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<a name="Invoking-cmp-1"></a>
<h1 class="chapter">12. Invoking <code>cmp</code></h1>
<a name="index-invoking-cmp"></a>
<a name="index-cmp-invocation"></a>
<p>The <code>cmp</code> command compares two files, and if they differ,
tells the first byte and line number where they differ or reports
that one file is a prefix of the other. Bytes and
lines are numbered starting with 1. The arguments of <code>cmp</code>
are as follows:
</p>
<table><tr><td> </td><td><pre class="example">cmp <var>options</var>… <var>from-file</var> <span class="roman">[</span><var>to-file</var> <span class="roman">[</span><var>from-skip</var> <span class="roman">[</span><var>to-skip</var><span class="roman">]</span><span class="roman">]</span><span class="roman">]</span>
</pre></td></tr></table>
<p>The file name ‘<tt>-</tt>’ is always the standard input. <code>cmp</code>
also uses the standard input if one file name is omitted. The
<var>from-skip</var> and <var>to-skip</var> operands specify how many bytes to
ignore at the start of each file; they are equivalent to the
‘<samp>--ignore-initial=<var>from-skip</var>:<var>to-skip</var></samp>’ option.
</p>
<p>By default, <code>cmp</code> outputs nothing if the two files have the
same contents. If one file is a prefix of the other, <code>cmp</code>
prints to standard error a message of the following form:
</p>
<table><tr><td> </td><td><pre class="example">cmp: EOF on <var>shorter-file</var>
</pre></td></tr></table>
<p>Otherwise, <code>cmp</code> prints to standard output a message of the
following form:
</p>
<table><tr><td> </td><td><pre class="example"><var>from-file</var> <var>to-file</var> differ: char <var>byte-number</var>, line <var>line-number</var>
</pre></td></tr></table>
<p>The message formats can differ outside the <acronym>POSIX</acronym> locale.
Also, <acronym>POSIX</acronym> allows the <acronym>EOF</acronym> message to be followed
by a blank and some additional information.
</p>
<p>An exit status of 0 means no differences were found, 1 means some
differences were found, and 2 means trouble.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#cmp-Options">12.1 Options to <code>cmp</code></a></td><td> </td><td align="left" valign="top"> Summary of options to <code>cmp</code>.
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<a name="Options-to-cmp"></a>
<h2 class="section">12.1 Options to <code>cmp</code></h2>
<a name="index-cmp-options"></a>
<a name="index-options-for-cmp"></a>
<p>Below is a summary of all of the options that <acronym>GNU</acronym>
<code>cmp</code> accepts. Most options have two equivalent names, one of
which is a single letter preceded by ‘<samp>-</samp>’, and the other of which
is a long name preceded by ‘<samp>--</samp>’. Multiple single letter options
(unless they take an argument) can be combined into a single command
line word: ‘<samp>-bl</samp>’ is equivalent to ‘<samp>-b -l</samp>’.
</p>
<dl compact="compact">
<dt> ‘<samp>-b</samp>’</dt>
<dt> ‘<samp>--print-bytes</samp>’</dt>
<dd><p>Print the differing bytes. Display control bytes as a
‘<samp>^</samp>’ followed by a letter of the alphabet and precede bytes
that have the high bit set with ‘<samp>M-</samp>’ (which stands for “meta”).
</p>
</dd>
<dt> ‘<samp>--help</samp>’</dt>
<dd><p>Output a summary of usage and then exit.
</p>
</dd>
<dt> ‘<samp>-i <var>skip</var></samp>’</dt>
<dt> ‘<samp>--ignore-initial=<var>skip</var></samp>’</dt>
<dd><p>Ignore any differences in the first <var>skip</var> bytes of the input
files. Treat files with fewer than <var>skip</var> bytes as if they are
empty. If <var>skip</var> is of the form
‘<samp><var>from-skip</var>:<var>to-skip</var></samp>’, skip the first <var>from-skip</var>
bytes of the first input file and the first <var>to-skip</var> bytes of the
second.
</p>
</dd>
<dt> ‘<samp>-l</samp>’</dt>
<dt> ‘<samp>--verbose</samp>’</dt>
<dd><p>Output the (decimal) byte numbers and (octal) values of all differing bytes,
instead of the default standard output.
Also, output the <acronym>EOF</acronym> message if one file is shorter than the other.
</p>
</dd>
<dt> ‘<samp>-n <var>count</var></samp>’</dt>
<dt> ‘<samp>--bytes=<var>count</var></samp>’</dt>
<dd><p>Compare at most <var>count</var> input bytes.
</p>
</dd>
<dt> ‘<samp>-s</samp>’</dt>
<dt> ‘<samp>--quiet</samp>’</dt>
<dt> ‘<samp>--silent</samp>’</dt>
<dd><p>Do not print anything; only return an exit status indicating whether
the files differ.
</p>
</dd>
<dt> ‘<samp>-v</samp>’</dt>
<dt> ‘<samp>--version</samp>’</dt>
<dd><p>Output version information and then exit.
</p></dd>
</dl>
<p>In the above table, operands that are byte counts are normally
decimal, but may be preceded by ‘<samp>0</samp>’ for octal and ‘<samp>0x</samp>’ for
hexadecimal.
</p>
<p>A byte count can be followed by a suffix to specify a multiple of that
count; in this case an omitted integer is understood to be 1. A bare
size letter, or one followed by ‘<samp>iB</samp>’, specifies a multiple using
powers of 1024. A size letter followed by ‘<samp>B</samp>’ specifies powers
of 1000 instead. For example, ‘<samp>-n 4M</samp>’ and ‘<samp>-n 4MiB</samp>’ are
equivalent to ‘<samp>-n 4194304</samp>’, whereas ‘<samp>-n 4MB</samp>’ is
equivalent to ‘<samp>-n 4000000</samp>’. This notation is upward compatible
with the <a href="http://www.bipm.fr/enus/3_SI/si-prefixes.html">SI prefixes</a> for decimal multiples and with the
<a href="http://physics.nist.gov/cuu/Units/binary.html">IEC 60027-2 prefixes for binary multiples</a>.
</p>
<p>The following suffixes are defined. Large sizes like <code>1Y</code> may be
rejected by your computer due to limitations of its arithmetic.
</p>
<dl compact="compact">
<dt> ‘<samp>kB</samp>’</dt>
<dd><a name="index-kilobyte_002c-definition-of"></a>
<p>kilobyte: <em>10^3 = 1000</em>.
</p></dd>
<dt> ‘<samp>k</samp>’</dt>
<dt> ‘<samp>K</samp>’</dt>
<dt> ‘<samp>KiB</samp>’</dt>
<dd><a name="index-kibibyte_002c-definition-of"></a>
<p>kibibyte: <em>2^10 = 1024</em>. ‘<samp>K</samp>’ is special: the SI prefix is
‘<samp>k</samp>’ and the IEC 60027-2 prefix is ‘<samp>Ki</samp>’, but tradition and
<acronym>POSIX</acronym> use ‘<samp>k</samp>’ to mean ‘<samp>KiB</samp>’.
</p></dd>
<dt> ‘<samp>MB</samp>’</dt>
<dd><a name="index-megabyte_002c-definition-of"></a>
<p>megabyte: <em>10^6 = 1,000,000</em>.
</p></dd>
<dt> ‘<samp>M</samp>’</dt>
<dt> ‘<samp>MiB</samp>’</dt>
<dd><a name="index-mebibyte_002c-definition-of"></a>
<p>mebibyte: <em>2^20 = 1,048,576</em>.
</p></dd>
<dt> ‘<samp>GB</samp>’</dt>
<dd><a name="index-gigabyte_002c-definition-of"></a>
<p>gigabyte: <em>10^9 = 1,000,000,000</em>.
</p></dd>
<dt> ‘<samp>G</samp>’</dt>
<dt> ‘<samp>GiB</samp>’</dt>
<dd><a name="index-gibibyte_002c-definition-of"></a>
<p>gibibyte: <em>2^30 = 1,073,741,824</em>.
</p></dd>
<dt> ‘<samp>TB</samp>’</dt>
<dd><a name="index-terabyte_002c-definition-of"></a>
<p>terabyte: <em>10^12 = 1,000,000,000,000</em>.
</p></dd>
<dt> ‘<samp>T</samp>’</dt>
<dt> ‘<samp>TiB</samp>’</dt>
<dd><a name="index-tebibyte_002c-definition-of"></a>
<p>tebibyte: <em>2^40 = 1,099,511,627,776</em>.
</p></dd>
<dt> ‘<samp>PB</samp>’</dt>
<dd><a name="index-petabyte_002c-definition-of"></a>
<p>petabyte: <em>10^15 = 1,000,000,000,000,000</em>.
</p></dd>
<dt> ‘<samp>P</samp>’</dt>
<dt> ‘<samp>PiB</samp>’</dt>
<dd><a name="index-pebibyte_002c-definition-of"></a>
<p>pebibyte: <em>2^50 = 1,125,899,906,842,624</em>.
</p></dd>
<dt> ‘<samp>EB</samp>’</dt>
<dd><a name="index-exabyte_002c-definition-of"></a>
<p>exabyte: <em>10^18 = 1,000,000,000,000,000,000</em>.
</p></dd>
<dt> ‘<samp>E</samp>’</dt>
<dt> ‘<samp>EiB</samp>’</dt>
<dd><a name="index-exbibyte_002c-definition-of"></a>
<p>exbibyte: <em>2^60 = 1,152,921,504,606,846,976</em>.
</p></dd>
<dt> ‘<samp>ZB</samp>’</dt>
<dd><a name="index-zettabyte_002c-definition-of"></a>
<p>zettabyte: <em>10^21 = 1,000,000,000,000,000,000,000</em>
</p></dd>
<dt> ‘<samp>Z</samp>’</dt>
<dt> ‘<samp>ZiB</samp>’</dt>
<dd><p><em>2^70 = 1,180,591,620,717,411,303,424</em>.
(‘<samp>Zi</samp>’ is a GNU extension to IEC 60027-2.)
</p></dd>
<dt> ‘<samp>YB</samp>’</dt>
<dd><a name="index-yottabyte_002c-definition-of"></a>
<p>yottabyte: <em>10^24 = 1,000,000,000,000,000,000,000,000</em>.
</p></dd>
<dt> ‘<samp>Y</samp>’</dt>
<dt> ‘<samp>YiB</samp>’</dt>
<dd><p><em>2^80 = 1,208,925,819,614,629,174,706,176</em>.
(‘<samp>Yi</samp>’ is a GNU extension to IEC 60027-2.)
</p></dd>
</dl>
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