In computing, diff is a file comparison utility that outputs the differences between two files, or the changes made to a current file by comparing it to a former version of the same file. Diff displays the changes made per line for text files. Modern implementations also support binary files^[1]. The output is called a diff or a patch since the output can be applied with the Unix program patch. The output of similar file comparison utilities are also called a "diff". Like the use of the word "grep" for describing the act of searching, the word diff is used in jargon as a verb for calculating any difference. For the formal concept of computation, see computation. ... File comparison in computing is the automatic comparing of data between files on a file system. ... A Hexdump of a JPEG image. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®, sometimes also written as or ® with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ... grep is a command line utility that was originally written for use with the Unix operating system. ... For the glossary of hacker slang, see Jargon File. ...

History

The diff utility was developed in the early 1970s on the Unix operating system which was emerging from AT&T Bell Labs in Murray Hill, New Jersey. The final version, first shipped with the 5th Edition of Unix in 1974, was entirely written by Douglas McIlroy. This research was published in a 1976 paper co-written with James W. Hunt who developed an initial prototype of diff.^[2] Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®, sometimes also written as or ® with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... This article is about the current AT&T. For the 1885-2005 company, see American Telephone & Telegraph. ... Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) was the main research and development arm of the United States Bell System. ... Portrait of Douglas McIlroy taken at the NATO conference in Garmisch 1968, courtesy of Brian Randell. ...

McIlroy's work was preceded and influenced by Steve Johnson's comparison program on GECOS and Mike Lesk's proof program. proof also originated on Unix and, like diff, produced line-by-line changes and even used angle-brackets (">" and "<") for presenting line insertions and deletions in the program's output. The heuristics used in these early applications were, however, deemed unreliable. The potential usefulness of a diff tool provoked McIlroy into researching and designing a more robust tool that could be used in a variety of tasks but perform well in the processing and size limitations of the PDP-11's hardware. His approach to the problem resulted from collaboration also with individuals at Bell Labs including Alfred Aho, Elliot Pinson, Jeffrey Ullman, and Harold S. Stone. Stephen Curtis Johnson spent nearly 20 years at Bell Labs and AT&T, where he wrote Yacc, Lint, and the Portable C Compiler. ... GECOS stands for General Electric Comprehensive Operating system. ... Mike Lesk is a computer program writer who created the lex programming tool, a program for Unix computers, with Eric E. Schmidt. ... For other uses, see Heuristic (disambiguation). ... The PDP-11 was a 16-bit minicomputer sold by Digital Equipment Corp. ... Dr. Alfred V. Aho is a computer scientist. ... Jeffrey D. Ullman (born November 22, 1942) is a renowned computer scientist. ...

In the context of Unix, the use of the ed line editor provided diff with the natural ability to create machine-usable "edit scripts". These edit scripts, when saved to a file, can, along with the original file, be reconstituted by ed into the modified file in its entirety. This greatly reduced the secondary storage necessary to maintain multiple versions of a file. McIlroy considered writing a post-processor for diff where a variety of output formats could be designed and implemented, but he found it more frugal and simpler to have diff be responsible for generating the syntax and reverse-order input accepted by the ed command. In 1985, Larry Wall composed a separate utility, patch, that generalized and extended the ability to modify files with diff output. Modes in Emacs also allow for converting the format of patches and even editing patches interactively. The text editor ed was the original standard on the Unix operating system. ... In computer storage, secondary storage, or external memory, is computer memory that is not directly accessible to the central processing unit of a computer, requiring the use of computers input/output channels. ... Larry Wall Larry Wall (born September 27, 1954) is a programmer, linguist, and author, most widely known for his creation of the Perl programming language in 1987. ... patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ... This article is about the text editor. ...

In diff's early years, common uses included comparing changes in the source of software code and markup for technical documents, verifying program debugging output, comparing filesystem listings and analyzing computer assembly code. The output targeted for ed was motivated to provide compression for a sequence of modifications made to a file. The Source Code Control System (SCCS) and its ability to archive revisions emerged in the late 1970s as a consequence of storing edit scripts from diff. Source Code Control System (SCCS) was the first source code revision control system. ...

Project Xanadu is a conceptual predecessor of diff. It was a hypertext project first conceived in 1960 that was to include a version tracking system necessary for its "transpointing windows" feature. The feature subsumed file differences in the expansive term "transclusion", where a document has included in it parts of other documents or revisions. Project Xanadu was founded by Ted Nelson in 1960 as the original hypertext project. ... In computer science, some hypertext systems, including Ted Nelsons Xanadu Project, have the capability for documents to include sections of other documents by reference, called transclusion. ...

Algorithm

The operation of diff is based on solving the Longest Common Subsequence (LCS) problem. The longest common subsequence problem (LCS) is finding a longest sequence which is a subsequence of all sequences in a set of sequences (often just two). ...

In the LCS problem, you have two sequences of items:

 a b c d f g h j q z 

 a b c d e f g i j k r x y z 

and you want to find the longest sequence of items that is present in both original sequences in the same order. That is, you want to find a new sequence which can be obtained from the first sequence by deleting some items, and from the second sequence by deleting other items. You also want this sequence to be as long as possible. In this case it is

 a b c d f g j z 

From the longest common subsequence it's only a small step to get diff-like output:

 e h i q k r x y + - + - + + + + 

Usage

It is invoked from the command line with the names of two files: diff original new. The output of the command represents the changes required to make the original file become the new file.

If original and new are directories, then diff will be run on each file that exists in both directories. An option, -r, will descend any matching subdirectories to compare files between directories.

Any of the examples in the article use the following two files, original and new:

 This part of the document has stayed the same from version to version. It shouldn't be shown if it doesn't change. Otherwise, that would not be helping to compress the size of the changes. This paragraph contains text that is outdated. It will be deleted in the near future. It is important to spell check this dokument. On the other hand, a misspelled word isn't the end of the world. Nothing in the rest of this paragraph needs to be changed. Things can be added after it. 

 This is an important notice! It should therefore be located at the beginning of this document! This part of the document has stayed the same from version to version. It shouldn't be shown if it doesn't change. Otherwise, that would not be helping to compress anything. It is important to spell check this document. On the other hand, a misspelled word isn't the end of the world. Nothing in the rest of this paragraph needs to be changed. Things can be added after it. This paragraph contains important new additions to this document. 

The command diff original new produces the following normal diff output:

 0a1,6 > This is an important > notice! It should > therefore be located at > the beginning of this > document! > 8,14c14 < compress the size of the < changes. < < This paragraph contains < text that is outdated. < It will be deleted in the < near future. --- > compress anything. 17c17 < check this dokument. On --- > check this document. On 24a25,28 > > This paragraph contains > important new additions > to this document. 

In this traditional output format, a stands for added, d for deleted and c for changed. Line numbers of the original file appear before a/d/c and those of the modified file appear after. Angle brackets appear at the beginning of lines that are added, deleted or changed. Addition lines are added to the original file to appear in the new file. Deletion lines are deleted from the original file to be missing in the new file. See parenthesis for an account of the rhetorical concept from which the name of the punctuation mark is derived. ...

By default, lines common to both files are not shown. Lines that have moved will show up as added on their new location and as deleted on their old location.^[3]

Variations

Most diff implementations remain outwardly unchanged since 1975. The modifications include improvements to the core algorithm, the addition of useful features to the command, and the design of new output formats. The basic algorithm is described in the papers An O(ND) Difference Algorithm and its Variations by Eugene W. Myers^[4] and in A File Comparison Program by Webb Miller and Myers.^[5] The algorithm was independently discovered and described in Algorithms for Approximate String Matching, by E. Ukkonen.^[6] The first editions of the diff program were designed for line comparisons of text files expecting the newline character to delimit lines. By the 1980s, support for binary files resulted in a shift in the application's design and implementation. In computing, a newline is a special character or sequence of characters signifying the end of a line of text. ...

Edit script

An edit script can still be generated by modern versions of diff with the -e option. The resulting edit script for this example is as follows:

 24a This paragraph contains important new additions to this document. . 17c check this document. On . 8,14c compress anything. . 0a This is an important notice! It should therefore be located at the beginning of this document! . 

Context format

The Berkeley distribution of Unix made a point of adding the context format (-C) and the ability to recurse on filesystem directory structures (-r), adding those features in 2.8 BSD, released in July 1981. The context format of diff introduced at Berkeley helped with distributing patches for source code that may have been changed minimally. BSD redirects here; for other uses see BSD (disambiguation). ...

In the context format, any changed lines are shown alongside unchanged lines before and after. The inclusion of any number of unchanged lines provides a context to the patch. The context consists of lines that have not changed between the two files, and so can be used as a reference to locate the chunk's place in a modified file and find the intended location a change should be applied regardless if the line numbers no longer correspond. The context format introduces greater readability for humans and reliability when applying the patch and an output which is accepted as input to the patch program. This intelligent behavior isn't possible with the traditional diff output. patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ...

The number of unchanged lines shown above and below a change hunk can be defined by the user, even zero, but three lines is typically the default. If the context of unchanged lines in a hunk overlap with an adjacent hunk, then diff will avoid duplicating the unchanged lines and merge the hunks into a single hunk.

A "!" represents a change between lines that correspond in the two files. A "+" represents the addition of a line, while a blank space represents an unchanged line. At the beginning of the patch is the file information, including the full path and a time stamp. At the beginning of each hunk are the line numbers that apply for the corresponding change in the files. A number range appearing between sets of three asterisks applies to the original file, while sets of three dashes apply to the new file. The chunk ranges specify the starting line number and the number of lines the change hunk applies to in the respective file. A space is a punctuation convention for providing interword separation in some scripts, including the Latin, Greek, Cyrillic, and Arabic. ... Timecode is also the title of a 2000 film directed by Mike Figgis which was shot in one continuous take. ...

The command diff -c original new produces the following output:

 *** /path/to/original timestamp --- /path/to/new timestamp *************** *** 1,3 **** --- 1,9 ---- + This is an important + notice! It should + therefore be located at + the beginning of this + document! + This part of the document has stayed the same from version to *************** *** 5,20 **** be shown if it doesn't change. Otherwise, that would not be helping to ! compress the size of the ! changes. ! ! This paragraph contains ! text that is outdated. ! It will be deleted in the ! near future. It is important to spell ! check this document. On the other hand, a misspelled word isn't the end of the world. --- 11,20 ---- be shown if it doesn't change. Otherwise, that would not be helping to ! compress anything. It is important to spell ! check this document. On the other hand, a misspelled word isn't the end of the world. *************** *** 22,24 **** --- 22,28 ---- this paragraph needs to be changed. Things can be added after it. + + This paragraph contains + important new additions + to this document. 

Unified format

The unified format (or unidiff) inherits the technical improvements made by the context format, but produces a smaller diff with old and new text presented immediately adjacent. Unified format is usually invoked using the "-u" command line option. This output is often used as input to the patch program. Many projects specifically request that "diffs" be submitted in the unified format, making unified diff format the most common format for exchange between software developers. In computer software, specifically command line interfaces, a switch (also known as option, command-line parameter, or command-line argument) is an indication by a user that a computer program should change its default behaviour. ... patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ...

Unified context diffs were originally developed by Wayne Davison in August 1990 (in unidiff which appeared in Volume 14 of comp.sources.misc). Richard Stallman added unified diff support to the GNU Project's diff utility one month later, and the feature debuted in GNU diff 1.15, released in January 1991. GNU diff has since generalized the context format to allow arbitrary formatting of diffs. Wayne Davison is a noted programmer and musician, born on 14 December in the US state of California. ... Richard Matthew Stallman (born March 16, 1953), often abbreviated rms,[2] is an American software freedom activist, hacker,[3] and software developer. ... GNU (pronounced ) is a computer operating system composed entirely of free software. ...

The format starts with the same two-line header as the context format, except that the original file is preceded by "---" and the new file is preceded by "+++". Following this are one or more change hunks (chunks) that contain the line differences in the file. The unchanged, contextual lines are preceded by a space character, addition lines are preceded by a plus sign, and deletion lines are preceded by a minus sign. Look up header in Wiktionary, the free dictionary. ... The plus (+) and minus (−) signs are used universally to represent the operations of addition and subtraction, and have been extended to many other meanings, more or less analogous. ... The plus (+) and minus (−) signs are used universally to represent the operations of addition and subtraction, and have been extended to many other meanings, more or less analogous. ...

A chunk begins with range information and is immediately followed with the line additions, line deletions, and any number of the contextual lines. The range information is surrounded by double-at signs and combines onto a single line what appears on two lines for the context format (see above). The format of the range information line is as follows: @ redirects here. ...

 @@ -R +R @@ 

The chunk range information contains two chunk ranges. The one preceded by a minus symbol is the range for the chunk of the original file, and the range for the new file is preceded by a plus symbol. Each chunk range, R, is of the format l,s where l is the starting line number and s is the number of lines the change hunk applies to for each respective file. In many versions of GNU diff, R can omit the comma and trailing value s, in which case s defaults to 1. Note that the only really interesting value is the l line number of the first range; all the other values can be computed from the diff.

The chunk range for the original should be the sum of all contextual and deletion (including changed) chunk lines. The chunk range for the new file should be a sum of all contextual and addition (including changed) chunk lines. If chunk size information does not correspond with the number of lines in the hunk, then the diff could be considered invalid and be rejected.

If a line is modified, it is represented as a deletion and addition. Since the hunks of the original and new file appear in the same hunk, such changes would appear adjacent to one another.^[7] An occurrence of this in the example below is:

 -check this dokument. On +check this document. On 

The command diff -u original new produces the following output:

 --- original timestamp +++ new timestamp @@ -1,3 +1,9 @@ +This is an important +notice! It should +therefore be located at +the beginning of this +document! + This part of the document has stayed the same from version to @@ -5,16 +11,10 @@ be shown if it doesn't change. Otherwise, that would not be helping to -compress the size of the -changes. - -This paragraph contains -text that is outdated. -It will be deleted in the -near future. +compress anything. It is important to spell -check this dokument. On +check this document. On the other hand, a misspelled word isn't the end of the world. @@ -22,3 +22,7 @@ this paragraph needs to be changed. Things can be added after it. + +This paragraph contains +important new additions +to this document. 

There are some modifications and extensions to the diff formats that are used and understood by certain programs and in certain contexts. For example, some revision control systems (such as Subversion) specify a revision/version number, "working copy", or any other comment instead of a timestamp in the diff's header section. Some tools allow diffs for several different files to be merged into one, using a header for each modified file that may look something like this: In computing, Subversion (SVN) is a version control system (VCS) initiated in 2000 by CollabNet Inc. ...

 Index: path/to/file.cpp =================================================================== 

As a special case, unified diff expects to work with files that end in a newline. If either file does not, unified diff will emit the special line

  No newline at end of file 

after the modifications. The patch program should be aware of this. patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ...

Others

Postprocessors sdiff and diffmk render side-by-side diff listings and applied change marks to printed documents, respectively. Both were developed elsewhere in Bell Labs in or before 1981.

Diff3 compares one file against two other files. It was originally developed by Paul Jensen to reconcile changes made by two persons editing a common source. It is seldom invoked directly and is largely subsumed by the merge program. However, it is used internally by many revision control systems. diff3 is a Unix utility to compare three files and show any differences among them. ... Revision control (also known as version control (system) (VCS), source control or (source) code management (SCM)) is the management of multiple revisions of the same unit of information. ...

GNU diff and diff3 are included in the diffutils package with other diff and patch related utilities. Emacs has Ediff for showing the changes a patch would provide in a user interface that combines interactive editing and merging capabilities for patch files. patch is a Unix program that updates text files according to instructions contained in a separate file, called a patch file. ... This article is about the text editor. ... Ediff is a tool available in Emacs for interactively creating and applying diff files without ever having to see their contents. ...

Wdiff makes it easy to see the words or phrases that changed in a text document, especially in the presence of word-wrapping or different column widths. Spiff goes yet further, ignoring floating point differences under a tunable precision and ignoring irrelevancies in program files such as whitespace and comment formatting. Daisy Diff diffs HTML documents and reconstructs the layout and style information.

See also

• Comparison of file comparison tools
• comm
• cmp
• Delta encoding
• History of software configuration management
• Kompare
• Levenshtein distance
• Longest common subsequence problem
• Microsoft File Compare
• Revision Control System
• rsync
• Software configuration management
• tkdiff
• List of Unix programs
• diff3

// Basic general information about file comparison software. ... The comm command in Unix is a utility that is used to compare two files. ... cmp is a command line utility for computer systems that use a Unix operating system. ... Delta encoding is a way of storing or transmitting data in form of differences between sequential data rather than complete files. ... This article or section does not cite its references or sources. ... Kompare is a free graphical computer application which helps the user to compare two different text files or two directories. ... In information theory and computer science, the Levenshtein distance is a string metric which is one way to measure edit distance. ... The longest common subsequence problem (LCS) is finding a longest sequence which is a subsequence of all sequences in a set of sequences (often just two). ... In MS-DOS 3. ... The Revision Control System (RCS) is a software implementation of revision control that automates the storing, retrieval, logging, identification, and merging of revisions. ... In computing, rsync is a computer program for Unix systems which synchronizes files and directories from one location to another while minimizing data transfer using delta encoding when appropriate. ... Software Configuration Management (SCM) is part of configuration management (CM). ... tkdiff is a graphical diff viewer based on the tk framework. ... This is a list of Unix programs. ... diff3 is a Unix utility to compare three files and show any differences among them. ...

References

• Paul Heckel (April 1978). "A technique for isolating differences between files". Communications of the ACM 21 (4): 264–268. [2]
• A generic implementation of the Myers SES/LCS algorithm with the Hirschberg linear space refinement (C source code)

is the 118th day of the year (119th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Guido van Rossum Guido van Rossum is a Dutch computer programmer who is best known as the author of the Python programming language. ... is the 165th day of the year (166th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ...

External links

• GNU Diff utilities. Made available by the Free Software Foundation. Free Documentation. Free source code.
• DiffUtils for Windows – part of GnuWin32
• Online interface to the diff program
• Algorithm::Diff – A diff library implemented in Perl
• java-diff – A diff library implemented in Java
• JavaScript diff algorithms: jsdiff, XinDiff, google-diff-match-patch
• diff algorithm in C# – Source code of the "An O(ND) Difference Algorithm and its Variations" in C#
• DaisyDiff – HTML differ
• Winmerge - GUI Diff-like tool
• KDIFF3 - Another GUI Diff-like tool
• Adobe Flex Diff - Diff app implemented in Adobe Flex