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<a name="Overview-1"></a>
<h2 class="unnumbered">Overview</h2>
<a name="index-overview-of-diff-and-patch"></a>

<p>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one file is a newer version of the other file.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.
</p>
<p>You can use the <code>diff</code> command to show differences between two
files, or each corresponding file in two directories.  <code>diff</code>
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 <em>diff</em> or <em>patch</em>.  For files that are identical,
<code>diff</code> normally produces no output; for binary (non-text) files,
<code>diff</code> normally reports only that they are different.
</p>
<p>You can use the <code>cmp</code> command to show the byte and line numbers
where two files differ.  <code>cmp</code> 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 <kbd>M-x
compare-windows</kbd>.  See <a href="http://www.gnu.org/software/emacs/manual/html_node/emacs/Other-Window.html#Other-Window">Other Window</a> in <cite>The <acronym>GNU</acronym>
Emacs Manual</cite>, for more information on that command.
</p>
<p>You can use the <code>diff3</code> command to show differences among three
files.  When two people have made independent changes to a common
original, <code>diff3</code> can report the differences between the original
and the two changed versions, and can produce a merged file that
contains both persons&rsquo; changes together with warnings about conflicts.
</p>
<p>You can use the <code>sdiff</code> command to merge two files interactively.
</p>
<p>You can use the set of differences produced by <code>diff</code> 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 <code>diff</code> output, you can use the
<code>patch</code> program to update, or <em>patch</em>, a copy of the file.  If you
think of <code>diff</code> as subtracting one file from another to produce
their difference, you can think of <code>patch</code> as adding the difference
to one file to reproduce the other.
</p>
<p>This manual first concentrates on making diffs, and later shows how to
use diffs to update files.
</p>
<p><acronym>GNU</acronym> <code>diff</code> 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
by Eugene W. Myers in &ldquo;An O(ND) Difference Algorithm and its Variations&rdquo;,
<cite>Algorithmica</cite> Vol. 1, 1986, pp. 251&ndash;266,
<a href="http://dx.doi.org/10.1007/BF01840446">http://dx.doi.org/10.1007/BF01840446</a>; and in &ldquo;A File
Comparison Program&rdquo;, Webb Miller and Eugene W. Myers,
<cite>Software&mdash;Practice and Experience</cite> Vol. 15, 1985,
pp. 1025&ndash;1040,
<a href="http://dx.doi.org/10.1002/spe.4380151102">http://dx.doi.org/10.1002/spe.4380151102</a>.
The algorithm was independently discovered as described by Esko Ukkonen in
&ldquo;Algorithms for Approximate String Matching&rdquo;,
<cite>Information and Control</cite> Vol. 64, 1985, pp. 100&ndash;118,
<a href="http://dx.doi.org/10.1016/S0019-9958(85)80046-2">http://dx.doi.org/10.1016/S0019-9958(85)80046-2</a>.
Related algorithms are surveyed by Alfred V. Aho in
section 6.3 of &ldquo;Algorithms for Finding Patterns in Strings&rdquo;,
<cite>Handbook of Theoretical Computer Science</cite> (Jan Van Leeuwen,
ed.), Vol. A, <cite>Algorithms and Complexity</cite>, Elsevier/MIT Press,
1990, pp. 255&ndash;300.
</p>
<p><acronym>GNU</acronym> <code>diff3</code> was written by Randy Smith.  <acronym>GNU</acronym>
<code>sdiff</code> was written by Thomas Lord.  <acronym>GNU</acronym> <code>cmp</code>
was written by Torbj&ouml;rn Granlund and David MacKenzie.
</p>
<p><acronym>GNU</acronym> <code>patch</code> was written mainly by Larry Wall and Paul Eggert;
several <acronym>GNU</acronym> 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.
</p>
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