@cindex fuzz factor when patching

For context diffs, and to a lesser extent normal diffs, @code{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, @code{patch} scans both forward and backward for a
set of lines matching the context given in the hunk.

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

If @code{patch} cannot find a place to install a hunk of the patch, it
writes the hunk out to a reject file (@pxref{Rejects}, for information
on how reject files are named).  It writes out rejected hunks in context
format no matter what form the input patch is in.  If the input is a
normal or @code{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 @code{patch}
thinks the failed hunks belong in the new file rather than in the old
one.

As it completes each hunk, @code{patch} tells you whether the hunk
succeeded or failed, and if it failed, on which line (in the new file)
@code{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 @emph{may} indicate that @code{patch} installed a hunk in
the wrong place.  @code{patch} also tells you if it used a fuzz factor
to make the match, in which case you should also be slightly suspicious.

@code{patch} cannot tell if the line numbers are off in an @code{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.

@code{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.

@node Empty Files, Multiple Patches, Imperfect, Merging with patch
@section Removing Empty Files
@cindex empty files, removing
@cindex removing empty files

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

@node Multiple Patches, patch Messages, Empty Files, Merging with patch
@section Multiple Patches in a File
@cindex multiple patches

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

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

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

@example
patch -p3 code.c + -p0 < patchfile
@end example

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

@example
patch -S + + -S + < patch file
@end example

@node patch Messages, , Multiple Patches, Merging with patch
@section Messages and Questions from @code{patch}
@cindex @code{patch} messages and questions
@cindex diagnostics from @code{patch}
@cindex messages from @code{patch}

@code{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, @code{patch} normally prompts you for more information from
the keyboard.  There are options to suppress printing non-fatal messages
and stopping for keyboard input.

The message @samp{Hmm...} indicates that @code{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 @code{patch}, unless an error
occurs, by using the @samp{-s}, @samp{--quiet}, or @samp{--silent}
option.

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

@itemize @bullet
@item
skip patches for which it can't find a file to patch;

@item
patch files even though they have the wrong version for the
@samp{Prereq:} line in the patch;

@item
assume that patches are not reversed even if they look like they are.
@end itemize

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

@itemize @bullet
@item
skip patches for which it can't find a file to patch (the same as @samp{-f});

@item
skip patches for which the file has the wrong version for the
@samp{Prereq:} line in the patch;

@item
assume that patches are reversed if they look like they are.
@end itemize

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

@node Making Patches, Invoking cmp, Merging with patch, Top
@chapter Tips for Making Patch Distributions
@cindex patch making tips
@cindex tips for patch making

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

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

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

An even clearer way to prevent this problem is to put a @samp{Prereq:}
line before the patch.  If the leading text in the patch file contains a
line that starts with @samp{Prereq:}, @code{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
whitespace or a newline.  If not, @code{patch} prompts you for
confirmation before proceeding.  This makes it difficult to accidentally
apply patches in the wrong order.

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

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

You might be able to reduce the size of the patch by renaming or
removing some files before making the patch.  If the older version of
the package contains any files that the newer version does not, or if
any files have been renamed between the two versions, make a list of
@code{rm} and @code{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,
@file{TAGS} and output from @code{yacc} and @code{makeinfo}), replace
the versions in the scratch directory with the newer versions, using
@code{rm} and @code{ln} or @code{cp}.

Now you can create the patch.  The de-facto standard @code{diff} format
for patch distributions is context format with two lines of context,
produced by giving @code{diff} the @samp{-C 2} option.  Give @code{diff}
the @samp{-N} option in case the newer version of the package contains
any files that the older one does not.  Make sure to specify the scratch
directory first and the newer directory second.

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

@node Invoking cmp, Invoking diff, Making Patches, Top
@chapter Invoking @code{cmp}
@cindex invoking @code{cmp}
@cindex @code{cmp} invocation

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

@example
cmp @var{options}@dots{} @var{from-file} @r{[}@var{to-file}@var{]}
@end example

The file name @samp{-} is always the standard input.  @code{cmp} also
uses the standard input if one file name is omitted.

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

@menu
* cmp Options::		Summary of options to @code{cmp}.
@end menu

@node cmp Options, , , Invoking cmp
@section Options to @code{cmp}
@cindex @code{cmp} options
@cindex options for @code{cmp}

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

@table @samp
@item -c
Print the differing characters.  Display control characters as a
@samp{^} followed by a letter of the alphabet and precede characters
that have the high bit set with @samp{M-} (which stands for ``meta'').

@item -l
Print the (decimal) offsets and (octal) values of all differing bytes.

@item --print-chars
Print the differing characters.  Display control characters as a
@samp{^} followed by a letter of the alphabet and precede characters
that have the high bit set with @samp{M-} (which stands for ``meta'').

@item --quiet
@itemx -s
@itemx --silent
Do not print anything; only return an exit status indicating whether
the files differ.

@item --verbose
Print the (decimal) offsets and (octal) values of all differing bytes.
@end table

@node Invoking diff, Invoking diff3, Invoking cmp, Top
@chapter Invoking @code{diff}
@cindex invoking @code{diff}
@cindex @code{diff} invocation

The format for running the @code{diff} command is:

@example
diff @var{options}@dots{} @var{from-file} @var{to-file}
@end example

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

If @var{from-file} is a directory and @var{to-file} is not, @code{diff}
compares the file in @var{from-file} whose file name is that of @var{to-file},
and vice versa.  The non-directory file must not be @samp{-}.

If both @var{from-file} and @var{to-file} are directories,
@code{diff} compares corresponding files in both directories, in
alphabetical order; this comparison is not recursive unless the
@samp{-r} or @samp{--recursive} option is given.  @code{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.

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

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

@menu
* diff Options::	Summary of options to @code{diff}.
@end menu

@node diff Options, , , Invoking diff
@section Options to @code{diff}
@cindex @code{diff} options
@cindex options for @code{diff}

Below is a summary of all of the options that GNU @code{diff} accepts.
Most options have two equivalent names, one of which is a single letter
preceded by @samp{-}, and the other of which is a long name preceded by
@samp{--}.  Multiple single letter options (unless they take an
argument) can be combined into a single command line word: @samp{-ac} is
equivalent to @samp{-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.

@table @samp
@item -@var{lines}
Show @var{lines} (an integer) lines of context.  This option does not
specify an output format by itself; it has no effect unless it is
combined with @samp{-c} (@pxref{Context Format}) or @samp{-u}
(@pxref{Unified Format}).  This option is obsolete.

@item -a
Treat all files as text and compare them line-by-line, even if they
do not seem to be text.  @xref{Binary}.

@item -b
Ignore changes in amount of blank and tab whitespace.  @xref{Whitespace}.

@item -B
Ignore changes that just insert or delete blank lines.  @xref{Blank
Lines}.

@item --brief
Report only whether the files differ, not the details of the
differences.  @xref{Brief}.

@item -c
Use the context output format.  @xref{Context Format}.

@item -C @var{lines}
@itemx --context@r{[}=@var{lines}@r{]}
Use the context output format, showing @var{lines} (an integer) lines of
context, or three if @var{lines} is not given.  @xref{Context Format}.

@item --changed-group-format=@var{format}
Use @var{format} to output a line group containing differing lines from
both files in if-then-else format.  @xref{Line Group Formats}.

@item -d
Change the algorithm perhaps find a smaller set of changes.  This makes
@code{diff} slower (sometimes much slower).  @xref{diff Performance}.

@item -D @var{name}
Make merged @samp{#ifdef} format output, conditional on the preprocessor
macro @var{name}.  @xref{If-then-else}.

@item -e
@itemx --ed
Make output that is a valid @code{ed} script.  @xref{ed Scripts}.

@item --exclude=@var{pattern}
When comparing directories, ignore files and subdirectories whose basenames
match @var{pattern}.  @xref{Comparing Directories}.

@item --exclude-from=@var{file}
When comparing directories, ignore files and subdirectories whose basenames
match any pattern contained in @var{file}.  @xref{Comparing Directories}.

@item --expand-tabs
Expand tabs to spaces in the output, to preserve the alignment of tabs
in the input files.  @xref{Tabs}.

@item -f
Make output that looks vaguely like an @code{ed} script but has changes
in the order they appear in the file.  @xref{Forward ed}.

@item -F @var{regexp}
In context and unified format, for each hunk of differences, show some
of the last preceding line that matches @var{regexp}.  @xref{Specified
Headings}.

@item --forward-ed
Make output that looks vaguely like an @code{ed} script but has changes
in the order they appear in the file.  @xref{Forward ed}.

@item -h
This option currently has no effect; it is present for Unix
compatibility.

@item -H
Use heuristics to speed handling of large files that have numerous
scattered small changes.  @xref{diff Performance}.

@item -i
Ignore changes in case; consider upper- and lower-case letters
equivalent.  @xref{Case Folding}.

@item -I @var{regexp}
Ignore changes that just insert or delete lines that match @var{regexp}.
@xref{Specified Folding}.

@item --ifdef=@var{name}
Make merged if-then-else output using @var{format}.  @xref{If-then-else}.

@item --ignore-all-space
Ignore whitespace when comparing lines.  @xref{Whitespace}.

@item --ignore-blank-lines
Ignore changes that just insert or delete blank lines.  @xref{Blank
Lines}.

@item --ignore-case
Ignore changes in case; consider upper- and lower-case to be the same.
@xref{Case Folding}.

@item --ignore-matching-lines=@var{regexp}
Ignore changes that just insert or delete lines that match @var{regexp}.
@xref{Specified Folding}.

@item --ignore-space-change
Ignore changes in amount of blank and tab whitespace.
@xref{Whitespace}.

@item --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.  @xref{Tabs}.

@item -l
Pass the output through @code{pr} to paginate it.  @xref{Pagination}.

@item -L @var{label}
Use @var{label} instead of the file name in the context format
(@pxref{Context Format}) and unified format (@pxref{Unified Format})
headers.  @xref{RCS}.

@item --label=@var{label}
Use @var{label} instead of the file name in the context format
(@pxref{Context Format}) and unified format (@pxref{Unified Format})
headers.

@item --left-column
Print only the left column of two common lines in side by side format.
@xref{Side by Side Format}.

@item --minimal
Change the algorithm to perhaps find a smaller set of changes.  This
makes @code{diff} slower (sometimes much slower).  @xref{diff
Performance}.

@item -n
Output RCS-format diffs; like @samp{-f} except that each command
specifies the number of lines affected.  @xref{RCS}.

@item -N
@itemx --new-file
In directory comparison, if a file is found in only one directory,
treat it as present but empty in the other directory.  @xref{Comparing
Directories}.

@item --new-group-format=@var{format}
Use @var{format} to output a group of lines taken from just the second
file in if-then-else format.  @xref{Line Group Formats}.

@item --new-line-format=@var{format}
Use @var{format} to output a line taken from just the second file in
if-then-else format.  @xref{Line Formats}.

@item --old-group-format=@var{format}
Use @var{format} to output a group of lines taken from just the first
file in if-then-else format.  @xref{Line Group Formats}.

@item --old-line-format=@var{format}
Use @var{format} to output a line taken from just the first file in
if-then-else format.  @xref{Line Formats}.

@item -p
Show which C function each change is in.  @xref{C Function Headings}.

@item -P
When comparing directories, if a file appears only in the second
directory of the two, treat it as present but empty in the other.

@item --paginate
Pass the output through @code{pr} to paginate it.  @xref{Pagination}.

@item -q
Report only whether the files differ, not the details of the
differences.  @xref{Brief}.

@item -r
When comparing directories, recursively compare any subdirectories
found.  @xref{Comparing Directories}.

@item --rcs
Output RCS-format diffs; like @samp{-f} except that each command
specifies the number of lines affected.  @xref{RCS}.

@item --recursive
When comparing directories, recursively compare any subdirectories
found.  @xref{Comparing Directories}.

@item --report-identical-files
Report when two files are the same.  @xref{Comparing Directories}.

@item -s
Report when two files are the same.  @xref{Comparing Directories}.

@item -S @var{file}
When comparing directories, start with the file @var{file}.  This is
used for resuming an aborted comparison.  @xref{Comparing Directories}.

@item --sdiff-merge-assist
Print extra information to help @code{sdiff}.  @code{sdiff} uses this
option when it runs @code{diff}.  This option is not intended for users
to use directly.

@item --show-c-function
Show which C function each change is in.  @xref{C Function Headings}.

@item --show-function-line=@var{regexp}
In context and unified format, for each hunk of differences, show some
of the last preceding line that matches @var{regexp}.  @xref{Specified
Headings}.

@item --side-by-side
Use the side by side output format.  @xref{Side by Side Format}.

@item --speed-large-files
Use heuristics to speed handling of large files that have numerous
scattered small changes.  @xref{diff Performance}.

@item --starting-file=@var{file}
When comparing directories, start with the file @var{file}.  This is
used for resuming an aborted comparison.  @xref{Comparing Directories}.

@item --suppress-common-lines
Do not print common lines in side by side format.
@xref{Side by Side Format}.

@item -t
Expand tabs to spaces in the output, to preserve the alignment of tabs
in the input files.  @xref{Tabs}.

@item -T
Output a tab rather than a space before the text of a line in normal or
context format.  This causes the alignment of tabs in the line to look
normal.  @xref{Tabs}.

@item --text
Treat all files as text and compare them line-by-line, even if they
do not appear to be text.  @xref{Binary}.

@item -u
Use the unified output format.  @xref{Unified Format}.

@item --unchanged-group-format=@var{format}
Use @var{format} to output a group of common lines taken from both files
in if-then-else format.  @xref{Line Group Formats}.

@item --unchanged-line-format=@var{format}
Use @var{format} to output a line common to both files in if-then-else
format.  @xref{Line Formats}.

@item --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.
@xref{Comparing Directories}.

@item -U @var{lines}
@itemx --unified@r{[}=@var{lines}@r{]}
Use the unified output format, showing @var{lines} (an integer) lines of
context, or three if @var{lines} is not given.  @xref{Unified Format}.

@item -v
@itemx --version
Output the version number of @code{diff}.

@item -w
Ignore horizontal whitespace when comparing lines.  @xref{Whitespace}.

@item -W @var{columns}
@itemx --width=@var{columns}
Use an output width of @var{columns} in side by side format.
@xref{Side by Side Format}.

@item -x @var{pattern}
When comparing directories, ignore files and subdirectories whose basenames
match @var{pattern}.  @xref{Comparing Directories}.

@item -X @var{file}
When comparing directories, ignore files and subdirectories whose basenames
match any pattern contained in @var{file}.  @xref{Comparing Directories}.

@item -y
Use the side by side output format.  @xref{Side by Side Format}.
@end table

@node Invoking diff3, Invoking patch, Invoking diff, Top
@chapter Invoking @code{diff3}
@cindex invoking @code{diff3}
@cindex @code{diff3} invocation

The @code{diff3} command compares three files and outputs descriptions
of their differences.  Its arguments are as follows:

@example
diff3 @var{options}@dots{} @var{mine} @var{older} @var{yours}
@end example

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

An exit status of 0 means @code{diff3} was successful, 1 means some
conflicts were found, and 2 means trouble.

@menu
* diff3 Options::		Summary of options to @code{diff3}.
@end menu

@node diff3 Options, , , Invoking diff3
@section Options to @code{diff3}
@cindex @code{diff3} options
@cindex options for @code{diff3}

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

@table @samp
@item -a
Treat all files as text and compare them line-by-line, even if they
do not appear to be text.  @xref{Binary}.

@item -A
Incorporate all changes from @var{older} to @var{yours} into @var{mine},
surrounding all conflicts with bracket lines.
@xref{Marking Conflicts}.

@item -e
Generate an @code{ed} script that incorporates all the changes from
@var{older} to @var{yours} into @var{mine}.  @xref{Which Changes}.

@item -E
Like @samp{-e}, except bracket lines from overlapping changes' first
and third files.
@xref{Marking Conflicts}.
With @samp{-e}, an overlapping change looks like this:

@example
<<<<<<< @var{mine}
@r{lines from @var{mine}}
=======
@r{lines from @var{yours}}
>>>>>>> @var{yours}
@end example

@item --ed
Generate an @code{ed} script that incorporates all the changes from
@var{older} to @var{yours} into @var{mine}.  @xref{Which Changes}.

@item --easy-only
Like @samp{-e}, except output only the nonoverlapping changes.
@xref{Which Changes}.

@item -i
Generate @samp{w} and @samp{q} commands at the end of the @code{ed}
script for System V compatibility.  This option must be combined with
one of the @samp{-AeExX3} options, and may not be combined with @samp{-m}.
@xref{Saving the Changed File}.

@item -L @var{label}
@itemx --label=@var{label}
Use the label @var{label} for the brackets output by the @samp{-A},
@samp{-E} and @samp{-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 @samp{diff3 -L X -L Y -L Z -m A B C} acts like
@samp{diff3 -m A B C}, except that the output looks like it came from
files named @samp{X}, @samp{Y} and @samp{Z} rather than from files
named @samp{A}, @samp{B} and @samp{C}.  @xref{Marking Conflicts}.

@item -m
@itemx --merge
Apply the edit script to the first file and send the result to standard
output.  Unlike piping the output from @code{diff3} to @code{ed}, this
works even for binary files and incomplete lines.  @samp{-A} is assumed
if no edit script option is specified.  @xref{Bypassing ed}.

@item --overlap-only
Like @samp{-e}, except output only the overlapping changes.
@xref{Which Changes}.

@item --show-all
Incorporate all unmerged changes from @var{older} to @var{yours} into
@var{mine}, surrounding all overlapping changes with bracket lines.
@xref{Marking Conflicts}.

@item --show-overlap
Like @samp{-e}, except bracket lines from overlapping changes' first
and third files.
@xref{Marking Conflicts}.

@item --text
Treat all files as text and compare them line-by-line, even if they
do not appear to be text.  @xref{Binary}.

@item -v
@itemx --version
Output the version number of @code{diff3}.

@item -x
Like @samp{-e}, except output only the overlapping changes.
@xref{Which Changes}.

@item -X
Like @samp{-E}, except output only the overlapping changes.
In other words, like @samp{-x}, except bracket changes as in @samp{-E}.
@xref{Marking Conflicts}.

@item -3
Like @samp{-e}, except output only the nonoverlapping changes.
@xref{Which Changes}.
@end table

@node Invoking patch, Invoking sdiff, Invoking diff3, Top
@chapter Invoking @code{patch}
@cindex invoking @code{patch}
@cindex @code{patch} invocation

Normally @code{patch} is invoked like this:

@example
patch <@var{patchfile}
@end example

The full format for invoking @code{patch} is:

@example
patch @var{options}@dots{} @r{[}@var{origfile} @r{[}@var{patchfile}@r{]}@r{]} @r{[}+ @var{options}@dots{} @r{[}@var{origfile}@r{]}@r{]}@dots{}
@end example

If you do not specify @var{patchfile}, or if @var{patchfile} is
@samp{-}, @code{patch} reads the patch (that is, the @code{diff} output)
from the standard input.

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

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

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

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

@menu
* patch Directories::	Changing directory and stripping paths.
* Backups::		Backup file names.
* Rejects::		Reject file names.
* patch Options::	Summary table of options to @code{patch}.
@end menu

@node patch Directories, Backups, , Invoking patch
@section Applying Patches in Other Directories
@cindex directories and patch
@cindex patching directories

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

@example
| patch -d /usr/src/emacs
@end example

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
@samp{-p@r{[}@var{number}@r{]}} or @samp{--strip@r{[}=@var{number}@r{]}}
option to set the file name strip count to @var{number}.  The strip
count tells @code{patch} how many slashes, along with the directory
names between them, to strip from the front of file names.  @samp{-p}
with no @var{number} given is equivalent to @samp{-p0}.  By default,
@code{patch} strips off all leading paths, leaving just the base file
names, except that when a file name given in the patch is a relative
path and all of its leading directories already exist, @code{patch} does
not strip off the leading path.  (A @dfn{relative} path is one that does
not start with a slash.)

@code{patch} looks for each file (after any slashes have been stripped)
in the current directory, or if you used the @samp{-d @var{directory}}
option, in that directory.

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

@node Backups, Rejects, patch Directories, Invoking patch
@section Backup File Names
@cindex backup file names

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

Alternately, you can specify the extension for backup files with the
@code{SIMPLE_BACKUP_SUFFIX} environment variable, which the options
override.

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

The @samp{-V @var{backup-style}} or
@samp{--version-control=@var{backup-style}} option takes as an argument
a method for creating backup file names.  You can alternately control
the type of backups that @code{patch} makes with the
@code{VERSION_CONTROL} environment variable, which the @samp{-V} option
overrides.  The value of the @code{VERSION_CONTROL} environment variable
and the argument to the @samp{-V} option are like the GNU Emacs
@code{version-control} variable (@pxref{Backups,
emacs, The GNU Emacs Manual}, 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.

@table @samp
@item t
@itemx numbered
Always make numbered backups.

@item nil
@itemx existing
Make numbered backups of files that already have them, simple backups of
the others.  This is the default.

@item never
@itemx simple
Always make simple backups.
@end table

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

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

If you specify the output file with the @samp{-o} option, that file is
the one that is backed up, not the input file.

@node Rejects, patch Options, Backups, Invoking patch
@section Reject File Names
@cindex reject file names

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

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

@node patch Options, , Rejects, Invoking patch
@section Options to @code{patch}
@cindex @code{patch} options
@cindex options for @code{patch}

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

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

@table @samp
@item -b @var{backup-suffix}
Use @var{backup-suffix} as the backup extension instead of
@samp{.orig} or @samp{~}.  @xref{Backups}.

@item -B @var{backup-prefix}
Use @var{backup-prefix} as a prefix to the backup file name.  If this
option is specified, any @samp{-b} option is ignored.  @xref{Backups}.

@item --batch
Do not ask any questions.  @xref{patch Messages}.

@item -c
@itemx --context
Interpret the patch file as a context diff.  @xref{patch Input}.

@item -d @var{directory}
@itemx --directory=@var{directory}
Makes directory @var{directory} the current directory for interpreting
both file names in the patch file, and file names given as arguments to
other options.  @xref{patch Directories}.

@item -D @var{name}
Make merged if-then-else output using @var{format}.  @xref{If-then-else}.

@item --debug=@var{number}
Set internal debugging flags.  Of interest only to @code{patch}
patchers.

@item -e
@itemx --ed
Interpret the patch file as an @code{ed} script.  @xref{patch Input}.

@item -E
Remove output files that are empty after the patches have been applied.
@xref{Empty Files}.

@item -f
Assume that the user knows exactly what he or she is doing, and do not
ask any questions.  @xref{patch Messages}.

@item -F @var{lines}
Set the maximum fuzz factor to @var{lines}.  @xref{Inexact}.

@item --force
Assume that the user knows exactly what he or she is doing, and do not
ask any questions.  @xref{patch Messages}.

@item --forward
Ignore patches that @code{patch} thinks are reversed or already applied.
See also @samp{-R}.  @xref{Reversed Patches}.

@item --fuzz=@var{lines}
Set the maximum fuzz factor to @var{lines}.  @xref{Inexact}.

@item --ifdef=@var{name}
Make merged if-then-else output using @var{format}.  @xref{If-then-else}.

@item --ignore-whitespace
@itemx -l
Let any sequence of whitespace in the patch file match any sequence of
whitespace in the input file.  @xref{Changed Whitespace}.

@item -n
@itemx --normal
Interpret the patch file as a normal diff.  @xref{patch Input}.

@item -N
Ignore patches that @code{patch} thinks are reversed or already applied.
See also @samp{-R}.  @xref{Reversed Patches}.

@item -o @var{output-file}
@itemx --output=@var{output-file}
Use @var{output-file} as the output file name.  @xref{patch Options}.

@item -p@r{[}@var{number}@r{]}
Set the file name strip count to @var{number}.  @xref{patch Directories}.

@item --prefix=@var{backup-prefix}
Use @var{backup-prefix} as a prefix to the backup file name.  If this
option is specified, any @samp{-b} option is ignored.  @xref{Backups}.

@item --quiet
Work silently unless an error occurs.  @xref{patch Messages}.

@item -r @var{reject-file}
Use @var{reject-file} as the reject file name.  @xref{Rejects}.

@item -R
Assume that this patch was created with the old and new files swapped.
@xref{Reversed Patches}.

@item --reject-file=@var{reject-file}
Use @var{reject-file} as the reject file name.  @xref{Rejects}.

@item --remove-empty-files
Remove output files that are empty after the patches have been applied.
@xref{Empty Files}.

@item --reverse
Assume that this patch was created with the old and new files swapped.
@xref{Reversed Patches}.

@item -s
Work silently unless an error occurs.  @xref{patch Messages}.

@item -S
Ignore this patch from the patch file, but continue looking for the next
patch in the file.  @xref{Multiple Patches}.

@item --silent
Work silently unless an error occurs.  @xref{patch Messages}.

@item --skip
Ignore this patch from the patch file, but continue looking for the next
patch in the file.  @xref{Multiple Patches}.

@item --strip@r{[}=@var{number}@r{]}
Set the file name strip count to @var{number}.  @xref{patch Directories}.

@item --suffix=@var{backup-suffix}
Use @var{backup-suffix} as the backup extension instead of
@samp{.orig} or @samp{~}.  @xref{Backups}.

@item -t
Do not ask any questions.  @xref{patch Messages}.

@item -u
@itemx --unified
Interpret the patch file as a unified diff.  @xref{patch Input}.

@item -v
Output the revision header and patch level of @code{patch}.

@item -V @var{backup-style}
Select the kind of backups to make.  @xref{Backups}.

@item --version
Output the revision header and patch level of @code{patch}.

@item --version=control=@var{backup-style}
Select the kind of backups to make.  @xref{Backups}.

@item -x @var{number}
Set internal debugging flags.  Of interest only to @code{patch}
patchers.
@end table

@node Invoking sdiff, Incomplete Lines, Invoking patch, Top
@chapter Invoking @code{sdiff}
@cindex invoking @code{sdiff}
@cindex @code{sdiff} invocation

The @code{sdiff} command merges two files and interactively outputs the
results.  Its arguments are as follows:

@example
sdiff -o @var{outfile} @var{options}@dots{} @var{from-file} @var{to-file}
@end example

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

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

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

@code{sdiff} without @samp{-o} (or @samp{--output}) produces a
side-by-side difference.  This usage is obsolete; use @samp{diff
--side-by-side} instead.

@menu
* sdiff Options::	Summary of options to @code{diff}.
@end menu

@node sdiff Options, , , Invoking sdiff
@section Options to @code{sdiff}
@cindex @code{sdiff} options
@cindex options for @code{sdiff}

Below is a summary of all of the options that GNU @code{sdiff} accepts.
Each option has two equivalent names, one of which is a single
letter preceded by @samp{-}, and the other of which is a long name
preceded by @samp{--}.  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.

@table @samp
@item -a
Treat all files as text and compare them line-by-line, even if they
do not appear to be text.  @xref{Binary}.

@item -b
Ignore changes in amount of blank and tab whitespace.  @xref{Whitespace}.

@item -B
Ignore changes that just insert or delete blank lines.  @xref{Blank
Lines}.

@item -d
Change the algorithm to perhaps find a smaller set of changes.  This
makes @code{sdiff} slower (sometimes much slower).  @xref{diff
Performance}.

@item -H
Use heuristics to speed handling of large files that have numerous
scattered small changes.  @xref{diff Performance}.

@item --expand-tabs
Expand tabs to spaces in the output, to preserve the alignment of tabs
in the input files.  @xref{Tabs}.

@item -i
Ignore changes in case; consider upper- and lower-case to be the same.
@xref{Case Folding}.

@item -I @var{regexp}
Ignore changes that just insert or delete lines that match @var{regexp}.
@xref{Specified Folding}.

@item --ignore-all-space
Ignore whitespace when comparing lines.  @xref{Whitespace}.

@item --ignore-blank-lines
Ignore changes that just insert or delete blank lines.  @xref{Blank
Lines}.

@item --ignore-case
Ignore changes in case; consider upper- and lower-case to be the same.
@xref{Case Folding}.

@item --ignore-matching-lines=@var{regexp}
Ignore changes that just insert or delete lines that match @var{regexp}.
@xref{Specified Folding}.

@item --ignore-space-change
Ignore changes in amount of blank and tab whitespace.
@xref{Whitespace}.

@item -l
@itemx --left-column
Print only the left column of two common lines.
@xref{Side by Side Format}.

@item --minimal
Change the algorithm to perhaps find a smaller set of changes.  This
makes @code{sdiff} slower (sometimes much slower).  @xref{diff
Performance}.

@item -o @var{file}
@itemx --output=@var{file}
Put merged output into @var{file}.  This option is required for merging.

@item -s
@itemx --suppress-common-lines
Do not print common lines.  @xref{Side by Side Format}.

@item --speed-large-files
Use heuristics to speed handling of large files that have numerous
scattered small changes.  @xref{diff Performance}.

@item -t
Expand tabs to spaces in the output, to preserve the alignment of tabs
in the input files.  @xref{Tabs}.

@item --text
Treat all files as text and compare them line-by-line, even if they
do not appear to be text.  @xref{Binary}.

@item -v
@itemx --version
Output the version number of @code{sdiff}.

@item -w @var{columns}
@itemx --width=@var{columns}
Use an output width of @var{columns}.  @xref{Side by Side Format}.
Note that for historical reasons, this option is @samp{-W} in @code{diff},
@samp{-w} in @code{sdiff}.

@item -W
Ignore horizontal whitespace when comparing lines.  @xref{Whitespace}.
Note that for historical reasons, this option is @samp{-w} in @code{diff},
@samp{-W} in @code{sdiff}.
@end table

@node Incomplete Lines, Projects, Invoking sdiff, Top
@chapter Incomplete Lines
@cindex incomplete lines
@cindex full lines
@cindex newline treatment by @code{diff}

When an input file ends in a non-newline character, its last line is
called an @dfn{incomplete line} because its last character is not a
newline.  All other lines are called @dfn{full lines} and end in a
newline character.  Incomplete lines do not match full lines unless
differences in blank lines are ignored (@pxref{Blank Lines}).

An incomplete line is normally distinguished on output from a full line
by a following line that starts with @samp{\}.  However, the RCS format
(@pxref{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 @samp{\} or @samp{/}
gutter marker; @xref{Side by Side}.  The if-then-else line format
preserves a line's incompleteness with @samp{%L}, and discards the
newline with @samp{%l}; @xref{Line Formats}.  Finally, with the
@code{ed} and forward @code{ed} output formats (@pxref{Output Formats})
@code{diff} cannot represent an incomplete line, so it pretends there
was a newline and reports an error.

For example, suppose @file{F} and @file{G} are one-byte files that
contain just @samp{f} and @samp{g}, respectively.  Then @samp{diff F G}
outputs

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

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

@example
d1 1
a1 1
g
@end example

@samp{diff -e F G} reports two errors and outputs the following:

@example
1c
g
.
@end example

@node Projects, Concept Index, Incomplete Lines, Top
@chapter Future Projects

Here are some ideas for improving GNU @code{diff} and @code{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
@samp{gnu@@prep.ai.mit.edu} to coordinate with other volunteers.

@menu
* Shortcomings::	Suggested projects for improvements.
* Bugs::		Reporting bugs.
@end menu

@node Shortcomings, Bugs, , Projects
@section Suggested Projects for Improving GNU @code{diff} and @code{patch}
@cindex projects for directories

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

@menu
* Changing Structure::	Handling changes to the directory structure.
* Special Files::	Handling symbolic links, device special files, etc.
* Unusual File Names::	Handling file names that contain unusual characters.
* Arbitrary Limits::	Patching non-text files.
* Large Files::		Handling files that do not fit in memory.
* Ignoring Changes::	Ignoring certain changes while showing others.
@end menu

@node Changing Structure, Special Files, , Shortcomings
@subsection Handling Changes to the Directory Structure
@cindex directory structure shanges

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

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

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

@node Special Files, Unusual File Names, Changing Structure, Shortcomings
@subsection Files that are Neither Directories Nor Regular Files
@cindex special files

Some files are neither directories nor regular files: they are unusual
files like symbolic links, device special files, named pipes, and
sockets.  Currently, @code{diff} treats all these files like regular
files.  However, this means that @code{patch} cannot represent changes
to such files.  For example, if you change which file a symbolic link
points to, @code{diff} outputs the difference between the two files,
instead of the change to the symbolic link.

@c This might not be a good idea; is it wise for root to install devices
@c this way?
@code{diff} should optionally report changes to special files specially,
and @code{patch} should be extended to understand these extensions.

@node Unusual File Names, Arbitrary Limits, Special Files, Shortcomings
@subsection File Names that Contain Unusual Characters
@cindex file names with unusual characters

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

@node Arbitrary Limits, Large Files, Unusual File Names, Shortcomings
@subsection Arbitrary Limits
@cindex binary file patching

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

@node Large Files, Ignoring Changes, Arbitrary Limits, Shortcomings
@subsection Handling Files that Do Not Fit in Memory
@cindex large files

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

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

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

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

@node Ignoring Changes,, Large Files, Shortcomings
@subsection Ignoring Certain Changes

It would be nice to have a feature for specifying two strings, one in
@var{from-file} and one in @var{to-file}, which should be considered to
match.  Thus, if the two strings are @samp{foo} and @samp{bar}, then if
two lines differ only in that @samp{foo} in file 1 corresponds to
@samp{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.

@node Bugs, , Shortcomings, Projects
@section Reporting Bugs
@cindex bug reports
@cindex reporting bugs

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

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

@node Concept Index, , Projects, Top
@unnumbered Concept Index

@printindex cp

@shortcontents
@contents
@bye
