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<a name="Standards-conformance"></a>
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<a name="Standards-conformance-1"></a>
<h2 class="chapter">17 Standards conformance</h2>
<a name="index-POSIX-1"></a>
<a name="index-POSIXLY_005fCORRECT-1"></a>
<p>In a few cases, the <acronym>GNU</acronym> utilities’ default behavior is
incompatible with the <acronym>POSIX</acronym> standard. To suppress these
incompatibilities, define the <code>POSIXLY_CORRECT</code> environment
variable. Unless you are checking for <acronym>POSIX</acronym> conformance, you
probably do not need to define <code>POSIXLY_CORRECT</code>.
</p>
<p>Normally options and operands can appear in any order, and programs act
as if all the options appear before any operands. For example,
‘<samp>diff lao tzu -C 2</samp>’ acts like ‘<samp>diff -C 2 lao tzu</samp>’, since
‘<samp>2</samp>’ is an option-argument of <samp>-C</samp>. However, if the
<code>POSIXLY_CORRECT</code> environment variable is set, options must appear
before operands, unless otherwise specified for a particular command.
</p>
<p>Newer versions of <acronym>POSIX</acronym> are occasionally incompatible with older
versions. For example, older versions of <acronym>POSIX</acronym> allowed the
command ‘<samp>diff -c -10</samp>’ to have the same meaning as ‘<samp>diff -C
10</samp>’, but <acronym>POSIX</acronym> 1003.1-2001 ‘<samp>diff</samp>’ no longer allows
digit-string options like <samp>-10</samp>.
</p>
<a name="index-_005fPOSIX2_005fVERSION"></a>
<p>The <acronym>GNU</acronym> utilities normally conform to the version of <acronym>POSIX</acronym>
that is standard for your system. To cause them to conform to a
different version of <acronym>POSIX</acronym>, define the <code>_POSIX2_VERSION</code>
environment variable to a value of the form <var>yyyymm</var> specifying
the year and month the standard was adopted. Two values are currently
supported for <code>_POSIX2_VERSION</code>: ‘<samp>199209</samp>’ stands for
<acronym>POSIX</acronym> 1003.2-1992, and ‘<samp>200112</samp>’ stands for <acronym>POSIX</acronym>
1003.1-2001. For example, if you are running older software that
assumes an older version of <acronym>POSIX</acronym> and uses ‘<samp>diff -c -10</samp>’,
you can work around the compatibility problems by setting
‘<samp>_POSIX2_VERSION=199209</samp>’ in your environment.
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