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<a name="Apply-Code"></a>
<h3 class="subsection">3.3.1 Apply Code</h3>

<p>The &quot;apply code&quot; selected determines the method of evaluating the
expression.  There are five apply codes, including the non-use
of an apply code.
</p>
<dl compact="compact">
<dt> &lsquo;<samp>no apply code</samp>&rsquo;</dt>
<dd><p>This is the most common expression type.
Expressions of this sort come in three flavors:
</p>
<dl compact="compact">
<dt> &lsquo;<samp>&lt;value-name&gt;</samp>&rsquo;</dt>
<dd><p>The result is the value of <var>value-name</var>, if defined.
Otherwise it is the empty string.
</p>
</dd>
<dt> &lsquo;<samp>&lt;basic-expr&gt;</samp>&rsquo;</dt>
<dd><p>The result of the basic expression is the result of the full expression,
See section <a href="basic-expression.html#basic-expression">Basic Expression</a>.
</p>
</dd>
<dt> &lsquo;<samp>&lt;value-name&gt; &lt;basic-expr&gt;</samp>&rsquo;</dt>
<dd><p>If there is a defined value for <var>value-name</var>, then the <var>basic-expr</var>
is evaluated.  Otherwise, the result is the empty string.
</p></dd>
</dl>

</dd>
<dt> &lsquo;<samp>% &lt;value-name&gt; &lt;basic-expr&gt;</samp>&rsquo;</dt>
<dd><p>If <var>value-name</var> is defined, use <var>basic-expr</var> as a format
string for sprintf.  Then, if the <var>basic-expr</var> is either a back-quoted
string or a parenthesized expression, then hand the result to the
appropriate interpreter for further evaluation.  Otherwise, for single
and double quote strings, the result is the result of the sprintf operation.
Naturally, if <var>value-name</var> is not defined, the result is the empty
string.
</p>
<p>For example, assume that &lsquo;<samp>fumble</samp>&rsquo; had the string value, &lsquo;<samp>stumble</samp>&rsquo;:
</p><table><tr><td>&nbsp;</td><td><pre class="example">[+ % fumble `printf '%%x\\n' $%s` +]
</pre></td></tr></table>
<p>This would cause the shell to evaluate &quot;&lsquo;<samp>printf '%x\n' $stumble</samp>&rsquo;&quot;.
Assuming that the shell variable &lsquo;<samp>stumble</samp>&rsquo; had a numeric value,
the expression result would be that number, in hex.  Note the need
for doubled percent characters and backslashes.
</p>
</dd>
<dt> &lsquo;<samp>? &lt;value-name&gt; &lt;basic-expr-1&gt; &lt;basic-expr-2&gt;</samp>&rsquo;</dt>
<dd><p>Two <var>basic-expr</var>-s are required.  If the <var>value-name</var> is
defined, then the first <var>basic-expr-1</var> is evaluated, otherwise
<var>basic-expr-2</var> is.
</p>
</dd>
<dt> &lsquo;<samp>- &lt;value-name&gt; &lt;basic-expr&gt;</samp>&rsquo;</dt>
<dd><p>Evaluate <var>basic-expr</var> only if <var>value-name</var> is <i>not</i> defined.
</p>
</dd>
<dt> &lsquo;<samp>?% &lt;value-name&gt; &lt;basic-expr-1&gt; &lt;basic-expr-2&gt;</samp>&rsquo;</dt>
<dd><p>This combines the functions of &lsquo;<samp>?</samp>&rsquo; and &lsquo;<samp>%</samp>&rsquo;.  If <var>value-name</var> is
defined, it behaves exactly like &lsquo;<samp>%</samp>&rsquo;, above, using <var>basic-expr-1</var>.
If not defined, then <var>basic-expr-2</var> is evaluated.
</p>
<p>For example, assume again that &lsquo;<samp>fumble</samp>&rsquo; had the string value,
&lsquo;<samp>stumble</samp>&rsquo;:
</p><table><tr><td>&nbsp;</td><td><pre class="example">[+ ?% fumble `cat $%s` `pwd` +]
</pre></td></tr></table>
<p>This would cause the shell to evaluate &quot;&lsquo;<samp>cat $stumble</samp>&rsquo;&quot;.
If &lsquo;<samp>fumble</samp>&rsquo; were not defined, then the result would be the name
of our current directory.
</p></dd>
</dl>

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