diff, diff3, sdiff, cmp, and patchdiff 2.5 and patch 2.1
diff Output Formats
diff Output Prettier
diff Performance Tradeoffs
sdiff
patch
cmp
diff
diff3
patch
sdiff
Computer users often find occasion to ask how two files differ. Perhaps one file is a newer version of the other file. Or maybe the two files started out as identical copies but were changed by different people.
You can use the diff command to show differences between two
files, or each corresponding file in two directories. diff
outputs differences between files line by line in any of several
formats, selectable by command line options. This set of differences is
often called a diff or patch. For files that are identical,
diff normally produces no output; for binary (non-text) files,
diff normally reports only that they are different.
You can use the cmp command to show the offsets and line numbers
where two files differ. cmp can also show all the characters
that differ between the two files, side by side. Another way to compare
two files character by character is the Emacs command M-x
compare-windows. See section `Other Window' in The GNU Emacs Manual, for more information on that command.
You can use the diff3 command to show differences among three
files. When two people have made independent changes to a common
original, diff3 can report the differences between the original
and the two changed versions, and can produce a merged file that
contains both persons' changes together with warnings about conflicts.
You can use the sdiff command to merge two files interactively.
You can use the set of differences produced by diff to distribute
updates to text files (such as program source code) to other people.
This method is especially useful when the differences are small compared
to the complete files. Given diff output, you can use the
patch program to update, or patch, a copy of the file. If you
think of diff as subtracting one file from another to produce
their difference, you can think of patch as adding the difference
to one file to reproduce the other.
This manual first concentrates on making diffs, and later shows how to use diffs to update files.
GNU diff was written by Mike Haertel, David Hayes, Richard
Stallman, Len Tower, and Paul Eggert. Wayne Davison designed and
implemented the unified output format. The basic algorithm is described
in "An O(ND) Difference Algorithm and its Variations", Eugene W. Myers,
Algorithmica Vol. 1 No. 2, 1986, pp. 251--266; and in "A File
Comparison Program", Webb Miller and Eugene W. Myers,
Software--Practice and Experience Vol. 15 No. 11, 1985,
pp. 1025--1040.
The algorithm was independently discovered as described in
"Algorithms for Approximate String Matching",
E. Ukkonen, Information and Control Vol. 64, 1985, pp. 100--118.
GNU diff3 was written by Randy Smith. GNU sdiff was
written by Thomas Lord. GNU cmp was written by Torbjorn Granlund
and David MacKenzie.
patch was written mainly by Larry Wall; the GNU enhancements were
written mainly by Wayne Davison and David MacKenzie. Parts of this
manual are adapted from a manual page written by Larry Wall, with his
permission.
There are several ways to think about the differences between two files.
One way to think of the differences is as a series of lines that were
deleted from, inserted in, or changed in one file to produce the other
file. diff compares two files line by line, finds groups of
lines that differ, and reports each group of differing lines. It can
report the differing lines in several formats, which have different
purposes.
GNU diff can show whether files are different without detailing
the differences. It also provides ways to suppress certain kinds of
differences that are not important to you. Most commonly, such
differences are changes in the amount of white space between words or
lines. diff also provides ways to suppress differences in
alphabetic case or in lines that match a regular expression that you
provide. These options can accumulate; for example, you can ignore
changes in both white space and alphabetic case.
Another way to think of the differences between two files is as a
sequence of pairs of characters that can be either identical or
different. cmp reports the differences between two files
character by character, instead of line by line. As a result, it is
more useful than diff for comparing binary files. For text
files, cmp is useful mainly when you want to know only whether
two files are identical.
To illustrate the effect that considering changes character by character
can have compared with considering them line by line, think of what
happens if a single newline character is added to the beginning of a
file. If that file is then compared with an otherwise identical file
that lacks the newline at the beginning, diff will report that a
blank line has been added to the file, while cmp will report that
almost every character of the two files differs.
diff3 normally compares three input files line by line, finds
groups of lines that differ, and reports each group of differing lines.
Its output is designed to make it easy to inspect two different sets of
changes to the same file.
When comparing two files, diff finds sequences of lines common to
both files, interspersed with groups of differing lines called
hunks. Comparing two identical files yields one sequence of
common lines and no hunks, because no lines differ. Comparing two
entirely different files yields no common lines and one large hunk that
contains all lines of both files. In general, there are many ways to
match up lines between two given files. diff tries to minimize
the total hunk size by finding large sequences of common lines
interspersed with small hunks of differing lines.
For example, suppose the file `F' contains the three lines
`a', `b', `c', and the file `G' contains the same
three lines in reverse order `c', `b', `a'. If
diff finds the line `c' as common, then the command
`diff F G' produces this output:
1,2d0 < a < b 3a2,3 > b > a
But if diff notices the common line `b' instead, it produces
this output:
1c1 < a --- > c 3c3 < c --- > a
It is also possible to find `a' as the common line. diff
does not always find an optimal matching between the files; it takes
shortcuts to run faster. But its output is usually close to the
shortest possible. You can adjust this tradeoff with the
`--minimal' option (see section diff Performance Tradeoffs).
The `-b' and `--ignore-space-change' options ignore white space
at line end, and considers all other sequences of one or more
white space characters to be equivalent. With these options,
diff considers the following two lines to be equivalent, where
`$' denotes the line end:
Here lyeth muche rychnesse in lytell space. -- John Heywood$ Here lyeth muche rychnesse in lytell space. -- John Heywood $
The `-w' and `--ignore-all-space' options are stronger than
`-b'. They ignore difference even if one file has white space where
the other file has none. White space characters include
tab, newline, vertical tab, form feed, carriage return, and space;
some locales may define additional characters to be white space.
With these options, diff considers the
following two lines to be equivalent, where `$' denotes the line
end and `^M' denotes a carriage return:
Here lyeth muche rychnesse in lytell space.-- John Heywood$ He relyeth much erychnes seinly tells pace. --John Heywood ^M$
The `-B' and `--ignore-blank-lines' options ignore insertions or deletions of blank lines. These options normally affect only lines that are completely empty; they do not affect lines that look empty but contain space or tab characters. With these options, for example, a file containing
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
GNU diff can treat lowercase letters as equivalent to their
uppercase counterparts, so that, for example, it considers `Funky
Stuff', `funky STUFF', and `fUNKy stuFf' to all be the same.
To request this, use the `-i' or `--ignore-case' option.
To ignore insertions and deletions of lines that match a regular expression, use the `-I regexp' or `--ignore-matching-lines=regexp' option. You should escape regular expressions that contain shell metacharacters to prevent the shell from expanding them. For example, `diff -I '^[0-9]'' ignores all changes to lines beginning with a digit.
However, `-I' only ignores the insertion or deletion of lines that
contain the regular expression if every changed line in the hunk--every
insertion and every deletion--matches the regular expression. In other
words, for each nonignorable change, diff prints the complete set
of changes in its vicinity, including the ignorable ones.
You can specify more than one regular expression for lines to ignore by
using more than one `-I' option. diff tries to match each
line against each regular expression, starting with the last one given.
When you only want to find out whether files are different, and you
don't care what the differences are, you can use the summary output
format. In this format, instead of showing the differences between the
files, diff simply reports whether files differ. The `-q'
and `--brief' options select this output format.
This format is especially useful when comparing the contents of two
directories. It is also much faster than doing the normal line by line
comparisons, because diff can stop analyzing the files as soon as
it knows that there are any differences.
You can also get a brief indication of whether two files differ by using
cmp. For files that are identical, cmp produces no
output. When the files differ, by default, cmp outputs the byte
offset and line number where the first difference occurs. You can use
the `-s' option to suppress that information, so that cmp
produces no output and reports whether the files differ using only its
exit status (see section Invoking cmp).
Unlike diff, cmp cannot compare directories; it can only
compare two files.
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 Summarizing Which Files Differ), and
reports only that the binary files are different. This is because line
by line comparisons are usually not meaningful for binary files.
diff determines whether a file is text or binary by checking the
first few bytes in the file; the exact number of bytes is system
dependent, but it is typically several thousand. If every character in
that part of the file is non-null, diff considers the file to be
text; otherwise it considers the file to be binary.
Sometimes you might want to force diff to consider files to be
text. For example, you might be comparing text files that contain
null characters; diff would erroneously decide that those are
non-text files. Or you might be comparing documents that are in a
format used by a word processing system that uses null characters to
indicate special formatting. You can force diff to consider all
files to be text files, and compare them line by line, by using the
`-a' or `--text' option. If the files you compare using this
option do not in fact contain text, they will probably contain few
newline characters, and the diff output will consist of hunks
showing differences between long lines of whatever characters the files
contain.
You can also force diff to consider all files to be binary files,
and report only whether they differ (but not how). Use the
`--brief' option for this.
In operating systems that distinguish between text and binary files,
diff normally reads and writes all data as text. Use the
`--binary' option to force diff to read and write binary
data instead. This option has no effect on a Posix-compliant system
like GNU or traditional Unix. However, many personal computer
operating systems represent the end of a line with a carriage return
followed by a newline. On such systems, diff normally ignores
these carriage returns on input and generates them at the end of each
output line, but with the `--binary' option diff treats
each carriage return as just another input character, and does not
generate a carriage return at the end of each output line. This can be
useful when dealing with non-text files that are meant to be
interchanged with Posix-compliant systems.
If you want to compare two files byte by byte, you can use the
cmp program with the `-l' option to show the values of each
differing byte in the two files. With GNU cmp, you can also use
the `-c' option to show the ASCII representation of those bytes.
See section Invoking cmp, for more information.
If diff3 thinks that any of the files it is comparing is binary
(a non-text file), it normally reports an error, because such
comparisons are usually not useful. diff3 uses the same test as
diff to decide whether a file is binary. As with diff, if
the input files contain a few non-text characters but otherwise are like
text files, you can force diff3 to consider all files to be text
files and compare them line by line by using the `-a' or
`--text' options.
diff Output Formats
diff has several mutually exclusive options for output format.
The following sections describe each format, illustrating how
diff reports the differences between two sample input files.
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'.
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.
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:
Here is the output of the command `diff lao tzu' (see section Two Sample Input Files, 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!
Usually, when you are looking at the differences between files, you will also want to see the parts of the files near the lines that differ, to help you understand exactly what has changed. These nearby parts of the files are called the context.
GNU diff provides two output formats that show context around the
differing lines: context format and unified format. It can
optionally show in which function or section of the file the differing
lines are found.
If you are distributing new versions of files to other people in the
form of diff output, you should use one of the output formats
that show context so that they can apply the diffs even if they have
made small changes of their own to the files. patch can apply
the diffs in this case by searching in the files for the lines of
context around the differing lines; if those lines are actually a few
lines away from where the diff says they are, patch can adjust
the line numbers accordingly and still apply the diff correctly.
See section Applying Imperfect Patches, for more information on using patch to apply
imperfect diffs.
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.
The context output format starts with a two-line header, which looks like this:
*** from-file from-file-modification-time --- to-file to-file-modification time
You can change the header's content with the `-L label' or `--label=label' option; see section Showing Alternate File Names.
Next come one or more hunks of differences; each hunk shows one area where the files differ. Context format hunks look like this:
*************** *** from-file-line-range **** from-file-line from-file-line... --- to-file-line-range ---- to-file-line to-file-line...
The lines of context around the lines that differ start with two space characters. The lines that differ between the two files start with one of the following indicator characters, followed by a space character:
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.
Here is the output of `diff -c lao tzu' (see section Two Sample Input Files, for the complete contents of the two files). Notice that up to three lines that are not different are shown around each line that is different; they are the context lines. Also notice that the first two hunks have run together, because their contents overlap.
*** lao Sat Jan 26 23:30:39 1991
--- tzu Sat Jan 26 23:30:50 1991
***************
*** 1,7 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
--- 1,6 ----
The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
!
Therefore let there always be non-being,
so we may see their subtlety,
And let there always be being,
***************
*** 9,11 ****
--- 8,13 ----
The two are the same,
But after they are produced,
they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!
Here is the output of `diff --context=1 lao tzu' (see section Two Sample Input Files, for the complete contents of the two files). Notice that at most one context line is reported here.
*** lao Sat Jan 26 23:30:39 1991
--- tzu Sat Jan 26 23:30:50 1991
***************
*** 1,5 ****
- The Way that can be told of is not the eternal Way;
- The name that can be named is not the eternal name.
The Nameless is the origin of Heaven and Earth;
! The Named is the mother of all things.
Therefore let there always be non-being,
--- 1,4 ----
The Nameless is the origin of Heaven and Earth;
! The named is the mother of all things.
!
Therefore let there always be non-being,
***************
*** 11 ****
--- 10,13 ----
they have different names.
+ They both may be called deep and profound.
+ Deeper and more profound,
+ The door of all subtleties!
The unified output format is a variation on the context format that is more compact because it omits redundant context lines. To select this output format, use the `-U lines', `--unified[=lines]', or `-u' option. The argument lines is the number of lines of context to show. When it is not given, it defaults to three.
At present, only GNU diff can produce this format and only GNU
patch can automatically apply diffs in this format. For proper
operation, patch typically needs at least two lines of context.
The unified output format starts with a two-line header, which looks like this:
--- from-file from-file-modification-time +++ to-file to-file-modification-time
You can change the header's content with the `-L label' or `--label=label' option; see See section Showing Alternate File Names.
Next come one or more hunks of differences; each hunk shows one area where the files differ. Unified format hunks look like this:
@@ from-file-range to-file-range @@ line-from-either-file line-from-either-file...
The lines common to both files begin with a space character. The lines that actually differ between the two files have one of the following indicator characters in the left column:
Here is the output of the command `diff -u lao tzu' (see section Two Sample Input Files, for the complete contents of the two files):
--- lao Sat Jan 26 23:30:39 1991 +++ tzu Sat Jan 26 23:30:50 1991 @@ -1,7 +1,6 @@ -The Way that can be told of is not the eternal Way; -The name that can be named is not the eternal name. The Nameless is the origin of Heaven and Earth; -The Named is the mother of all things. +The named is the mother of all things. + Therefore let there always be non-being, so we may see their subtlety, And let there always be being, @@ -9,3 +8,6 @@ The two are the same, But after they are produced, they have different names. +They both may be called deep and profound. +Deeper and more profound, +The door of all subtleties!
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.
To show in which sections differences occur for files that are not
source code for C or similar languages, use the `-F regexp'
or `--show-function-line=regexp' option. diff
considers lines that match the argument regexp to be the beginning
of a section of the file. Here are suggested regular expressions for
some common languages:
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' and `--show-function-line' options find the nearest
unchanged line that precedes each hunk of differences and matches the
given regular expression. Then they add that line to the end of the
line of asterisks in the context format, or to the `@@' line in
unified format. If no matching line exists, they leave the output for
that hunk unchanged. If that line is more than 40 characters long, they
output only the first 40 characters. You can specify more than one
regular expression for such lines; diff tries to match each line
against each regular expression, starting with the last one given. This
means that you can use `-p' and `-F' together, if you wish.
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' and `--show-c-function' options are equivalent to
`-F'^[_a-zA-Z$]'' if the unified format is specified, otherwise
`-c -F'^[_a-zA-Z$]'' (see section Showing Lines That Match Regular Expressions). GNU diff
provides them for the sake of convenience.
If you are comparing two files that have meaningless or uninformative
names, you might want diff to show alternate names in the header
of the context and unified output formats. To do this, use the `-L
label' or `--label=label' option. The first time
you give this option, its argument replaces the name and date of the
first file in the header; the second time, its argument replaces the
name and date of the second file. If you give this option more than
twice, diff reports an error. The `-L' option does not
affect the file names in the pr header when the `-l' or
`--paginate' option is used (see section Paginating diff Output).
Here are the first two lines of the output from `diff -C2 -Loriginal -Lmodified lao tzu':
*** original --- modified
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:
Normally, an output line is incomplete if and only if the lines that it contains are incomplete; See section Incomplete Lines. However, when an output line represents two differing lines, one might be incomplete while the other is not. In this case, the output line is complete, but its the gutter is marked `\' if the first line is incomplete, `/' if the second line is.
Side by side format is sometimes easiest to read, but it has limitations. It generates much wider output than usual, and truncates lines that are too long to fit. Also, it relies on lining up output more heavily than usual, so its output looks particularly bad if you use varying width fonts, nonstandard tab stops, or nonprinting characters.
You can use the sdiff command to interactively merge side by side
differences. See section Interactive Merging with sdiff, for more information on merging files.
The `-y' or `--side-by-side' option selects side by side format. Because side by side output lines contain two input lines, they are wider than usual. They are normally 130 columns, which can fit onto a traditional printer line. You can set the length of output lines with the `-W columns' or `--width=columns' option. The output line is split into two halves of equal length, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.
The `--left-column' option prints only the left column of two common lines. The `--suppress-common-lines' option suppresses common lines entirely.
Here is the output of the command `diff -y -W 72 lao tzu' (see section Two Sample Input Files, 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!
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 Showing Differences Without Context), 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'.
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:
ed ScriptHere is the output of `diff -e lao tzu' (see section Two Sample Input Files, 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
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.
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 Scripts), 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 Two Sample Input Files, 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!
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 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.
In a line group format, ordinary characters represent themselves; conversion specifications start with `%' and have one of the following forms.
printf conversion specification and n is one
of the following letters, stands for n's value formatted with F.
printf conversion specification can be `%d',
`%o', `%x', or `%X', specifying decimal, octal,
lower case hexadecimal, or upper case hexadecimal output
respectively. After the `%' the following options can appear in
sequence: a `-' specifying left-justification; an integer
specifying the minimum field width; and a period followed by an
optional integer specifying the minimum number of digits.
For example, `%5dN' prints the number of new lines in the group
in a field of width 5 characters, using the printf format "%5d".
Line formats control how each line taken from an input file is output as part of a line group in if-then-else format.
For example, the following command outputs text with a one-column change indicator to the left of the text. The first column of output is `-' for deleted lines, `|' for added lines, and a space for unchanged lines. The formats contain newline characters where newlines are desired on output.
diff \ --old-line-format='-%l ' \ --new-line-format='|%l ' \ --unchanged-line-format=' %l ' \ old new
To specify a line format, use one of the following options. You should quote format, since it often contains shell metacharacters.
In a line format, ordinary characters represent themselves; conversion specifications start with `%' and have one of the following forms.
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 Line Group Formats, for
more about printf conversion specifications.
The default line format is `%l' followed by a newline character.
If the input contains tab characters and it is important that they line up on output, you should ensure that `%l' or `%L' in a line format is just after a tab stop (e.g. by preceding `%l' or `%L' with a tab character), or you should use the `-t' or `--expand-tabs' option.
Taken together, the line and line group formats let you specify many
different formats. For example, the following command uses a format
similar to diff's normal format. You can tailor this command
to get fine control over diff's output.
diff \ --old-line-format='< %l ' \ --new-line-format='> %l ' \ --old-group-format='%df%(f=l?:,%dl)d%dE %<' \ --new-group-format='%dea%dF%(F=L?:,%dL) %>' \ --changed-group-format='%df%(f=l?:,%dl)c%dF%(F=L?:,%dL) %<--- %>' \ --unchanged-group-format='' \ old new
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 /* not name */ ' \ --new-group-format='#ifdef name %>#endif /* name */ ' \ --unchanged-group-format='%=' \ --changed-group-format='#ifndef name %<#else /* name */ %>#endif /* name */ '
You should carefully check the diff output for proper nesting.
For example, when using the the `-D name' or
`--ifdef=name' option, you should check that if the
differing lines contain any of the C preprocessor directives
`#ifdef', `#ifndef', `#else', `#elif', or
`#endif', they are nested properly and match. If they don't, you
must make corrections manually. It is a good idea to carefully check
the resulting code anyway to make sure that it really does what you
want it to; depending on how the input files were produced, the output
might contain duplicate or otherwise incorrect code.
The patch `-D name' option behaves just like
the diff `-D name' option, except it operates on
a file and a diff to produce a merged file; See section Options to patch.
Here is the output of `diff -DTWO lao tzu' (see section Two Sample Input Files, 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 /* not TWO */ The Nameless is the origin of Heaven and Earth; #ifndef TWO The Named is the mother of all things. #else /* TWO */ The named is the mother of all things. #endif /* TWO */ Therefore let there always be non-being, so we may see their subtlety, And let there always be being, so we may see their outcome. The two are the same, But after they are produced, they have different names. #ifdef TWO They both may be called deep and profound. Deeper and more profound, The door of all subtleties! #endif /* TWO */
You can use diff to compare some or all of the files in two
directory trees. When both file name arguments to diff are
directories, it compares each file that is contained in both
directories, examining file names in alphabetical order. Normally
diff is silent about pairs of files that contain no differences,
but if you use the `-s' or `--report-identical-files' option,
it reports pairs of identical files. Normally diff reports
subdirectories common to both directories without comparing
subdirectories' files, but if you use the `-r' or
`--recursive' option, it compares every corresponding pair of files
in the directory trees, as many levels deep as they go.
For file names that are in only one of the directories, diff
normally does not show the contents of the file that exists; it reports
only that the file exists in that directory and not in the other. You
can make diff act as though the file existed but was empty in the
other directory, so that it outputs the entire contents of the file that
actually exists. (It is output as either an insertion or a
deletion, depending on whether it is in the first or the second
directory given.) To do this, use the `-N' or `--new-file'
option.
If the older directory contains one or more large files that are not in the newer directory, you can make the patch smaller by using the `-P' or `--unidirectional-new-file' option instead of `-N'. This option is like `-N' except that it only inserts the contents of files that appear in the second directory but not the first (that is, files that were added). At the top of the patch, write instructions for the user applying the patch to remove the files that were deleted before applying the patch. See section Tips for Making Patch Distributions, for more discussion of making patches for distribution.
To ignore some files while comparing directories, use the `-x pattern' or `--exclude=pattern' option. This option ignores any files or subdirectories whose base names match the shell pattern pattern. Unlike in the shell, a period at the start of the base of a file name matches a wildcard at the start of a pattern. You should enclose pattern in quotes so that the shell does not expand it. For example, the option `-x '*.[ao]'' ignores any file whose name ends with `.a' or `.o'.
This option accumulates if you specify it more than once. For example, using the options `-x 'RCS' -x '*,v'' ignores any file or subdirectory whose base name is `RCS' or ends with `,v'.
If you need to give this option many times, you can instead put the patterns in a file, one pattern per line, and use the `-X file' or `--exclude-from=file' option.
If you have been comparing two directories and stopped partway through, later you might want to continue where you left off. You can do this by using the `-S file' or `--starting-file=file' option. This compares only the file file and all alphabetically later files in the topmost directory level.
diff Output Prettier
diff provides several ways to adjust the appearance of its output.
These adjustments can be applied to any output format.
The lines of text in some of the diff output formats are preceded
by one or two characters that indicate whether the text is inserted,
deleted, or changed. The addition of those characters can cause tabs to
move to the next tabstop, throwing off the alignment of columns in the
line. GNU diff provides two ways to make tab-aligned columns
line up correctly.
The first way is to have diff convert all tabs into the correct
number of spaces before outputting them; select this method with the
`-t' or `--expand-tabs' option. diff assumes that
tabstops are set every 8 columns. To use this form of output with
patch, you must give patch the `-l' or
`--ignore-white-space' option (see section Applying Patches with Changed White Space, for more
information).
The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line. This ensures that all following tab characters are in the same position relative to tabstops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the `-T' or `--initial-tab' option.
diff Output
It can be convenient to have long output page-numbered and time-stamped.
The `-l' and `--paginate' options do this by sending the
diff output through the pr program. Here is what the page
header might look like for `diff -lc lao tzu':
Mar 11 13:37 1991 diff -lc lao tzu Page 1
diff Performance Tradeoffs
GNU diff runs quite efficiently; however, in some circumstances
you can cause it to run faster or produce a more compact set of changes.
There are two ways that you can affect the performance of GNU
diff by changing the way it compares files.
Performance has more than one dimension. These options improve one aspect of performance at the cost of another, or they improve performance in some cases while hurting it in others.
The way that GNU diff determines which lines have changed always
comes up with a near-minimal set of differences. Usually it is good
enough for practical purposes. If the diff output is large, you
might want diff to use a modified algorithm that sometimes
produces a smaller set of differences. The `-d' or
`--minimal' option does this; however, it can also cause
diff to run more slowly than usual, so it is not the default
behavior.
When the files you are comparing are large and have small groups of
changes scattered throughout them, you can use the `-H' or
`--speed-large-files' option to make a different modification to
the algorithm that diff uses. If the input files have a constant
small density of changes, this option speeds up the comparisons without
changing the output. If not, diff might produce a larger set of
differences; however, the output will still be correct.
Normally diff discards the prefix and suffix that is common to
both files before it attempts to find a minimal set of differences.
This makes diff run faster, but occasionally it may produce
non-minimal output. The `--horizon-lines=lines' option
prevents diff from discarding the last lines lines of the
prefix and the first lines lines of the suffix. This gives
diff further opportunities to find a minimal output.
Use the program diff3 to compare three files and show any
differences among them. (diff3 can also merge files; see
section Merging From a Common Ancestor).
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 Invoking diff3, for more information on how to run diff3.
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 Two Sample Input Files). 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
diff3 Normal FormatEach 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 Preserving Tabstop Alignment, for more information.
Commands take the following forms:
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.
diff3 Normal FormatHere is the output of the command `diff3 lao tzu tao' (see section A Third Sample Input File, 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
When two people have made changes to copies of the same file,
diff3 can produce a merged output that contains both sets of
changes together with warnings about conflicts.
One might imagine programs with names like diff4 and diff5
to compare more than three files simultaneously, but in practice the
need rarely arises. You can use diff3 to merge three or more
sets of changes to a file by merging two change sets at a time.
diff3 can incorporate changes from two modified versions into a
common preceding version. This lets you merge the sets of changes
represented by the two newer files. Specify the common ancestor version
as the second argument and the two newer versions as the first and third
arguments, like this:
diff3 mine older yours
You can remember the order of the arguments by noting that they are in alphabetical order.
You can think of this as subtracting older from yours and adding the result to mine, or as merging into mine the changes that would turn older into yours. This merging is well-defined as long as mine and older match in the neighborhood of each such change. This fails to be true when all three input files differ or when only older differs; we call this a conflict. When all three input files differ, we call the conflict an overlap.
diff3 gives you several ways to handle overlaps and conflicts.
You can omit overlaps or conflicts, or select only overlaps,
or mark conflicts with special `<<<<<<<' and `>>>>>>>' lines.
diff3 can output the merge results as an ed script that
that can be applied to the first file to yield the merged output.
However, it is usually better to have diff3 generate the merged
output directly; this bypasses some problems with ed.
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 A Third Sample Input File, 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 .
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 A Third Sample Input File), `diff3 -A lao tzu tao' puts brackets around the conflict where only `tzu' differs:
<<<<<<< tzu ======= The Way that can be told of is not the eternal Way; The name that can be named is not the eternal name. >>>>>>> tao
And it outputs the three-way conflict as follows:
<<<<<<< lao ||||||| tzu They both may be called deep and profound. Deeper and more profound, The door of all subtleties! ======= -- The Way of Lao-Tzu, tr. Wing-tsit Chan >>>>>>> tao
The `-E' or `--show-overlap' option outputs less information than the `-A' or `--show-all' option, because it outputs only unmerged changes, and it never outputs the contents of the second file. Thus the `-E' option acts like the `-e' option, except that it brackets the first and third files from three-way overlapping changes. Similarly, `-X' acts like `-x', except it brackets all its (necessarily overlapping) changes. For example, for the three-way overlapping change above, the `-E' and `-X' options output the following:
<<<<<<< lao ======= -- The Way of Lao-Tzu, tr. Wing-tsit Chan >>>>>>> tao
If you are comparing files that have meaningless or uninformative names, you can use the `-L label' or `--label=label' option to show alternate names in the `<<<<<<<', `|||||||' and `>>>>>>>' brackets. This option can be given up to three times, once for each input file. Thus `diff3 -A -L X -L Y -L Z A B C' acts like `diff3 -A A B C', except that the output looks like it came from files named `X', `Y' and `Z' rather than from files named `A', `B' and `C'.
With the `-m' or `--merge' option, diff3 outputs the
merged file directly. This is more efficient than using ed to
generate it, and works even with non-text files that ed would
reject. If you specify `-m' without an ed script option,
`-A' (`--show-all') is assumed.
For example, the command `diff3 -m lao tzu tao' (see section A Third Sample Input File 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
diff3 Merges Incomplete LinesWith `-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.
Traditional Unix diff3 generates an ed script without the
trailing `w' and and `q' commands that save the changes.
System V diff3 generates these extra commands. GNU diff3
normally behaves like traditional Unix diff3, but with the
`-i' option it behaves like System V diff3 and appends the
`w' and `q' commands.
The `-i' option requires one of the ed script options
`-AeExX3', and is incompatible with the merged output option
`-m'.
sdiff
With sdiff, you can merge two files interactively based on a
side-by-side `-y' format comparison (see section Showing Differences Side by Side). Use
`-o file' or `--output=file' to specify where to
put the merged text. See section Invoking sdiff, for more details on the
options to sdiff.
Another way to merge files interactively is to use the Emacs Lisp
package emerge. See section `emerge' in The GNU Emacs Manual, for more information.
diff Options to sdiff
The following sdiff options have the same meaning as for
diff. See section Options to diff, for the use of these options.
-a -b -d -i -t -v -B -H -I regexp --ignore-blank-lines --ignore-case --ignore-matching-lines=regexp --ignore-space-change --left-column --minimal --speed-large-files --suppress-common-lines --expand-tabs --text --version --width=columns
For historical reasons, sdiff has alternate names for some
options. The `-l' option is equivalent to the `--left-column'
option, and similarly `-s' is equivalent to
`--suppress-common-lines'. The meaning of the sdiff
`-w' and `-W' options is interchanged from that of
diff: with sdiff, `-w columns' is equivalent to
`--width=columns', and `-W' is equivalent to
`--ignore-all-space'. sdiff without the `-o' option is
equivalent to diff with the `-y' or `--side-by-side'
option (see section Showing Differences Side by Side).
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.
The text editor invoked is specified by the EDITOR environment
variable if it is set. The default is system-dependent.
patch
patch takes comparison output produced by diff and applies
the differences to a copy of the original file, producing a patched
version. With patch, you can distribute just the changes to a
set of files instead of distributing the entire file set; your
correspondents can apply patch to update their copy of the files
with your changes. patch automatically determines the diff
format, skips any leading or trailing headers, and uses the headers to
determine which file to patch. This lets your correspondents feed an
article or message containing a difference listing directly to
patch.
patch detects and warns about common problems like forward
patches. It saves the original version of the files it patches, and
saves any patches that it could not apply. It can also maintain a
patchlevel.h file to ensures that your correspondents apply
diffs in the proper order.
patch accepts a series of diffs in its standard input, usually
separated by headers that specify which file to patch. It applies
diff hunks (see 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, saving the
original file in `f.orig', and putting reject hunks (if any)
into `f.rej'.
See section Invoking patch, for detailed information on the options to
patch. See section Backup File Names, for more information on how
patch names backup files. See section Reject File Names, for more information
on where patch puts reject hunks.
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.
ed script.
patch tries to skip any leading text in the patch file, apply the
diff, and then skip any trailing text. Thus you can feed a news article
or mail message directly to patch, and it should work. If the
entire diff is indented by a constant amount of white space, patch
automatically ignores the indentation.
However, certain other types of imperfect input require user intervention.
Sometimes mailers, editors, or other programs change spaces into tabs,
or vice versa. If this happens to a patch file or an input file, the
files might look the same, but patch will not be able to match
them properly. If this problem occurs, use the `-l' or
`--ignore-white-space' option, which makes patch compare
white space loosely so that any sequence of white space in the patch file
matches any sequence of white space in the input files. Non-white-space
characters must still match exactly. Each line of the context must
still match a line in the input file.
Sometimes people run diff with the new file first instead of
second. This creates a diff that is "reversed". To apply such
patches, give patch the `-R' or `--reverse' option.
patch then attempts to swap each hunk around before applying it.
Rejects come out in the swapped format. The `-R' option does not
work with ed scripts because there is too little information in
them to reconstruct the reverse operation.
Often patch can guess that the patch is reversed. If the first
hunk of a patch fails, patch reverses the hunk to see if it can
apply it that way. If it can, patch asks you if you want to have
the `-R' option set; if it can't, patch continues to apply
the patch normally. This method cannot detect a reversed patch if it is
a normal diff and the first command is an append (which should have been
a delete) since appends always succeed, because a null context matches
anywhere. But most patches add or change lines rather than delete them,
so most reversed normal diffs begin with a delete, which fails, and
patch notices.
If you apply a patch that you have already applied, patch thinks
it is a reversed patch and offers to un-apply the patch. This could be
construed as a feature. If you did this inadvertently and you don't
want to un-apply the patch, just answer `n' to this offer and to
the subsequent "apply anyway" question--or type C-c to kill the
patch process.
patch Find Inexact Matches
For context diffs, and to a lesser extent normal diffs, patch can
detect when the line numbers mentioned in the patch are incorrect, and
it attempts to find the correct place to apply each hunk of the patch.
As a first guess, it takes the line number mentioned in the hunk, plus
or minus any offset used in applying the previous hunk. If that is not
the correct place, patch scans both forward and backward for a
set of lines matching the context given in the hunk.
First patch looks for a place where all lines of the context
match. If it cannot find such a place, and it is reading a context or
unified diff, and the maximum fuzz factor is set to 1 or more, then
patch makes another scan, ignoring the first and last line of
context. If that fails, and the maximum fuzz factor is set to 2 or
more, it makes another scan, ignoring the first two and last two lines
of context are ignored. It continues similarly if the maximum fuzz
factor is larger.
The `-F lines' or `--fuzz=lines' option sets the maximum fuzz factor to lines. This option only applies to context and unified diffs; it ignores up to lines lines while looking for the place to install a hunk. Note that a larger fuzz factor increases the odds of making a faulty patch. The default fuzz factor is 2; it may not be set to more than the number of lines of context in the diff, ordinarily 3.
If patch cannot find a place to install a hunk of the patch, it
writes the hunk out to a reject file (see section Reject File 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.
As it completes each hunk, patch tells you whether the hunk
succeeded or failed, and if it failed, on which line (in the new file)
patch thinks the hunk should go. If this is different from the
line number specified in the diff, it tells you the offset. A single
large offset may indicate that patch installed a hunk in
the wrong place. patch also tells you if it used a fuzz factor
to make the match, in which case you should also be slightly suspicious.
patch cannot tell if the line numbers are off in an ed
script, and can only detect wrong line numbers in a normal diff when it
finds a change or delete command. It may have the same problem with a
context diff using a fuzz factor equal to or greater than the number of
lines of context shown in the diff (typically 3). In these cases, you
should probably look at a context diff between your original and patched
input files to see if the changes make sense. Compiling without errors
is a pretty good indication that the patch worked, but not a guarantee.
patch usually produces the correct results, even when it must
make many guesses. However, the results are guaranteed only when
the patch is applied to an exact copy of the file that the patch was
generated from.
Sometimes when comparing two directories, the first directory contains a
file that the second directory does not. If you give diff the
`-N' or `--new-file' option, it outputs a diff that deletes
the contents of this file. By default, patch leaves an empty
file after applying such a diff. The `-E' or
`--remove-empty-files' option to patch deletes output files
that are empty after applying the diff.
If the patch file contains more than one patch, patch tries to
apply each of them as if they came from separate patch files. This
means that it determines the name of the file to patch for each patch,
and that it examines the leading text before each patch for file names
and prerequisite revision level (see section Tips for Making Patch Distributions, for more on
that topic).
For the second and subsequent patches in the patch file, you can give options and another original file name by separating their argument lists with a `+'. However, the argument list for a second or subsequent patch may not specify a new patch file, since that does not make sense.
For example, to tell patch to strip the first three slashes from
the name of the first patch in the patch file and none from subsequent
patches, and to use `code.c' as the first input file, you can use:
patch -p3 code.c + -p0 < patchfile
The `-S' or `--skip' option ignores the current patch from the patch file, but continue looking for the next patch in the file. Thus, to ignore the first and third patches in the patch file, you can use:
patch -S + + -S + < patch file
patch
patch can produce a variety of messages, especially if it has
trouble decoding its input. In a few situations where it's not sure how
to proceed, patch normally prompts you for more information from
the keyboard. There are options to suppress printing non-fatal messages
and stopping for keyboard input.
The message `Hmm...' indicates that patch is reading text in
the patch file, attempting to determine whether there is a patch in that
text, and if so, what kind of patch it is.
You can inhibit all terminal output from patch, unless an error
occurs, by using the `-s', `--quiet', or `--silent'
option.
There are two ways you can prevent patch from asking you any
questions. The `-f' or `--force' option assumes that you know
what you are doing. It assumes the following:
The `-t' or `--batch' option is similar to `-f', in that it suppresses questions, but it makes somewhat different assumptions:
patch exits with a non-zero status if it creates any reject
files. When applying a set of patches in a loop, you should check the
exit status, so you don't apply a later patch to a partially patched
file.
Here are some things you should keep in mind if you are going to distribute patches for updating a software package.
Make sure you have specified the file names correctly, either in a context diff header or with an `Index:' line. If you are patching files in a subdirectory, be sure to tell the patch user to specify a `-p' or `--strip' option as needed. Take care to not send out reversed patches, since these make people wonder whether they have already applied the patch.
To save people from partially applying a patch before other patches that
should have gone before it, you can make the first patch in the patch
file update a file with a name like `patchlevel.h' or
`version.c', which contains a patch level or version number. If
the input file contains the wrong version number, patch will
complain immediately.
An even clearer way to prevent this problem is to put a `Prereq:'
line before the patch. If the leading text in the patch file contains a
line that starts with `Prereq:', patch takes the next word
from that line (normally a version number) and checks whether the next
input file contains that word, preceded and followed by either
white space or a newline. If not, patch prompts you for
confirmation before proceeding. This makes it difficult to accidentally
apply patches in the wrong order.
Since patch does not handle incomplete lines properly, make sure
that all the source files in your program end with a newline whenever
you release a version.
To create a patch that changes an older version of a package into a
newer version, first make a copy of the older version in a scratch
directory. Typically you do that by unpacking a tar or
shar archive of the older version.
You might be able to reduce the size of the patch by renaming or
removing some files before making the patch. If the older version of
the package contains any files that the newer version does not, or if
any files have been renamed between the two versions, make a list of
rm and mv commands for the user to execute in the old
version directory before applying the patch. Then run those commands
yourself in the scratch directory.
If there are any files that you don't need to include in the patch
because they can easily be rebuilt from other files (for example,
`TAGS' and output from yacc and makeinfo), replace
the versions in the scratch directory with the newer versions, using
rm and ln or cp.
Now you can create the patch. The de-facto standard diff format
for patch distributions is context format with two lines of context,
produced by giving diff the `-C 2' option. Do not use less
than two lines of context, because patch typically needs at
least two lines for proper operation. Give diff the `-P'
option in case the newer version of the package contains any files that
the older one does not. Make sure to specify the scratch directory
first and the newer directory second.
Add to the top of the patch a note telling the user any rm and
mv commands to run before applying the patch. Then you can
remove the scratch directory.
cmp
The cmp command compares two files, and if they differ, tells the
first byte and line number where they differ. Its arguments are as
follows:
cmp options... from-file [to-file]
The file name `-' is always the standard input. cmp also
uses the standard input if one file name is omitted.
An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.
cmp
Below is a summary of all of the options that GNU cmp accepts.
Most options have two equivalent names, one of which is a single letter
preceded by `-', and the other of which is a long name preceded by
`--'. Multiple single letter options (unless they take an
argument) can be combined into a single command line word: `-cl' is
equivalent to `-c -l'.
cmp.
diff
The format for running the diff command is:
diff options... from-file to-file
In the simplest case, diff compares the contents of the two files
from-file and to-file. A file name of `-' stands for
text read from the standard input. As a special case, `diff - -'
compares a copy of standard input to itself.
If from-file is a directory and to-file is not, diff
compares the file in from-file whose file name is that of to-file,
and vice versa. The non-directory file must not be `-'.
If both from-file and to-file are directories,
diff compares corresponding files in both directories, in
alphabetical order; this comparison is not recursive unless the
`-r' or `--recursive' option is given. diff never
compares the actual contents of a directory as if it were a file. The
file that is fully specified may not be standard input, because standard
input is nameless and the notion of "file with the same name" does not
apply.
diff options begin with `-', so normally from-file and
to-file may not begin with `-'. However, `--' as an
argument by itself treats the remaining arguments as file names even if
they begin with `-'.
An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.
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.
patch typically needs at least two lines of context.
patch typically needs at least two lines of
context.
diff slower (sometimes much slower). See section diff Performance Tradeoffs.
ed script. See section ed Scripts.
ed script but has changes
in the order they appear in the file. See section Forward ed Scripts.
ed script but has changes
in the order they appear in the file. See section Forward ed Scripts.
diff Performance Tradeoffs.
diff Performance Tradeoffs.
pr to paginate it. See section Paginating diff Output.
diff slower (sometimes much slower). See section diff Performance Tradeoffs.
pr to paginate it. See section Paginating diff Output.
sdiff. sdiff uses this
option when it runs diff. This option is not intended for users
to use directly.
diff Performance Tradeoffs.
patch typically needs at least two lines of
context.
diff.
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.
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.
ed script that incorporates all the changes from
older to yours into mine. See section Selecting Which Changes to Incorporate.
<<<<<<< mine lines from mine ======= lines from yours >>>>>>> yours
ed script that incorporates all the changes from
older to yours into mine. See section Selecting Which Changes to Incorporate.
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 Saving the Changed File.
diff3 to ed, this
works even for binary files and incomplete lines. `-A' is assumed
if no edit script option is specified. See section Generating the Merged Output Directly.
diff3.
patch
Normally patch is invoked like this:
patch <patchfile
The full format for invoking patch is:
patch options... [origfile [patchfile]] [+ options... [origfile]]...
If you do not specify patchfile, or if patchfile is
`-', patch reads the patch (that is, the diff output)
from the standard input.
You can specify one or more of the original files as orig arguments; each one and options for interpreting it is separated from the others with a `+'. See section Multiple Patches in a File, for more information.
If you do not specify an input file on the command line, patch
tries to figure out from the leading text (any text in the patch
that comes before the diff output) which file to edit. In the
header of a context or unified diff, patch looks in lines
beginning with `***', `---', or `+++'; among those, it
chooses the shortest name of an existing file. Otherwise, if there is
an `Index:' line in the leading text, patch tries to use the
file name from that line. If patch cannot figure out the name of
an existing file from the leading text, it prompts you for the name of
the file to patch.
If the input file does not exist or is read-only, and a suitable RCS or
SCCS file exists, patch attempts to check out or get the file
before proceeding.
By default, patch replaces the original input file with the
patched version, after renaming the original file into a backup file
(see section Backup File Names, for a description of how patch names backup
files). You can also specify where to put the output with the `-o
output-file' or `--output=output-file' option.
The `-d directory' or `--directory=directory'
option to patch makes directory directory the current
directory for interpreting both file names in the patch file, and file
names given as arguments to other options (such as `-B' and
`-o'). For example, while in a news reading program, you can patch
a file in the `/usr/src/emacs' directory directly from the article
containing the patch like this:
| patch -d /usr/src/emacs
Sometimes the file names given in a patch contain leading directories,
but you keep your files in a directory different from the one given in
the patch. In those cases, you can use the
`-p[number]' or `--strip[=number]'
option to set the file name strip count to number. The strip
count tells patch how many slashes, along with the directory
names between them, to strip from the front of file names. `-p'
with no number given is equivalent to `-p0'. By default,
patch strips off all leading directories, leaving just the base file
names, except that when a file name given in the patch is a relative
file name and all of its leading directories already exist, patch does
not strip off the leading directory. (A relative file name is one
that does not start with a slash.)
patch looks for each file (after any slashes have been stripped)
in the current directory, or if you used the `-d directory'
option, in that directory.
For example, suppose the file name in the patch file is `/gnu/src/emacs/etc/NEWS'. Using `-p' or `-p0' gives the entire file name unmodified, `-p1' gives `gnu/src/emacs/etc/NEWS' (no leading slash), `-p4' gives `etc/NEWS', and not specifying `-p' at all gives `NEWS'.
Normally, patch renames an original input file into a backup file
by appending to its name the extension `.orig', or `~' on
systems that do not support long file names. The `-b
backup-suffix' or `--suffix=backup-suffix' option uses
backup-suffix as the backup extension instead.
Alternately, you can specify the extension for backup files with the
SIMPLE_BACKUP_SUFFIX environment variable, which the options
override.
patch can also create numbered backup files the way GNU Emacs
does. With this method, instead of having a single backup of each file,
patch makes a new backup file name each time it patches a file.
For example, the backups of a file named `sink' would be called,
successively, `sink.~1~', `sink.~2~', `sink.~3~', etc.
The `-V backup-style' or
`--version-control=backup-style' option takes as an argument
a method for creating backup file names. You can alternately control
the type of backups that patch makes with the
VERSION_CONTROL environment variable, which the `-V' option
overrides. The value of the VERSION_CONTROL environment variable
and the argument to the `-V' option are like the GNU Emacs
version-control variable (see section Backup File Names, for more information on backup versions in
Emacs). They also recognize synonyms that are more descriptive. The
valid values are listed below; unique abbreviations are acceptable.
Alternately, you can tell patch to prepend a prefix, such as a
directory name, to produce backup file names. The `-B
backup-prefix' or `--prefix=backup-prefix' option makes
backup files by prepending backup-prefix to them. If you use this
option, patch ignores any `-b' option that you give.
If the backup file already exists, patch creates a new backup
file name by changing the first lowercase letter in the last component
of the file name into uppercase. If there are no more lowercase letters
in the name, it removes the first character from the name. It repeats
this process until it comes up with a backup file name that does not
already exist.
If you specify the output file with the `-o' option, that file is the one that is backed up, not the input file.
The names for reject files (files containing patches that patch
could not find a place to apply) are normally the name of the output
file with `.rej' appended (or `#' on systems that do not
support long file names).
Alternatively, you can tell patch to place all of the rejected
patches in a single file. The `-r reject-file' or
`--reject-file=reject-file' option uses reject-file as
the reject file name.
patch
Here is a summary of all of the options that patch accepts.
Older versions of patch do not accept long-named options or the
`-t', `-E', or `-V' options.
Multiple single-letter options that do not take an argument can be combined into a single command line argument (with only one dash). Brackets ([ and ]) indicate that an option takes an optional argument.
patch.
patch Input Format.
patch
patchers.
ed script. See section Selecting the patch Input Format.
patch.
patch Find Inexact Matches.
patch.
patch thinks are reversed or already applied.
See also `-R'. See section Applying Reversed Patches.
patch Find Inexact Matches.
patch recognizes, then exit.
patch Input Format.
patch thinks are reversed or already applied.
See also `-R'. See section Applying Reversed Patches.
patch.
patch.
patch.
patch.
patch.
patch Input Format.
patch.
patch, then exit.
patch
patchers.
sdiff
The sdiff command merges two files and interactively outputs the
results. Its arguments are as follows:
sdiff -o outfile options... from-file to-file
This merges from-file with to-file, with output to outfile.
If from-file is a directory and to-file is not, sdiff
compares the file in from-file whose file name is that of to-file,
and vice versa. from-file and to-file may not both be
directories.
sdiff options begin with `-', so normally from-file
and to-file may not begin with `-'. However, `--' as an
argument by itself treats the remaining arguments as file names even if
they begin with `-'. You may not use `-' as an input file.
An exit status of 0 means no differences were found, 1 means some differences were found, and 2 means trouble.
sdiff without `-o' (or `--output') produces a
side-by-side difference. This usage is obsolete; use `diff
--side-by-side' instead.
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.
sdiff slower (sometimes much slower). See section diff Performance Tradeoffs.
diff Performance Tradeoffs.
sdiff slower (sometimes much slower). See section diff Performance Tradeoffs.
diff Performance Tradeoffs.
sdiff.
diff,
`-w' in sdiff.
diff,
`-W' in sdiff.
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 Suppressing Differences in Blank and Tab Spacing).
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 Scripts) 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 Showing Differences 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 diff 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 .
Here are some ideas for improving GNU diff and patch. The
GNU project has identified some improvements as potential programming
projects for volunteers. You can also help by reporting any bugs that
you find.
If you are a programmer and would like to contribute something to the GNU project, please consider volunteering for one of these projects. If you are seriously contemplating work, please write to `gnu@prep.ai.mit.edu' to coordinate with other volunteers.
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.
diff and patch do not handle some changes to directory
structure. For example, suppose one directory tree contains a directory
named `D' with some subsidiary files, and another contains a file
with the same name `D'. `diff -r' does not output enough
information for patch to transform the the directory subtree into
the file.
There should be a way to specify that a file has been deleted without
having to include its entire contents in the patch file. There should
also be a way to tell patch that a file was renamed, even if
there is no way for diff to generate such information.
These problems can be fixed by extending the diff output format
to represent changes in directory structure, and extending patch
to understand these extensions.
Some files are neither directories nor regular files: they are unusual
files like symbolic links, device special files, named pipes, and
sockets. Currently, diff treats symbolic links like regular files;
it treats other special files like regular files if they are specified
at the top level, but simply reports their presence when comparing
directories. This means that patch cannot represent changes
to such files. For example, if you change which file a symbolic link
points to, diff outputs the difference between the two files,
instead of the change to the symbolic link.
diff should optionally report changes to special files specially,
and patch should be extended to understand these extensions.
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.
GNU diff can analyze files with arbitrarily long lines and files
that end in incomplete lines. However, patch cannot patch such
files. The patch internal limits on line lengths should be
removed, and patch should be extended to parse diff
reports of incomplete lines.
diff operates by reading both files into memory. This method
fails if the files are too large, and diff should have a fallback.
One way to do this is to scan the files sequentially to compute hash codes of the lines and put the lines in equivalence classes based only on hash code. Then compare the files normally. This does produce some false matches.
Then scan the two files sequentially again, checking each match to see whether it is real. When a match is not real, mark both the "matching" lines as changed. Then build an edit script as usual.
The output routines would have to be changed to scan the files sequentially looking for the text to print.
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.
If you think you have found a bug in GNU cmp, diff,
diff3, sdiff, or patch, please report it by
electronic mail to `bug-gnu-utils@prep.ai.mit.edu'. Send as
precise a description of the problem as you can, including sample input
files that produce the bug, if applicable.
Because Larry Wall has not released a new version of patch since
mid 1988 and the GNU version of patch has been changed since
then, please send bug reports for patch by electronic mail to
both `bug-gnu-utils@prep.ai.mit.edu' and
`lwall@netlabs.com'.
Jump to: ! - + - < - a - b - c - d - e - f - h - i - l - m - n - o - p - r - s - t - u - w
cmp invocation
cmp options
patch
diff invocation
diff options
diff sample input
diff3 hunks
diff3 invocation
diff3 options
diff3 sample input
ed script output format
diff output
diff3 output
ed script output format
diff3
ifdef output format
cmp
diff
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patch
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cmp
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patch
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diff and patch
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patch messages and questions
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