Comparing and Merging Files

diff, diff3, sdiff, cmp, and patch

Edition {No value for ‘EDITION’}, for diffutils {No value for ‘VERSION’} and patch 2.1

{No value for ‘UPDATED’}

Copyright © 1992, 1993, 1994 Free Software Foundation, Inc.

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

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

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

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 Perhaps one file is a newer version of the other file.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 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 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.

1 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 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.

1.1 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 ‘--minimal’ option (see section diff Performance Tradeoffs).

1.2 Suppressing Differences in Blank and Tab Spacing

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$

1.3 Suppressing Differences in Blank Lines

The ‘-B’ and ‘--ignore-blank-lines’ options ignore insertions or deletions of blank lines. These options 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

1.4 Suppressing Case Differences

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.

1.5 Suppressing Lines Matching a Regular Expression

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.

1.6 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’ 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.

1.7 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 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.

2 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.

2.1 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’.

2.2 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.

2.2.1 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:

‘lar’

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.

‘fct’

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.

‘rdl’

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.

2.2.2 An Example of Normal Format

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!

2.3 Showing Differences in Their Context

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

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

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

2.3.1 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.

2.3.1.1 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

You can change the header’s content with the ‘-L label’ or ‘--label=label’ option; see 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:

‘!’

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.

2.3.1.2 An Example of Context Format

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!

2.3.1.3 An Example of Context Format with Less Context

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!

2.3.2 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 two lines of context.

2.3.2.1 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

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:

‘+’

A line was added here to the first file.

‘-’

A line was removed here from the first file.

2.3.2.2 An Example of Unified Format

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!

2.3.3 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.

2.3.3.1 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 argument regexp to be the beginning of a section of the file. Here are suggested regular expressions for some common languages:

‘^[A-Za-z_]’

C, C++, Prolog

‘^(’

Lisp

‘^@\(chapter\|appendix\|unnumbered\|chapheading\)’

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’ 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.

2.3.3.2 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’ 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.

2.3.4 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 ‘-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

2.4 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 Suppressing Differences in Blank and Tab Spacing).

‘|’

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

‘<’

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

‘>’

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

‘(’

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

‘)’

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

‘\’

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

‘/’

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

Normally, an output line is incomplete if and only if the lines that it contains are incomplete; See section Incomplete Lines. However, when an output line represents two differing lines, one might be incomplete while the other is not. In this case, the output line is complete, but its the gutter is marked ‘\’ if the first line is incomplete, ‘/’ if the second line is.

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

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

2.5 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, 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.

2.5.1 An Example of Side by Side Format

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!

2.6 Making Edit Scripts

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

2.6.1 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’.

2.6.1.1 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.

2.6.1.2 Example ed Script

Here 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

2.6.2 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.

2.6.3 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 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!

2.7 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.

2.7.1 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 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.

‘Fn’

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 ‘-’ 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".

‘(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 the new file) is 0, to ‘1 line’ if N is 1, and to ‘%dN lines’ otherwise.

2.7.2 Line Formats

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

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

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

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

‘--old-line-format=format’

formats lines just from the first file.

‘--new-line-format=format’

formats lines just from the second file.

‘--unchanged-line-format=format’

formats lines common to both files.

‘--line-format=format’

formats all lines; in effect, it sets all three above options simultaneously.

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

‘%l’

stands for the 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 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.

‘Fn’

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 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

2.7.3 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 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.

2.7.4 An Example of If-then-else Format

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 /* ! 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 */

3 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. 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.

4 Making diff Output Prettier

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

4.1 Preserving Tabstop 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 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.

4.2 Paginating 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

5 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.

6 Comparing Three Files

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

6.1 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 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

6.2 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 Preserving Tabstop Alignment, for more information.

Commands take the following forms:

‘file:la’

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

‘file:rc’

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

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

6.3 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.

6.4 An Example of diff3 Normal Format

Here 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

7 Merging From a Common Ancestor

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

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

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

diff3 mine older yours

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

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

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

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

7.1 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 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
.

7.2 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 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’.

7.3 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’ (‘--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

7.4 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.

7.5 Saving the Changed File

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’.

8 Interactive Merging with 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 emerge in The GNU Emacs Manual, for more information.

8.1 Specifying 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).

8.2 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.

‘el’

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

‘er’

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

‘l’

Copy the left version to the output.

‘q’

Quit.

‘r’

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.

9 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 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.

9.1 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.

9.2 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 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.

9.2.1 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 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.

9.2.2 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. 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.

9.2.3 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; 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.

9.3 Removing Empty Files

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.

9.4 Multiple Patches in a File

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

9.5 Messages and Questions from patch

patch can produce a variety of messages, especially if it has trouble decoding its input. In a few situations where it’s not sure how to proceed, patch normally prompts you for more information from the keyboard. There are options to 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:

• 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 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.

10 Tips for Making Patch Distributions

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.

11 Invoking 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.

11.1 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: ‘-cl’ is equivalent to ‘-c -l’.

‘-c’

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

‘--ignore-initial=bytes’

Ignore any differences in the the first bytes bytes of the input files. Treat files with fewer than bytes bytes as if they are empty.

‘-l’

Print the (decimal) offsets and (octal) values of all differing bytes.

‘--print-chars’

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

‘--quiet’

‘-s’

‘--silent’

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

‘--verbose’

Print the (decimal) offsets and (octal) values of all differing bytes.

‘-v’

‘--version’

Output the version number of cmp.

12 Invoking 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.

12.1 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.

‘-lines’

Show lines (an integer) lines of context. This option does not specify an output format by itself; it has no effect unless it is combined with ‘-c’ (see section Context Format) or ‘-u’ (see section Unified Format). This option is obsolete. For proper operation, patch typically needs at least two lines of context.

‘-a’

Treat all files as text and compare them line-by-line, even if they do not seem to be text. See section Binary Files and Forcing Text Comparisons.

‘-b’

Ignore changes in amount of white space. See section Suppressing Differences in Blank and Tab Spacing.

‘-B’

Ignore changes that just insert or delete blank lines. See section Suppressing Differences in Blank Lines.

‘--binary’

Read and write data in binary mode. See section Binary Files and Forcing Text Comparisons.

‘--brief’

Report only whether the files differ, not the details of the differences. See section Summarizing Which Files Differ.

‘-c’

Use the context output format. 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.

‘--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’

Change the algorithm perhaps find a smaller set of changes. This makes diff slower (sometimes much slower). See section diff Performance Tradeoffs.

‘-D name’

Make merged ‘#ifdef’ format output, conditional on the preprocessor macro name. See section Merging Files with If-then-else.

‘-e’

‘--ed’

Make output that is a valid ed script. See section ed Scripts.

‘--exclude=pattern’

When comparing directories, ignore files and subdirectories whose basenames match pattern. See section Comparing Directories.

‘--exclude-from=file’

When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file. See section Comparing Directories.

‘--expand-tabs’

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See section Preserving Tabstop Alignment.

‘-f’

Make output that looks vaguely like an ed script but has changes in the order they appear in the file. See section Forward ed Scripts.

‘-F regexp’

In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp. See section Showing Lines That Match Regular Expressions.

‘--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 Scripts.

‘-h’

This option currently has no effect; it is present for Unix compatibility.

‘-H’

Use heuristics to speed handling of large files that have numerous scattered small changes. See section diff Performance Tradeoffs.

‘--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 Tradeoffs.

‘-i’

Ignore changes in case; consider upper- and lower-case letters equivalent. See section Suppressing Case Differences.

‘-I regexp’

Ignore changes that just insert or delete lines that match regexp. See section Suppressing Lines Matching a Regular Expression.

‘--ifdef=name’

Make merged if-then-else output using name. See section Merging Files with If-then-else.

‘--ignore-all-space’

Ignore white space when comparing lines. See section Suppressing Differences in Blank and Tab Spacing.

‘--ignore-blank-lines’

Ignore changes that just insert or delete blank lines. See section Suppressing Differences in Blank Lines.

‘--ignore-case’

Ignore changes in case; consider upper- and lower-case to be the same. See section Suppressing Case Differences.

‘--ignore-matching-lines=regexp’

Ignore changes that just insert or delete lines that match regexp. See section Suppressing Lines Matching a Regular Expression.

‘--ignore-space-change’

Ignore changes in amount of white space. See section Suppressing Differences in Blank and Tab Spacing.

‘--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 Preserving Tabstop Alignment.

‘-l’

Pass the output through pr to paginate it. See section Paginating diff Output.

‘-L label’

Use label instead of the file name in the context format (see section Context Format) and unified format (see section Unified Format) headers. See section RCS Scripts.

‘--label=label’

Use label instead of the file name in the context format (see section Context Format) and unified format (see section Unified Format) headers.

‘--left-column’

Print only the left column of two common lines in side by side format. See section Controlling Side by Side Format.

‘--line-format=format’

Use format to output all input lines in if-then-else format. See section Line Formats.

‘--minimal’

Change the algorithm to perhaps find a smaller set of changes. This makes diff slower (sometimes much slower). See section diff Performance Tradeoffs.

‘-n’

Output RCS-format diffs; like ‘-f’ except that each command specifies the number of lines affected. See section RCS Scripts.

‘-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 which C function each change is in. See section Showing C Function Headings.

‘-P’

When comparing directories, if a file appears only in the second directory of the two, treat it as present but empty in the other. See section Comparing Directories.

‘--paginate’

Pass the output through pr to paginate it. See section Paginating diff Output.

‘-q’

Report only whether the files differ, not the details of the differences. See section Summarizing Which Files Differ.

‘-r’

When comparing directories, recursively compare any subdirectories found. See section Comparing Directories.

‘--rcs’

Output RCS-format diffs; like ‘-f’ except that each command specifies the number of lines affected. See section RCS Scripts.

‘--recursive’

When comparing directories, recursively compare any subdirectories found. See section Comparing Directories.

‘--report-identical-files’

Report when two files are the same. See section Comparing Directories.

‘-s’

Report when two files are the same. See section Comparing Directories.

‘-S file’

When comparing directories, start with the file file. This is used for resuming an aborted comparison. See section Comparing Directories.

‘--sdiff-merge-assist’

Print extra information to help sdiff. sdiff uses this option when it runs diff. This option is not intended for users to use directly.

‘--show-c-function’

Show which C function each change is in. See section Showing C Function Headings.

‘--show-function-line=regexp’

In context and unified format, for each hunk of differences, show some of the last preceding line that matches regexp. See section Showing Lines That Match Regular Expressions.

‘--side-by-side’

Use the side by side output format. See section Controlling 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 Tradeoffs.

‘--starting-file=file’

When comparing directories, start with the file file. This is used for resuming an aborted comparison. See section Comparing Directories.

‘--suppress-common-lines’

Do not print common lines in side by side format. See section Controlling Side by Side Format.

‘-t’

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See section Preserving Tabstop Alignment.

‘-T’

Output a tab rather than a space before the text of a line in normal or context format. This causes the alignment of tabs in the line to look normal. See section Preserving Tabstop Alignment.

‘--text’

Treat all files as text and compare them line-by-line, even if they do not appear to be text. See section Binary Files and Forcing Text Comparisons.

‘-u’

Use the unified output format. 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.

‘-v’

‘--version’

Output the version number of diff.

‘-w’

Ignore white space when comparing lines. See section Suppressing Differences in Blank and Tab Spacing.

‘-W columns’

‘--width=columns’

Use an output width of columns in side by side format. See section Controlling Side by Side Format.

‘-x pattern’

When comparing directories, ignore files and subdirectories whose basenames match pattern. See section Comparing Directories.

‘-X file’

When comparing directories, ignore files and subdirectories whose basenames match any pattern contained in file. See section Comparing Directories.

‘-y’

Use the side by side output format. See section Controlling Side by Side Format.

13 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.

13.1 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’

Treat all files as text and compare them line-by-line, even if they do not appear to be text. See section Binary Files and Forcing Text Comparisons.

‘-A’

Incorporate all changes from older to yours into mine, surrounding all conflicts with bracket lines. See section Marking Conflicts.

‘-e’

Generate an ed script that incorporates all the changes from older to yours into mine. See section Selecting Which Changes to Incorporate.

‘-E’

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

‘--ed’

Generate an ed script that incorporates all the changes from older to yours into mine. See section Selecting Which Changes to Incorporate.

‘--easy-only’

Like ‘-e’, except output only the nonoverlapping changes. See section Selecting Which Changes to Incorporate.

‘-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 Saving the Changed File.

‘--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 Preserving Tabstop Alignment.

‘-L label’

‘--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 -L X -L Y -L 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 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 Generating the Merged Output Directly.

‘--overlap-only’

Like ‘-e’, except output only the overlapping changes. See section Selecting Which Changes to Incorporate.

‘--show-all’

Incorporate all unmerged changes from older to yours into mine, surrounding all overlapping changes with bracket lines. See section Marking Conflicts.

‘--show-overlap’

Like ‘-e’, except bracket lines from overlapping changes’ first and third files. See section Marking Conflicts.

‘-T’

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 Preserving Tabstop Alignment.

‘--text’

Treat all files as text and compare them line-by-line, even if they do not appear to be text. See section Binary Files and Forcing Text Comparisons.

‘-v’

‘--version’

Output the version number of diff3.

‘-x’

Like ‘-e’, except output only the overlapping changes. See section Selecting Which Changes to Incorporate.

‘-X’

Like ‘-E’, except output only the overlapping changes. In other words, like ‘-x’, except bracket changes as in ‘-E’. See section Marking Conflicts.

‘-3’

Like ‘-e’, except output only the nonoverlapping changes. See section Selecting Which Changes to Incorporate.

14 Invoking 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.

14.1 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 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’.

14.2 Backup File Names

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 The GNU Emacs Manual, for more information on backup versions in Emacs). They also recognize synonyms that are more descriptive. The valid values are listed below; unique abbreviations are acceptable.

‘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.

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.

14.3 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 ‘#’ 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.

14.4 Options to 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.

‘-b backup-suffix’

Use backup-suffix as the backup extension instead of ‘.orig’ or ‘~’. See section Backup File Names.

‘-B backup-prefix’

Use backup-prefix as a prefix to the backup file name. If this option is specified, any ‘-b’ option is ignored. See section Backup File Names.

‘--batch’

Do not ask any questions. See section Messages and Questions from patch.

‘-c’

‘--context’

Interpret the patch file as a context diff. See section Selecting the patch Input Format.

‘-d directory’

‘--directory=directory’

Makes 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 Applying Patches in Other Directories.

‘-D name’

Make merged if-then-else output using format. See section Merging Files with If-then-else.

‘--debug=number’

Set internal debugging flags. Of interest only to patch patchers.

‘-e’

‘--ed’

Interpret the patch file as an ed script. See section Selecting the patch Input Format.

‘-E’

Remove output files that are empty after the patches have been applied. See section Removing Empty Files.

‘-f’

Assume that the user knows exactly what he or she is doing, and do not ask any questions. See section Messages and Questions from patch.

‘-F lines’

Set the maximum fuzz factor to lines. See section Helping patch Find Inexact Matches.

‘--force’

Assume that the user knows exactly what he or she is doing, and do not ask any questions. See section Messages and Questions from patch.

‘--forward’

Ignore patches that patch thinks are reversed or already applied. See also ‘-R’. See section Applying Reversed Patches.

‘--fuzz=lines’

Set the maximum fuzz factor to lines. See section Helping patch Find Inexact Matches.

‘--help’

Print a summary of the options that patch recognizes, then exit.

‘--ifdef=name’

Make merged if-then-else output using format. See section Merging Files with If-then-else.

‘--ignore-white-space’

‘-l’

Let any sequence of white space in the patch file match any sequence of white space in the input file. See section Applying Patches with Changed White Space.

‘-n’

‘--normal’

Interpret the patch file as a normal diff. See section Selecting the patch Input Format.

‘-N’

Ignore patches that patch thinks are reversed or already applied. See also ‘-R’. See section Applying Reversed Patches.

‘-o output-file’

‘--output=output-file’

Use output-file as the output file name. See section Options to patch.

‘-p[number]’

Set the file name strip count to number. See section Applying Patches in Other Directories.

‘--prefix=backup-prefix’

Use backup-prefix as a prefix to the backup file name. If this option is specified, any ‘-b’ option is ignored. See section Backup File Names.

‘--quiet’

Work silently unless an error occurs. See section Messages and Questions from patch.

‘-r reject-file’

Use reject-file as the reject file name. See section Reject File Names.

‘-R’

Assume that this patch was created with the old and new files swapped. See section Applying Reversed Patches.

‘--reject-file=reject-file’

Use reject-file as the reject file name. See section Reject File Names.

‘--remove-empty-files’

Remove output files that are empty after the patches have been applied. See section Removing Empty Files.

‘--reverse’

Assume that this patch was created with the old and new files swapped. See section Applying Reversed Patches.

‘-s’

Work silently unless an error occurs. See section Messages and Questions from patch.

‘-S’

Ignore this patch from the patch file, but continue looking for the next patch in the file. See section Multiple Patches in a File.

‘--silent’

Work silently unless an error occurs. See section Messages and Questions from patch.

‘--skip’

Ignore this patch from the patch file, but continue looking for the next patch in the file. See section Multiple Patches in a File.

‘--strip[=number]’

Set the file name strip count to number. See section Applying Patches in Other Directories.

‘--suffix=backup-suffix’

Use backup-suffix as the backup extension instead of ‘.orig’ or ‘~’. See section Backup File Names.

‘-t’

Do not ask any questions. See section Messages and Questions from patch.

‘-u’

‘--unified’

Interpret the patch file as a unified diff. See section Selecting the patch Input Format.

‘-v’

Output the revision header and patch level of patch.

‘-V backup-style’

Select the kind of backups to make. See section Backup File Names.

‘--version’

Output the revision header and patch level of patch, then exit.

‘--version=control=backup-style’

Select the kind of backups to make. See section Backup File Names.

‘-x number’

Set internal debugging flags. Of interest only to patch patchers.

15 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.

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.

15.1 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’

Treat all files as text and compare them line-by-line, even if they do not appear to be text. See section Binary Files and Forcing Text Comparisons.

‘-b’

Ignore changes in amount of white space. See section Suppressing Differences in Blank and Tab Spacing.

‘-B’

Ignore changes that just insert or delete blank lines. See section Suppressing Differences in Blank Lines.

‘-d’

Change the algorithm to perhaps find a smaller set of changes. This makes sdiff slower (sometimes much slower). See section diff Performance Tradeoffs.

‘-H’

Use heuristics to speed handling of large files that have numerous scattered small changes. See section diff Performance Tradeoffs.

‘--expand-tabs’

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See section Preserving Tabstop Alignment.

‘-i’

Ignore changes in case; consider upper- and lower-case to be the same. See section Suppressing Case Differences.

‘-I regexp’

Ignore changes that just insert or delete lines that match regexp. See section Suppressing Lines Matching a Regular Expression.

‘--ignore-all-space’

Ignore white space when comparing lines. See section Suppressing Differences in Blank and Tab Spacing.

‘--ignore-blank-lines’

Ignore changes that just insert or delete blank lines. See section Suppressing Differences in Blank Lines.

‘--ignore-case’

Ignore changes in case; consider upper- and lower-case to be the same. See section Suppressing Case Differences.

‘--ignore-matching-lines=regexp’

Ignore changes that just insert or delete lines that match regexp. See section Suppressing Lines Matching a Regular Expression.

‘--ignore-space-change’

Ignore changes in amount of white space. See section Suppressing Differences in Blank and Tab Spacing.

‘-l’

‘--left-column’

Print only the left column of two common lines. See section Controlling Side by Side Format.

‘--minimal’

Change the algorithm to perhaps find a smaller set of changes. This makes sdiff slower (sometimes much slower). See section diff Performance Tradeoffs.

‘-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 Controlling 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 Tradeoffs.

‘-t’

Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files. See section Preserving Tabstop Alignment.

‘--text’

Treat all files as text and compare them line-by-line, even if they do not appear to be text. See section Binary Files and Forcing Text Comparisons.

‘-v’

‘--version’

Output the version number of sdiff.

‘-w columns’

‘--width=columns’

Use an output width of columns. See section Controlling Side by Side Format. Note that for historical reasons, this option is ‘-W’ in diff, ‘-w’ in sdiff.

‘-W’

Ignore horizontal white space when comparing lines. See section Suppressing Differences in Blank and Tab Spacing. Note that for historical reasons, this option is ‘-w’ in diff, ‘-W’ in sdiff.

16 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 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
.

17 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 ‘gnu@prep.ai.mit.edu’ to coordinate with other volunteers.

17.1 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.

17.1.1 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 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.

17.1.2 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 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.

17.1.3 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.

17.1.4 Arbitrary Limits

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.

17.1.5 Handling Files that Do Not Fit in Memory

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.

17.1.6 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.

17.2 Reporting Bugs

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’.

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