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## Overview of diff

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 Perhaps one file is a newer version of the other file.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 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 Paul Eggert, Mike Haertel, David Hayes, Richard Stallman, and Len Tower. 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.