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    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 Perhaps one file is a newer version of the other file.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
    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 byte and line numbers
 where two files differ.  `cmp' can also show all the bytes that differ
 between the two files, side by side.  A way to compare two files
 character by character is the Emacs command `M-x compare-windows'.
  Other Window (emacs)Other Window, for more information on that
    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 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.
    GNU `diff3' was written by Randy Smith.  GNU `sdiff' was written by
 Thomas Lord.  GNU `cmp' was written by Torbjorn Granlund and David
    `patch' was written mainly by Larry Wall and Paul Eggert; several
 GNU enhancements were contributed by Wayne Davison and David MacKenzie.
 Parts of this manual are adapted from a manual page written by Larry
 Wall, with his permission.
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