computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, file comparison is the calculation and display of the differences and similarities between data objects, typically

text file A text file (sometimes spelled textfile; an old alternative name is flatfile) is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists stored as data within a computer file system. In operat ...

s such as

source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the ...

. The methods, implementations, and results are typically called a diff, after the

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...

diff utility. The output may be presented in a

graphical user interface The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, ins ...

or used as part of larger tasks in

networks Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

s, or

revision control In software engineering, version control (also known as revision control, source control, or source code management) is a class of systems responsible for managing changes to computer programs, documents, large web sites, or other collections o ...

. Some widely used file comparison programs are

diff In computing, the utility diff is a data comparison tool that computes and displays the differences between the contents of files. Unlike edit distance notions used for other purposes, diff is line-oriented rather than character-oriented, but ...

, cmp, FileMerge, WinMerge,

Beyond Compare Beyond Compare is a proprietary data comparison utility. Aside from comparing files, the program is capable of doing side-by-side comparison of directories, FTP and SFTP directories, Dropbox directories, Amazon S3 directories, and archives. It ...

, and

File Compare In computing, fc (File Compare) is a command-line program in DOS, IBM OS/2 and Microsoft Windows operating systems, that compares multiple files and outputs the differences between them. It is similar to the Unix commands comm, cmp and diff. ...

. Many

text editor A text editor is a type of computer program that edits plain text. Such programs are sometimes known as "notepad" software (e.g. Windows Notepad). Text editors are provided with operating systems and software development packages, and can be ...

s and

word processor A word processor (WP) is a device or computer program that provides for input, editing, formatting, and output of text, often with some additional features. Early word processors were stand-alone devices dedicated to the function, but current ...

s perform file comparison to highlight the changes to a file or document.

Method types

Most file comparison tools find the longest common subsequence between two files. Any data not in the longest common subsequence is presented as a change or an insertion or a deletion. In 1978, Paul Heckel published an algorithm that identifies most moved blocks of text. This is used in the IBM History Flow tool. Other file comparison programs find block moves. Some specialized file comparison tools find the

longest increasing subsequence In computer science, the longest increasing subsequence problem is to find a subsequence of a given sequence in which the subsequence's elements are in sorted order, lowest to highest, and in which the subsequence is as long as possible. This subseq ...

between two files. The

rsync rsync is a utility for efficiently transferring and synchronizing files between a computer and a storage drive and across networked computers by comparing the modification times and sizes of files. It is commonly found on Unix-like opera ...

protocol uses a

rolling hash A rolling hash (also known as recursive hashing or rolling checksum) is a hash function where the input is hashed in a window that moves through the input. A few hash functions allow a rolling hash to be computed very quickly—the new hash value ...

function to compare two files on two distant computers with low communication overhead. File comparison in word processors is typically at the word level, while comparison in most programming tools is at the line level. Byte or character-level comparison is useful in some specialized applications.

Display

Display of file comparison varies, with the main approaches being either showing two files side-by-side, or showing a single file, with markup showing the changes from one file to the other. In either case, particularly side-by-side viewing,

code folding Code or text folding, or less commonly holophrasting, is a feature of some graphical user interfaces that allows the user to selectively hide ("fold") or display ("unfold") parts of a document. This allows the user to manage large amounts of tex ...

or text folding may be used to hide unchanged portions of the file, only showing the changed portions.

Reasoning

Comparison tools are used for various reasons. When one wishes to compare binary files, byte-level is probably best. But if one wishes to compare

s or

computer program A computer program is a sequence or set of instructions in a programming language for a computer to Execution (computing), execute. Computer programs are one component of software, which also includes software documentation, documentation and oth ...

s, a side-by-side visual comparison is usually best. This gives the user the chance to decide which file is the preferred one to retain, if the files should be merged to create one containing all of the differences, or perhaps to keep them both as-is for later reference, through some form of "versioning" control. File comparison is an important, and most likely integral, part of

file synchronization File synchronization (or syncing) in computing is the process of ensuring that computer files in two or more locations are updated via certain rules. In ''one-way file synchronization'', also called mirroring, updated files are copied from a sourc ...

and

backup In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", ...

. In backup methodologies, the issue of

data corruption In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...

is an important one. Corruption occurs without warning and without one's knowledge; at least usually until too late to recover the missing parts. Usually, the only way to know for sure if a file has become corrupted is when it is next used or opened. Barring that, one must use a comparison tool to at least recognize that a difference has occurred. Therefore, all file sync or backup programs must include file comparison if these programs are to be actually useful and trusted.

Historical uses

Prior to file comparison, machines existed to compare magnetic tapes or punch cards. The IBM 519 Card Reproducer could determine whether a deck of

punched card A punched card (also punch card or punched-card) is a piece of stiff paper that holds digital data represented by the presence or absence of holes in predefined positions. Punched cards were once common in data processing applications or to di ...

s were equivalent. In 1957, John Van Gardner developed a system to compare the

check sum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

s of loaded sections of Fortran programs to

debug In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve i ...

compilation problems on the

IBM 704 The IBM 704 is a large digital mainframe computer introduced by IBM in 1954. It was the first mass-produced computer with hardware for floating-point arithmetic. The IBM 704 ''Manual of operation'' states: The type 704 Electronic Data-Proce ...

References

External links