# diff

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

NAME
diff — Comparing and Merging Files

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file is a newer version of the other file.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

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

You can use the cmp command to show the byte and line numbers where two
files differ.  cmp can also show all the bytes that differ between the two
files, side by side.  A way to compare two files character by character is
the Emacs command M-x compare-windows.  See Section.Dq Other Window , for

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

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

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

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

GNU diff was written by Paul Eggert, Mike Haertel, David Hayes, Richard
Stallman, and Len Tower. Wayne Davison designed and implemented the unified
output format.  The basic algorithm is described by Eugene W. Myers in “An
O(ND) Difference Algorithm and its Variations”, Algorithmica Vol. 1 No. 2,
1986, pp. 251--266; and in “A File Comparison Program”, Webb Miller and
Eugene W. Myers, Software---Practice and Experience Vol. 15 No. 11, 1985,
pp. 1025--1040. The algorithm was independently discovered as described by
E. Ukkonen in “Algorithms for Approximate String Matching”, Information and
Control Vol. 64, 1985, pp. 100--118. Unless the [--minimal] option is used,
diff uses a heuristic by Paul Eggert that limits the cost to O(N^1.5 log N)
at the price of producing suboptimal output for large inputs with many
differences.  Related algorithms are surveyed by Alfred V. Aho in section
6.3 of “Algorithms for Finding Patterns in Strings”, Handbook of
Theoretical Computer Science (Jan Van Leeuwen, ed.), Vol. A, Algorithms and
Complexity, Elsevier/MIT Press, 1990, pp. 255--300.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The [-b] or [--ignore-space-change] option is stronger. It ignores white
space at line end, and considers all other sequences of one or more white
space characters within a line to be equivalent. With this option, diff
considers the following two lines to be equivalent, where $denotes the line end: Here lyeth muche rychnesse in lytell space. -- John Heywood$
Here lyeth muche rychnesse in lytell space. -- John Heywood   $The [-w] or [--ignore-all-space] option is stronger still. It ignores differences even if one line has white space where the other line has none. White space characters include tab, newline, vertical tab, form feed, carriage return, and space; some locales may define additional characters to be white space. With this option, diff considers the following two lines to be equivalent, where$ denotes the line end and ^M denotes a
carriage return:

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

Suppressing Differences Whose Lines Are All Blank
The [-B] or [--ignore-blank-lines] option ignores changes that consist
entirely of blank lines. With this option, for example, a file containing

1.  A point is that which has no part.

2.  A line is breadthless length.
-- Euclid, The Elements, I
is considered identical to a file containing

1.  A point is that which has no part.
2.  A line is breadthless length.

-- Euclid, The Elements, I

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

Suppressing Differences Whose Lines All Match a Regular Expression
To ignore insertions and deletions of lines that match a grep -style
regular expression, use the [-I regexp] or [--ignore-matching-lines=
regexp] option. You should escape regular expressions that contain shell
metacharacters to prevent the shell from expanding them. For example, diff
-I '^[[:digit:]]' ignores all changes to lines beginning with a digit.

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

You can specify more than one regular expression for lines to ignore by
using more than one [-I] option.  diff tries to match each line against
each regular expression.

Suppressing Case Differences
GNU diff can treat lower case letters as equivalent to their upper case
counterparts, so that, for example, it considers Funky Stuff, funky STUFF,
and fUNKy stuFf to all be the same. To request this, use the [-i] or
[--ignore-case] option.

Summarizing Which Files Differ
When you only want to find out whether files are different, and you don't
care what the differences are, you can use the summary output format. In
this format, instead of showing the differences between the files, diff
simply reports whether files differ. The [-q] or [--brief] option selects
this output format.

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

You can also get a brief indication of whether two files differ by using
cmp.  For files that are identical, cmp produces no output. When the files
differ, by default, cmp outputs the byte and line number where the first
difference occurs, or reports that one file is a prefix of the other. You
can use the [-s], [--quiet], or [--silent] option to suppress that
information, so that cmp produces no output and reports whether the files
differ using only its exit status (see Section “Invoking cmp”).

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

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

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

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

You can also force diff to report only whether files differ (but not how).
Use the [-q] or [--brief] option for this.

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

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

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

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

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

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

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

This is the file lao:

The Way that can be told of is not the eternal Way;
The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
The Named is the mother of all things.
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
so we may see their outcome.
The two are the same,
But after they are produced,
they have different names.

This is the file tzu:

The Nameless is the origin of Heaven and Earth;
The named is the mother of all things.

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

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

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

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

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

Context Format

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

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

Example of Context Format

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

*** lao 2002-02-21 23:30:39.942229878 -0800
--- tzu 2002-02-21 23:30:50.442260588 -0800
***************
*** 1,7 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
--- 1,6 ----
The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
!
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
***************
*** 9,11 ****
--- 8,13 ----
The two are the same,
But after they are produced,
they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!

Example of Context Format with Less Context

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

*** lao 2002-02-21 23:30:39.942229878 -0800
--- tzu 2002-02-21 23:30:50.442260588 -0800
***************
*** 1,5 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
Therefore let there always be non-being,
--- 1,4 ----
The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
!
Therefore let there always be non-being,
***************
*** 11 ****
--- 10,13 ----
they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!

Detailed Description of Context Format

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

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

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

You can change the header's content with the [--label= label] option; see
Alternate Names.

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

***************
*** from-file-line-numbers ****
from-file-line
from-file-line...
--- to-file-line-numbers ----
to-file-line
to-file-line...

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

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

!       A line that is part of a group of one or more lines that changed
between the two files. There is a corresponding group of lines
marked with !  in the part of this hunk for the other file.

+       An “inserted” line in the second file that corresponds to nothing
in the first file.

-       A “deleted” line in the first file that corresponds to nothing in
the second file.

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

Unified Format

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

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

Example of Unified Format

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

--- lao 2002-02-21 23:30:39.942229878 -0800
+++ tzu 2002-02-21 23:30:50.442260588 -0800
@@ -1,7 +1,6 @@
-The Way that can be told of is not the eternal Way;
-The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
-The Named is the mother of all things.
+The named is the mother of all things.
+
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
@@ -9,3 +8,6 @@
The two are the same,
But after they are produced,
they have different names.
+They both may be called deep and profound.
+Deeper and more profound,
+The door of all subtleties!

Detailed Description of Unified Format

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

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

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

You can change the header's content with the [--label= label] option;
seeSee Section “Alternate Names”.

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

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

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

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

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

+       A line was added here to the first file.

-       A line was removed here from the first file.

Showing Which Sections Differences Are in

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

Showing Lines That Match Regular Expressions

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

^[[:alpha:]$_] C, C++, Prolog ^( Lisp ^@node Texinfo This option does not automatically select an output format; in order to use it, you must select the context format (see Section “Context Format”) or unified format (see Section “Unified Format”). In other output formats it has no effect. The [-F] or [--show-function-line] option finds the nearest unchanged line that precedes each hunk of differences and matches the given regular expression. Then it adds that line to the end of the line of asterisks in the context format, or to the @@ line in unified format. If no matching line exists, this option leaves the output for that hunk unchanged. If that line is more than 40 characters long, it outputs only the first 40 characters. You can specify more than one regular expression for such lines; diff tries to match each line against each regular expression, starting with the last one given. This means that you can use [-p] and [-F] together, if you wish. Showing C Function Headings To show in which functions differences occur for C and similar languages, you can use the [-p] or [--show-c-function] option. This option automatically defaults to the context output format (see Section “Context Format”), with the default number of lines of context. You can override that number with [-C lines] elsewhere in the command line. You can override both the format and the number with [-U lines] elsewhere in the command line. The [-p] or [--show-c-function] option is equivalent to [-F '^[[:alpha:]$_]'] if the unified format is specified, otherwise [-c -F
'^[[:alpha:]\$_]'] (see Section “Specified Headings”).  GNU diff provides
this option for the sake of convenience.

Showing Alternate File Names

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

Here are the first two lines of the output from diff -C 2 --label=original
--label=modified lao tzu:

*** original
--- modified

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

white space
The corresponding lines are in common. That is, either the lines
are identical, or the difference is ignored because of one of the
[--ignore] options (see Section “White Space”).

|       The corresponding lines differ, and they are either both complete
or both incomplete.

<       The files differ and only the first file contains the line.

>       The files differ and only the second file contains the line.

(       Only the first file contains the line, but the difference is
ignored.

)       Only the second file contains the line, but the difference is
ignored.

\       The corresponding lines differ, and only the first line is
incomplete.

/       The corresponding lines differ, and only the second line is
incomplete.

Normally, an output line is incomplete if and only if the lines that it
contains are incomplete;See Section “Incomplete Lines”.  However, when an
the other is not. In this case, the output line is complete, but its the
gutter is marked \ if the first line is incomplete, / if the second line
is.

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

You can use the sdiff command to interactively merge side by side
merging files.

Controlling Side by Side Format

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

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

Example of Side by Side Format

Here is the output of the command diff -y -W 72 lao tzu (see Section
“Sample diff Input”, for the complete contents of the two files).

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

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

Example of Normal Format

Here is the output of the command diff lao tzu (see Section “Sample diff
Input”, for the complete contents of the two files). Notice that it shows
only the lines that are different between the two files.

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

Detailed Description of Normal Format

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

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

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

la r    Add the lines in range r of the second file after line l of the
first file. For example, 8a12,15 means append lines 12--15 of file
2 after line 8 of file 1; or, if changing file 2 into file 1,
delete lines 12--15 of file 2.

fc t    Replace the lines in range f of the first file with lines in range
t of the second file. This is like a combined add and delete, but
more compact.  For example, 5,7c8,10 means change lines 5--7 of
file 1 to read as lines 8--10 of file 2; or, if changing file 2
into file 1, change lines 8--10 of file 2 to read as lines 5--7 of
file 1.

rd l    Delete the lines in range r from the first file; line l is where
they would have appeared in the second file had they not been
deleted.  For example, 5,7d3 means delete lines 5--7 of file 1; or,
if changing file 2 into file 1, append lines 5--7 of file 1 after
line 3 of file 2.

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

ed(Scripts)

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

Like the normal format (see Section “Normal”), this output format does not
show any context; unlike the normal format, it does not include the
information necessary to apply the diff in reverse (to produce the first
file if all you have is the second file and the diff).

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

Example ed(Script)

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

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

.
1,2d

Detailed Description of ed(Format)

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

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

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

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

la      Add text from the second file after line l in the first file. For
example, 8a means to add the following lines after line 8 of file
1.

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

rd      Delete the lines in range r from the first file. For example, 5,7d
means delete lines 5--7 of file 1.

Forward ed(Scripts)

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

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

RCS Scripts

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

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

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

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

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

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

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

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

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

Line Group Formats

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

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

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

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

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

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

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

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

--old-group-format= format
These line groups are hunks containing only lines from the first
file. The default old group format is the same as the changed group
format if it is specified; otherwise it is a format that outputs
the line group as-is.

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

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

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

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

%<      stands for the lines from the first file, including the trailing
newline.  Each line is formatted according to the old line format
(see Section “Line Formats”).

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

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

%%      stands for %.

%c' C'  where C is a single character, stands for C.  C may not be a
backslash or an apostrophe. For example, %c':' stands for a colon,
even inside the then-part of an if-then-else format, which a colon
would normally terminate.

%c'\ O'
where O is a string of 1, 2, or 3 octal digits, stands for the
character with octal code O.  For example, %c'\0' stands for a null
character.

F n     where F is a printf conversion specification and n is one of the
following letters, stands for n 's value formatted with F.

e       The line number of the line just before the group in the
old file.

f       The line number of the first line in the group in the old
file; equals e + 1.

l       The line number of the last line in the group in the old
file.

m       The line number of the line just after the group in the old
file; equals l + 1.

n       The number of lines in the group in the old file; equals l
- f + 1.

E, F, L, M, N
Likewise, for lines in the new file.

The printf conversion specification can be %d, %o, %x, or %X,
specifying decimal, octal, lower case hexadecimal, or upper case
hexadecimal output respectively. After the % the following options
can appear in sequence: a series of zero or more flags; an integer
specifying the minimum field width; and a period followed by an
optional integer specifying the minimum number of digits. The flags
are - for left-justification, ' for separating the digit into
groups as specified by the LC_NUMERIC locale category, and 0 for
number of new lines in the group in a field of width 5 characters,
using the printf format %5d.

(A= B? T: E)
If A equals B then T else E.  A and B are each either a decimal
constant or a single letter interpreted as above.  This format spec
is equivalent to T if A 's value equals B 's; otherwise it is
equivalent to E.

For example, %(N=0?no:%dN) line%(N=1?:s) is equivalent to no lines
if N (the number of lines in the group in the new file) is 0, to 1
line if N is 1, and to %dN lines otherwise.

Line Formats

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

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

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

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

--old-line-format= format
formats lines just from the first file.

--new-line-format= format
formats lines just from the second file.

--unchanged-line-format= format
formats lines common to both files.

--line-format= format
formats all lines; in effect, it sets all three above options
simultaneously.

In a line format, ordinary characters represent themselves; conversion

%l      stands for the contents of the line, not counting its trailing
newline (if any). This format ignores whether the line is
incomplete;See Section “Incomplete Lines”.

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

%%      stands for %.

%c' C'  where C is a single character, stands for C.  C may not be a
backslash or an apostrophe. For example, %c':' stands for a colon.

%c'\ O'
where O is a string of 1, 2, or 3 octal digits, stands for the
character with octal code O.  For example, %c'\0' stands for a null
character.

where F is a printf conversion specification, stands for the line
number formatted with F.  For example, %.5dn prints the line number
using the printf format %.5d.  See Section.Dq Line Group Formats ,
for more about printf conversion specifications.

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

If the input contains tab characters and it is important that they line up
on output, you should ensure that %l or %L in a line format is just after a
tab stop (e.g. by preceding %l or %L with a tab character), or you should
use the [-t] or [--expand-tabs] option.

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

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

Example of If-then-else Format

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

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

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

Detailed Description of If-then-else Format

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

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

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

--old-group-format='#ifndef name
%<#endif /* ! name */
' \
--new-group-format='#ifdef name
%>#endif /* name */
' \
--unchanged-group-format='%=' \
--changed-group-format='#ifndef name
%<#else /* name */
%>#endif /* name */
'

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

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

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

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

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

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

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

d1 1
a1 1
g

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

1c
g
.

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

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

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

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

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

If you need to give this option many times, you can instead put the
patterns in a file, one pattern per line, and use the [-X file] or
[--exclude-from= file] option. Trailing white space and empty lines are
ignored in the pattern file.

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

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

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

Making diff(Output) Prettier
diff provides several ways to adjust the appearance of its output. These
adjustments can be applied to any output format.

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

The first way is to have diff convert all tabs into the correct number of
spaces before outputting them; select this method with the [-t] or
[--expand-tabs] option. To use this form of output with patch, you must
give patch the [-l] or [--ignore-white-space] option (see Section “Changed
are set every 8 print columns, but this can be altered by the [--tabsize=
columns] option.

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

Paginating diff(Output)
It can be convenient to have long output page-numbered and time-stamped.
The [-l] or [--paginate] option does this by sending the diff output
through the pr program. Here is what the page header might look like for
diff -lc lao tzu:

2002-02-22 14:20                 diff -lc lao tzu                 Page 1

GNU diff runs quite efficiently; however, in some circumstances you can
cause it to run faster or produce a more compact set of changes.

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

You can also affect the performance of GNU diff by giving it options that
change the way it compares files. Performance has more than one dimension.
These options improve one aspect of performance at the cost of another, or
they improve performance in some cases while hurting it in others.

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

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

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

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

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

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

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

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

The Way that can be told of is not the eternal Way;
The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
The named is the mother of all things.

Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
so we may see their result.
The two are the same,
But after they are produced,
they have different names.

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

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

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

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

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

Detailed Description of diff3(Normal) Format
Each hunk begins with a line marked ====.  Three-way hunks have plain ====
lines, and two-way hunks have 1, 2, or 3 appended to specify which of the
three input files differ in that hunk. The hunks contain copies of two or
three sets of input lines each preceded by one or two commands identifying
where the lines came from.

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

Commands take the following forms:

file: la
This hunk appears after line l of file file, and contains no lines
in that file. To edit this file to yield the other files, one must
append hunk lines taken from the other files. For example, 1:11a
means that the hunk follows line 11 in the first file and contains
no lines from that file.

file: rc
This hunk contains the lines in the range r of file file.  The
range r is a comma-separated pair of line numbers, or just one
number if the range is a singleton. To edit this file to yield the
other files, one must change the specified lines to be the lines
taken from the other files. For example, 2:11,13c means that the
hunk contains lines 11 through 13 from the second file.

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

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

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

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

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

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

Merging From a Common Ancestor
When two people have made changes to copies of the same file, diff3 can
produce a merged output that contains both sets of changes together with

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

diff3 can incorporate changes from two modified versions into a common
preceding version. This lets you merge the sets of changes represented by
the two newer files. Specify the common ancestor version as the second
argument and the two newer versions as the first and third arguments, like
this:

diff3 mine older yours

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

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

diff3 gives you several ways to handle overlaps and conflicts. You can omit
overlaps or conflicts, or select only overlaps, or mark conflicts with
special <<<<<<< and >>>>>>> lines.

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

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

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

Here is the output of the command diff3 with each of these three options
(see Section “Sample diff3 Input”, for the complete contents of the files).
Notice that [-e] outputs the union of the disjoint sets of changes output
by [-3] and [-x].

Output of diff3 -e lao tzu tao:

11a

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

Output of diff3 -3 lao tzu tao:

8c
so we may see their result.
.

Output of diff3 -x lao tzu tao:

11a

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

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

<<<<<<< A
lines from A
=======
lines from B
>>>>>>> B

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

<<<<<<< A
lines from A
||||||| B
lines from B
=======
lines from C
>>>>>>> C

The [-A] or [--show-all] option acts like the [-e] option, except that it
brackets conflicts, and it outputs all changes from older to yours, not
just the unmerged changes. Thus, given the sample input files (see Section
“Sample diff3 Input”), diff3 -A lao tzu tao puts brackets around the
conflict where only tzu differs:

<<<<<<< tzu
=======
The Way that can be told of is not the eternal Way;
The name that can be named is not the eternal name.
>>>>>>> tao

And it outputs the three-way conflict as follows:

<<<<<<< lao
||||||| tzu
They both may be called deep and profound.
Deeper and more profound,
The door of all subtleties!
=======

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

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

<<<<<<< lao
=======

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

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

Generating the Merged Output Directly
With the [-m] or [--merge] option, diff3 outputs the merged file directly.
This is more efficient than using ed to generate it, and works even with
non-text files that ed would reject. If you specify [-m] without an ed
script option, [-A] is assumed.

For example, the command diff3 -m lao tzu tao (see Section “Sample diff3
Input” for a copy of the input files) would output the following:

<<<<<<< tzu
=======
The Way that can be told of is not the eternal Way;
The name that can be named is not the eternal name.
>>>>>>> tao
The Nameless is the origin of Heaven and Earth;
The Named is the mother of all things.
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
so we may see their result.
The two are the same,
But after they are produced,
they have different names.
<<<<<<< lao
||||||| tzu
They both may be called deep and profound.
Deeper and more profound,
The door of all subtleties!
=======

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

How diff3(Merges) Incomplete Lines
With [-m], incomplete lines (see Section “Incomplete Lines”) are simply
copied to the output as they are found; if the merged output ends in an
conflict and one of the input files ends in an incomplete line, succeeding
|||||||, ======= or >>>>>>> brackets appear somewhere other than the start
of a line because they are appended to the incomplete line.

Without [-m], if an ed script option is specified and an incomplete line is
found, diff3 generates a warning and acts as if a newline had been present.

Saving the Changed File
Traditional Unix diff3 generates an ed script without the trailing w and q
commands that save the changes. System V diff3 generates these extra
commands. GNU diff3 normally behaves like traditional Unix diff3, but with
the [-i] option it behaves like System V diff3 and appends the w and q
commands.

The [-i] option requires one of the ed script options [-AeExX3], and is
incompatible with the merged output option [-m].

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

Another way to merge files interactively is to use the Emacs Lisp package

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

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

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

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

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

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

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

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

el

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

er

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

l

1       Copy the left version to the output.

q       Quit.

r

2       Copy the right version to the output.

s       Silently copy common lines.

v       Verbosely copy common lines. This is the default.

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

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

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

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

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

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

-c

--context
context diff.

-e

--ed    ed script.

-n

--normal
normal diff.

-u

--unified
unified diff.

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

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

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

Applying Imperfect Patches
patch tries to skip any leading text in the patch file, apply the diff, and
then skip any trailing text. Thus you can feed a mail message directly to
patch, and it should work. If the entire diff is indented by a constant
amount of white space, patch automatically ignores the indentation. If a
context diff contains trailing carriage return on each line, patch
automatically ignores the carriage return. If a context diff has been
encapsulated by prepending - to lines beginning with - as per
ftp://ftp.isi.edu/in-notes/rfc934.txt, patch automatically unencapsulates
the input.

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

Applying Patches with Changed White Space

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

Applying Reversed Patches

Sometimes people run diff with the new file first instead of second. This
creates a diff that is “reversed”.  To apply such patches, give patch the
[-R] or [--reverse] option.  patch then attempts to swap each hunk around
before applying it. Rejects come out in the swapped format.

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

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

Helping patch(Find) Inexact Matches

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

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

The [-F lines] or [--fuzz= lines] option sets the maximum fuzz factor to
lines.  This option only applies to context and unified diffs; it ignores
up to lines lines while looking for the place to install a hunk. Note that
a larger fuzz factor increases the odds of making a faulty patch. The
default fuzz factor is 2; there is no point to setting it to more than the
number of lines of context in the diff, ordinarily 3.

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

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

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

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

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

Predicting what patch(will) do

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

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

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

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

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

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

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

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

Multiple Patches in a File
If the patch file contains more than one patch, and if you do not specify
an input file on the command line, patch tries to apply each patch as if
they came from separate patch files. This means that it determines the name
of the file to patch for each patch, and that it examines the leading text
before each patch for file names and prerequisite revision level (see
Section “Making Patches”, for more on that topic).

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

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

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

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

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

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

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

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

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

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

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

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

| patch -d /usr/src/emacs

Sometimes the file names given in a patch contain leading directories, but
you keep your files in a directory different from the one given in the
patch.  In those cases, you can use the [-p number] or [--strip= number]
option to set the file name strip count to number.  The strip count tells
patch how many slashes, along with the directory names between them, to
strip from the front of file names. A sequence of one or more adjacent
slashes is counted as a single slash. By default, patch strips off all
leading directories, leaving just the base file names.

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

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

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

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

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

Backup File Names
Normally, patch renames an original input file into a backup file by
appending to its name the extension .orig, or ~ if using .orig would make
the backup file name too long. The [-z backup-suffix] or [--suffix=
backup-suffix] option causes patch to use backup-suffix as the backup

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

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

The [-V backup-style] or [--version-control= backup-style] option takes as
an argument a method for creating backup file names. You can alternately
control the type of backups that patch makes with the PATCH_VERSION_CONTROL
environment variable, which the [-V] option overrides. If
PATCH_VERSION_CONTROL is not set, the VERSION_CONTROL environment variable
backup file names; they do not affect the choice of revision control system
(see Section “Revision Control”).

The values of these environment variables and the argument to the [-V]
option are like the GNU Emacs version-control variable (see Section “Backup
recognize synonyms that are more descriptive. The valid values are listed
below; unique abbreviations are acceptable.

t

numbered
Always make numbered backups.

nil

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

never

simple  Always make simple backups.

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

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

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

Reject File Names
The names for reject files (files containing patches that patch could not
find a place to apply) are normally the name of the output file with .rej
appended (or # if using .rej would make the backup file name too long).

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

Messages and Questions from patch
patch can produce a variety of messages, especially if it has trouble
decoding its input. In a few situations where it's not sure how to proceed,
are options to produce more or fewer messages, to have it not ask for
keyboard input, and to affect the way that file names are quoted in
messages.

patch exits with status 0 if all hunks are applied successfully, 1 if some
hunks cannot be applied, and 2 if there is more serious trouble. When
applying a set of patches in a loop, you should check the exit status, so
you don't apply a later patch to a partially patched file.

Controlling the Verbosity of patch

You can cause patch to produce more messages by using the [--verbose]
option. For example, when you give this option, the message Hmm...
indicates that patch is reading text in the patch file, attempting to
determine whether there is a patch in that text, and if so, what kind of
patch it is.

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

Inhibiting Keyboard Input

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

Skip patches that do not contain file names in their headers.

Patch files even though they have the wrong version for the Prereq:
line in the patch;

Assume that patches are not reversed even if they look like they are.

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

Skip patches that do not contain file names in their headers (the same
as [-f]).

Skip patches for which the file has the wrong version for the Prereq:
line in the patch;

Assume that patches are reversed if they look like they are.

patch(Quoting) Style

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

literal
Output names as-is.

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

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

c       Quote names as for a C language string.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

diff -Naur gcc-3.0.3 gcc-3.0.4

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

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

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

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

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

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

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

Avoid sending patches that compare backup file names like README.orig or
README~, since this might confuse patch into patching a backup file instead
of the real file. Instead, send patches that compare the same base file

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

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

Generating Smaller Patches
The simplest way to generate a patch is to use diff -Naur (see Section
“Tips for Patch Producers”), but you might be able to reduce the size of
the patch by renaming or removing some files before making the patch. If
the older version of the package contains any files that the newer version
does not, or if any files have been renamed between the two versions, make
a list of rm and mv commands for the user to execute in the old version
directory before applying the patch. Then run those commands yourself in
the scratch directory.

If there are any files that you don't need to include in the patch because
they can easily be rebuilt from other files (for example, TAGS and output
from yacc and makeinfo), exclude them from the patch by giving diff the [-x
pattern] option (see Section “Comparing Directories”).  If you want your
patch to modify a derived file because your recipients lack tools to build
it, make sure that the patch for the derived file follows any patches for
files that it depends on, so that the recipients' time stamps will not
confuse make.

Now you can create the patch using diff -Naur.  Make sure to specify the
scratch directory first and the newer directory second.

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

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

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

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

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

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

cmp: EOF on shorter-file

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

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

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

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

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

-b

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

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

-i skip

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

-l

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

-n count

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

-s

--quiet

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

-v

--version
Output version information and then exit.

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

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

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

kB      kilobyte: 10^3 = 1000.

k

K

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

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

M

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

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

G

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

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

T

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

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

P

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

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

E

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

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

Z

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

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

Y

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

Invoking diff
The format for running the diff command is:

diff options... files...

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

If one file is a directory and the other is not, diff compares the file in
the directory whose name is that of the non-directory.  The non-directory
file must not be -.

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

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

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

An exit status of 0 means no differences were found, 1 means some
differences were found, and 2 means trouble. Normally, differing binary
files count as trouble, but this can be altered by using the [-a] or
[--text] option, or the [-q] or [--brief] option.

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

-a

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

-b

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

-B

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

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

-c      Use the context output format, showing three lines of context.See
Section “Context Format”.

-C lines

--context[= lines]
Use the context output format, showing lines (an integer) lines of
context, or three if lines is not given.See Section “Context
Format”.  For proper operation, patch typically needs at least two
lines of context.

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

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

-d

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

-D name

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

-e

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

-E

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

-f

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

-F regexp

--show-function-line= regexp
In context and unified format, for each hunk of differences, show
some of the last preceding line that matches regexp.  See

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

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

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

-i

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

-I regexp

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

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

-l

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

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

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

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

-n

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

-N

--new-file
In directory comparison, if a file is found in only one directory,
treat it as present but empty in the other directory.See Section
“Comparing Directories”.

--new-group-format= format
Use format to output a group of lines taken from just the second
file in if-then-else format.See Section “Line Group Formats”.

--new-line-format= format
Use format to output a line taken from just the second file in if-
then-else format.See Section “Line Formats”.

--old-group-format= format
Use format to output a group of lines taken from just the first
file in if-then-else format.See Section “Line Group Formats”.

--old-line-format= format
Use format to output a line taken from just the first file in if-
then-else format.See Section “Line Formats”.

-p

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

-q

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

-r

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

-s

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

-S file

--starting-file= file
for resuming an aborted comparison.See Section “Comparing
Directories”.

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

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

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

-t

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

-T

--initial-tab
Output a tab rather than a space before the text of a line in
normal or context format. This causes the alignment of tabs in the
line to look normal.See Section “Tabs”.

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

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

-u      Use the unified output format, showing three lines of context.See
Section “Unified Format”.

--unchanged-group-format= format
Use format to output a group of common lines taken from both files
in if-then-else format.See Section “Line Group Formats”.

--unchanged-line-format= format
Use format to output a line common to both files in if-then-else
format.See Section “Line Formats”.

--unidirectional-new-file
When comparing directories, if a file appears only in the second
directory of the two, treat it as present but empty in the
other.See Section “Comparing Directories”.

-U lines

--unified[= lines]
Use the unified output format, showing lines (an integer) lines of
context, or three if lines is not given.See Section “Unified
Format”.  For proper operation, patch typically needs at least two
lines of context.

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

-v

--version
Output version information and then exit.

-w

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

-W columns

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

-x pattern

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

-X file

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

-y

--side-by-side
Use the side by side output format.See Section “Side by Side
Format”.

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

diff3 options... mine older yours

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

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

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

-a

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

-A

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

--diff-program= program
Use the compatible comparison program program to compare files

-e

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

-E

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

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

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

-i      Generate w and q commands at the end of the ed script for System V
compatibility. This option must be combined with one of the
[-AeExX3] options, and may not be combined with [-m].  See
Section.Dq Saving the Changed File .

--label= label
Use the label label for the brackets output by the [-A], [-E] and
[-X] options. This option may be given up to three times, one for
each input file.  The default labels are the names of the input
files. Thus diff3 --label X --label Y --label Z -m A B C acts like
diff3 -m A B C, except that the output looks like it came from
files named X, Y and Z rather than from files named A, B and C.
See Section.Dq Marking Conflicts .

-m

--merge
Apply the edit script to the first file and send the result to
standard output.  Unlike piping the output from diff3 to ed, this
works even for binary files and incomplete lines.  [-A] is assumed
if no edit script option is specified.See Section “Bypassing ed”.

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

-T

--initial-tab
Output a tab rather than two spaces before the text of a line in
normal format.  This causes the alignment of tabs in the line to
look normal.See Section “Tabs”.

-v

--version
Output version information and then exit.

-x

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

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

-3

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

Invoking patch
Normally patch is invoked like this:

patch <patchfile

The full format for invoking patch is:

patch options... [origfile [patchfile]]

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

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

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

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

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

-b

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

-B prefix

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

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

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

-c

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

-d directory

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

-D name

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

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

-e

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

-E

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

-f

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

-F lines

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

-g num

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

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

-i patchfile

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

-l

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

-n

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

-N

--forward
Ignore patches that patch thinks are reversed or already applied.

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

-o file

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

-p number

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

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

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

-r reject-file

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

-R

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

-s

--quiet

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

-t

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

-T

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

-u

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

-v

--version
Output version information and then exit.

-V backup-style

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

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

-x number

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

-Y prefix

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

-z suffix

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

-Z

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

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

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

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

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

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

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

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

-a

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

-b

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

-B

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

-d

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

--diff-program= program
Use the compatible comparison program program to compare files

-E

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

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

-i

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

-I regexp

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

-l

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

-o file

--output= file
Put merged output into file.  This option is required for merging.

-s

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

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

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

-t

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

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

-v

--version
Output version information and then exit.

-w columns

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

-W

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

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

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

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

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

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

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

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

Handling Multibyte and Varying-Width Characters

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

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

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

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

Handling Changes to the Directory Structure

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

There should be a way to specify that a file has been removed without
having to include its entire contents in the patch file. There should also
be a way to tell patch that a file was renamed, even if there is no way for
diff to generate such information. There should be a way to tell patch that
a file's time stamp has changed, even if its contents have not changed.

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

Files that are Neither Directories Nor Regular Files

Some files are neither directories nor regular files: they are unusual
files like symbolic links, device special files, named pipes, and sockets.
Currently, diff treats symbolic links as if they were the pointed-to files,
except that a recursive diff reports an error if it detects infinite loops
of symbolic links (e.g., symbolic links to ..).  diff treats other special
files like regular files if they are specified at the top level, but simply
reports their presence when comparing directories. This means that patch
cannot represent changes to such files. For example, if you change which
file a symbolic link points to, diff outputs the difference between the two

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

File Names that Contain Unusual Characters

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

Outputting Diffs in Time Stamp Order

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

Ignoring Certain Changes

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

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

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

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

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

Improving Performance

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

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

Copying This Manual
Place, Suite 330, Boston, MA 02111-1307, USA

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

1.   PREAMBLE

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

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

6.   COMBINING DOCUMENTS

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

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

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

7.   COLLECTIONS OF DOCUMENTS

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

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

8.   AGGREGATION WITH INDEPENDENT WORKS

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

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

9.   TRANSLATION

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

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

10.  TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided for under this License. Any other attempt
to copy, modify, sublicense or distribute the Document is void, and
parties who have received copies, or rights, from you under this
parties remain in full compliance.

11.  FUTURE REVISIONS OF THIS LICENSE

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

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

To use this License in a document you have written, include a copy of the
just after the title page:

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.  A copy of the license is included in the section entitled “GNU

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

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

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

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

Translations of This Manual
Nishio Futoshi of the GNUjdoc project has prepared a Japanese translation