expr.html

IEEE 1003.1-2003, Single Unix Specification v3

<p class="tent">Matching expression; see below.</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">(&nbsp;<i>expr</i>&nbsp;)</p>
</td>
<td align="left">
<p class="tent">Grouping symbols. Any expression can be placed within parentheses. Parentheses can be nested to a depth of
{EXPR_NEST_MAX}.</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><i>integer</i></p>
</td>
<td align="left">
<p class="tent">An argument consisting only of an (optional) unary minus followed by digits.</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><i>string</i></p>
</td>
<td align="left">
<p class="tent">A string argument; see below.</p>
</td>
</tr>
</table>
</center>

<h5><a name="tag_04_50_13_01"></a>Matching Expression</h5>

<p>The <tt>':'</tt> matching operator shall compare the string resulting from the evaluation of <i>expr1</i> with the regular
expression pattern resulting from the evaluation of <i>expr2</i>. Regular expression syntax shall be that defined in the Base
Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../basedefs/xbd_chap09.html#tag_09_03">Section 9.3, Basic Regular
Expressions</a>, except that all patterns are anchored to the beginning of the string (that is, only sequences starting at the
first character of a string are matched by the regular expression) and, therefore, it is unspecified whether <tt>'^'</tt> is a
special character in that context. Usually, the matching operator shall return a string representing the number of characters
matched ( <tt>'0'</tt> on failure). Alternatively, if the pattern contains at least one regular expression subexpression
<tt>"[\(...\)]"</tt> , the string corresponding to <tt>"\1"</tt> shall be returned.</p>

<h5><a name="tag_04_50_13_02"></a>String Operand</h5>

<p>A string argument is an argument that cannot be identified as an <i>integer</i> argument or as one of the expression operator
symbols shown in the OPERANDS section.</p>

<p>The use of string arguments <b>length</b>, <b>substr</b>, <b>index</b>, or <b>match</b> produces unspecified results.</p>
</blockquote>

<h4><a name="tag_04_50_14"></a>EXIT STATUS</h4>

<blockquote>
<p>The following exit values shall be returned:</p>

<dl compact>
<dt>&nbsp;0</dt>

<dd>The <i>expression</i> evaluates to neither null nor zero.</dd>

<dt>&nbsp;1</dt>

<dd>The <i>expression</i> evaluates to null or zero.</dd>

<dt>&nbsp;2</dt>

<dd>Invalid <i>expression</i>.</dd>

<dt>&gt;2</dt>

<dd>An error occurred.</dd>
</dl>
</blockquote>

<h4><a name="tag_04_50_15"></a>CONSEQUENCES OF ERRORS</h4>

<blockquote>
<p>Default.</p>
</blockquote>

<hr>
<div class="box"><em>The following sections are informative.</em></div>

<h4><a name="tag_04_50_16"></a>APPLICATION USAGE</h4>

<blockquote>
<p>After argument processing by the shell, <i>expr</i> is not required to be able to tell the difference between an operator and an
operand except by the value. If <tt>"$a"</tt> is <tt>'='</tt> , the command:</p>

<pre>
<tt>expr $a = '='
</tt>
</pre>

<p>looks like:</p>

<pre>
<tt>expr = = =
</tt>
</pre>

<p>as the arguments are passed to <i>expr</i> (and they all may be taken as the <tt>'='</tt> operator). The following works
reliably:</p>

<pre>
<tt>expr X$a = X=
</tt>
</pre>

<p>Also note that this volume of IEEE&nbsp;Std&nbsp;1003.1-2001 permits implementations to extend utilities. The <i>expr</i>
utility permits the integer arguments to be preceded with a unary minus. This means that an integer argument could look like an
option. Therefore, the conforming application must employ the <tt>"--"</tt> construct of Guideline 10 of the Base Definitions
volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../basedefs/xbd_chap12.html#tag_12_02">Section 12.2, Utility Syntax
Guidelines</a> to protect its operands if there is any chance the first operand might be a negative integer (or any string with a
leading minus).</p>
</blockquote>

<h4><a name="tag_04_50_17"></a>EXAMPLES</h4>

<blockquote>
<p>The <i>expr</i> utility has a rather difficult syntax:</p>

<ul>
<li>
<p>Many of the operators are also shell control operators or reserved words, so they have to be escaped on the command line.</p>
</li>

<li>
<p>Each part of the expression is composed of separate arguments, so liberal usage of &lt;blank&gt;s is required. For example:</p>

<center>
<table border="1" cellpadding="3" align="center">
<tr valign="top">
<th align="center">
<p class="tent"><b>Invalid</b></p>
</th>
<th align="center">
<p class="tent"><b>Valid</b></p>
</th>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><i>expr</i> 1+2</p>
</td>
<td align="left">
<p class="tent"><i>expr</i> 1 + 2</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><i>expr</i> "1 + 2"</p>
</td>
<td align="left">
<p class="tent"><i>expr</i> 1 + 2</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><i>expr</i> 1 + (2 * 3)</p>
</td>
<td align="left">
<p class="tent"><i>expr</i> 1 + \( 2 \* 3 \)</p>
</td>
</tr>
</table>
</center>
</li>
</ul>

<p>In many cases, the arithmetic and string features provided as part of the shell command language are easier to use than their
equivalents in <i>expr</i>. Newly written scripts should avoid <i>expr</i> in favor of the new features within the shell; see <a
href="xcu_chap02.html#tag_02_05"><i>Parameters and Variables</i></a> and <a href="xcu_chap02.html#tag_02_06_04"><i>Arithmetic
Expansion</i></a> .</p>

<p>The following command:</p>

<pre>
<tt>a=$(expr $a + 1)
</tt>
</pre>

<p>adds 1 to the variable <i>a</i>.</p>

<p>The following command, for <tt>"$a"</tt> equal to either <b>/usr/abc/file</b> or just <b>file</b>:</p>

<pre>
<tt>expr $a : '.*/\(.*\)' \| $a
</tt>
</pre>

<p>returns the last segment of a pathname (that is, <b>file</b>). Applications should avoid the character <tt>'/'</tt> used alone
as an argument; <i>expr</i> may interpret it as the division operator.</p>

<p>The following command:</p>

<pre>
<tt>expr "//$a" : '.*/\(.*\)'
</tt>
</pre>

<p>is a better representation of the previous example. The addition of the <tt>"//"</tt> characters eliminates any ambiguity about
the division operator and simplifies the whole expression. Also note that pathnames may contain characters contained in the
<i>IFS</i> variable and should be quoted to avoid having <tt>"$a"</tt> expand into multiple arguments.</p>

<p>The following command:</p>

<pre>
<tt>expr "$VAR" : '.*'
</tt>
</pre>

<p>returns the number of characters in <i>VAR</i>.</p>
</blockquote>

<h4><a name="tag_04_50_18"></a>RATIONALE</h4>

<blockquote>
<p>In an early proposal, EREs were used in the matching expression syntax. This was changed to BREs to avoid breaking historical
applications.</p>

<p>The use of a leading circumflex in the BRE is unspecified because many historical implementations have treated it as a special
character, despite their system documentation. For example:</p>

<pre>
<tt>expr foo : ^foo     expr ^foo : ^foo
</tt>
</pre>

<p>return 3 and 0, respectively, on those systems; their documentation would imply the reverse. Thus, the anchoring condition is
left unspecified to avoid breaking historical scripts relying on this undocumented feature.</p>
</blockquote>

<h4><a name="tag_04_50_19"></a>FUTURE DIRECTIONS</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_04_50_20"></a>SEE ALSO</h4>

<blockquote>
<p><a href="xcu_chap02.html#tag_02_05"><i>Parameters and Variables</i></a> , <a href="xcu_chap02.html#tag_02_06_04"><i>Arithmetic
Expansion</i></a></p>
</blockquote>

<h4><a name="tag_04_50_21"></a>CHANGE HISTORY</h4>

<blockquote>
<p>First released in Issue 2.</p>
</blockquote>

<h4><a name="tag_04_50_22"></a>Issue 5</h4>

<blockquote>
<p>The FUTURE DIRECTIONS section is added.</p>
</blockquote>

<h4><a name="tag_04_50_23"></a>Issue 6</h4>

<blockquote>
<p>The <i>expr</i> utility is aligned with the IEEE&nbsp;P1003.2b draft standard, to include resolution of IEEE PASC Interpretation
1003.2 #104.</p>

<p>The normative text is reworded to avoid use of the term &quot;must&quot; for application requirements.</p>
</blockquote>

<div class="box"><em>End of informative text.</em></div>

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