Diff utility

In computing, the diff utility is a data comparison tool that calculates and displays the differences between two files. Unlike edit distance notions used for other purposes, diff is line-oriented rather than character-oriented, but it is like Levenshtein distance in that it tries to determine the smallest set of deletions and insertions to create one file from the other. The diff command displays the changes made in a standard format, such that both humans and machines can understand the changes and apply them: given one file and the changes, the other file can be created.

Typically, diff is used to show the changes between two versions of the same file. Modern implementations also support binary files. 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 became a generic term for calculating data difference and the results thereof.

The diff utility was developed in the early 1970s on the Unix operating system which was emerging from 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. The algorithm this paper described became known as the Hunt–McIlroy algorithm.

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.

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