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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>expr</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_001_014_624"> </a>NAME</h4><blockquote>expr - evaluate arguments as an expression</blockquote><h4><a name = "tag_001_014_625"> </a>SYNOPSIS</h4><blockquote><pre><code>expr <i>operand</i></code></pre></blockquote><h4><a name = "tag_001_014_626"> </a>DESCRIPTION</h4><blockquote>The<i>expr</i>utility will evaluate an expressionand write the result to standard output.</blockquote><h4><a name = "tag_001_014_627"> </a>OPTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_001_014_628"> </a>OPERANDS</h4><blockquote>The single expression evaluated by<i>expr</i>will be formed from the operands, as described in theEXTENDED DESCRIPTION section.Each of the expression operator symbols:<pre><code>( ) | & = > >= < <= != + - * / % :</code></pre>and the symbols<i>integer</i>and<i>string</i>in the table must be providedas separate arguments to<i>expr</i>.</blockquote><h4><a name = "tag_001_014_629"> </a>STDIN</h4><blockquote>Not used.</blockquote><h4><a name = "tag_001_014_630"> </a>INPUT FILES</h4><blockquote>None.</blockquote><h4><a name = "tag_001_014_631"> </a>ENVIRONMENT VARIABLES</h4><blockquote>The following environment variables affect the execution of<i>expr</i>:<dl compact><dt><i>LANG</i><dd>Provide a default value for the internationalisation variablesthat are unset or null.If<i>LANG</i>is unset or null, the corresponding value from theimplementation-dependent default locale will be used.If any of the internationalisation variables contains an invalid setting, theutility will behave as if none of the variables had been defined.<dt><i>LC_ALL</i><dd>If set to a non-empty string value,override the values of all the other internationalisation variables.<dt><i>LC_COLLATE</i><dd>Determine the locale for thebehaviour of ranges, equivalence classesand multi-character collating elementswithin regular expressions and by thestring comparison operators.<dt><i>LC_CTYPE</i><dd>Determine thelocale for the interpretation of sequences of bytes of text data ascharacters (for example, single- as opposed to multi-byte charactersin arguments) andthe behaviour of character classes within regular expressions.<dt><i>LC_MESSAGES</i><dd>Determine the locale that should be used to affectthe format and contents of diagnosticmessages written to standard error.<dt><i>NLSPATH</i><dd>Determine the location of message cataloguesfor the processing of<i>LC_MESSAGES .</i></dl></blockquote><h4><a name = "tag_001_014_632"> </a>ASYNCHRONOUS EVENTS</h4><blockquote>Default.<br></blockquote><h4><a name = "tag_001_014_633"> </a>STDOUT</h4><blockquote>The<i>expr</i>utility will evaluate the expressionand write the result to standard output.The character0will be written to indicate a zero value and nothing will be writtento indicate a null string.</blockquote><h4><a name = "tag_001_014_634"> </a>STDERR</h4><blockquote>Used only for diagnostic messages.</blockquote><h4><a name = "tag_001_014_635"> </a>OUTPUT FILES</h4><blockquote>None.</blockquote><h4><a name = "tag_001_014_636"> </a>EXTENDED DESCRIPTION</h4><blockquote>The formation of the expression to be evaluated is shown inthe following table.The symbols<i>expr</i>,<i>expr1</i>and<i>expr2</i>represent expressions formed from<i>integer</i>and<i>string</i>symbols and the expression operator symbols (all separate arguments)by recursive application of the constructs described in the table.The expressionsare listed in order of increasing precedence,with equal-precedence operators grouped between horizontal lines.All of the operators are left-associative.<pre><table bordercolor=#000000 border=1 align=center><tr valign=top><th align=center><b>Expression</b><th align=center><b>Description</b><tr valign=top><td align=left><i>expr1</i> <b>|</b> <i>expr2</i><td align=left> Returns the evaluation of if it is neither null nor zero; otherwise, returns the evaluation of <tr valign=top><td align=left><i>expr1</i> & <i>expr2</i><td align=left> Returns the evaluation of if neither expression evaluates to null or zero; otherwise, returns zero. <tr valign=top><td align=left> <td align=left> Returns the result of a decimal integer comparison if both arguments are integers; otherwise, returns the result of a string comparison using the locale-specific collation sequence. The result of each comparison will be 1 if the specified relationship is true, or 0 if the relationship is false. <tr valign=top><td align=left><i>expr1</i> <b>=</b> <i>expr2</i><td align=left>Equal.<tr valign=top><td align=left><i>expr1</i> <b>></b> <i>expr2</i><td align=left>Greater than.<tr valign=top><td align=left><i>expr1</i> <b>>=</b> <i>expr2</i><td align=left>Greater than or equal.<tr valign=top><td align=left><i>expr1</i> <b><</b> <i>expr2</i><td align=left>Less than.<tr valign=top><td align=left><i>expr1</i> <b><=</b> <i>expr2</i><td align=left>Less than or equal.<tr valign=top><td align=left><i>expr1</i> <b>!=</b> <i>expr2</i><td align=left>Not equal.<tr valign=top><td align=left><i>expr1</i> <b>+</b> <i>expr2</i><td align=left> Addition of decimal integer-valued arguments. <tr valign=top><td align=left><i>expr1</i> <b>-</b> <i>expr2</i><td align=left> Subtraction of decimal integer-valued arguments. <tr valign=top><td align=left><i>expr1</i> <b>*</b> <i>expr2</i><td align=left> Multiplication of decimal integer-valued arguments. <tr valign=top><td align=left><i>expr1</i> <b>/</b> <i>expr2</i><td align=left> Integer division of decimal integer-valued arguments, producing an integer result. <tr valign=top><td align=left><i>expr1</i> % <i>expr2</i><td align=left> Remainder of integer division of decimal integer-valued arguments. <tr valign=top><td align=left><i>expr1</i> <b>:</b> <i>expr2</i><td align=left> Matching expression. See below. <tr valign=top><td align=left>( expr )<td align=left> Grouping symbols. Any expression can be placed within parentheses. Parentheses can be nested to a depth of <tr valign=top><td align=left><i>integer</i><td align=left> An argument consisting only of an (optional) unary minus followed by digits. <tr valign=top><td align=left><i>string</i><td align=left> A string argument. See below. </table></pre><h5><a name = "tag_001_014_636_001"> </a>Matching Expression</h5>The ":" matching operator will compare the string resulting fromthe evaluation of<i>expr1</i>with the regular expression pattern resulting from the evaluation of<i>expr2</i>.Regular expression syntax is that defined in the <b>XBD</b> specification, <a href="../xbd/re.html#tag_007_003"><b>Basic Regular Expressions</b> </a> ,except that all patterns are anchored to the beginning of the string(that is, only sequences starting at thefirst character of a string will be matched by the regular expression)and, therefore, it is unspecified whether "^" is a special characterin that context.Usually, the matching operator will returna string representing the number of characters matched(0on failure).Alternatively, if the pattern contains at least oneregular expression subexpression [<b>\(...</b>\)],the string corresponding to\1will be returned.<h5><a name = "tag_001_014_636_002"> </a>String Operand</h5>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 inthe OPERANDS section.<p>The use of string arguments<b>length</b>,<b>substr</b>,<b>index</b>or<b>match</b>produces unspecified results.</blockquote><h4><a name = "tag_001_014_637"> </a>EXIT STATUS</h4><blockquote>The following exit values are returned:<dl compact><dt>0<dd>The<i>expression</i>evaluates to neither null nor zero.<dt>1<dd>The<i>expression</i>evaluates to null or zero.<dt>2<dd>Invalid<i>expression</i>.<dt>>2<dd>An error occurred.</dl></blockquote><h4><a name = "tag_001_014_638"> </a>CONSEQUENCES OF ERRORS</h4><blockquote>Default.</blockquote><h4><a name = "tag_001_014_639"> </a>APPLICATION USAGE</h4><blockquote>After argument processing by the shell,<i>expr</i>is not required to be able to tell the differencebetween an operator and an operandexcept by the value.If<b>$a</b>is"=",the command:<pre><code>expr $a = '='</code></pre>looks like:<pre><code>expr = = =</code></pre>as the arguments are passed to<i>expr</i>(and they all may be taken as the"="operator).The following works reliably:<pre><code>expr X$a = X=</code></pre><p>Also note that this specification permits implementations to extend utilities.The<i>expr</i>utility permits the integer argumentsto be preceded with a unary minus.This means that an integer argument could look like an option.Therefore, the portable application must employ the --construct of Guideline 10 of the <b>XBD</b> specification, <a href="../xbd/utilconv.html#usg"><b>Utility Syntax Guidelines</b> </a> to protect its operands if there is any chance the first operandmight be a negative integer (or any string with a leading minus).</blockquote><h4><a name = "tag_001_014_640"> </a>EXAMPLES</h4><blockquote>The<i>expr</i>utility has a rather difficult syntax:<ul><p><li>Many of the operators are also shell control operatorsor reserved words, so they have to be escaped on the command line.<br><p><li>Each part of the expression is composed of separate arguments,so liberal usage ofblank charactersis required.For example:<br><p><table bordercolor=#000000 border=1 align=center><tr valign=top><th align=center><b>Invalid</b><th align=center><b>Valid</b><tr valign=top><td align=left>expr 1+2<td align=left>expr 1 + 2<tr valign=top><td align=left>expr "1 + 2"<td align=left>expr 1 + 2<tr valign=top><td align=left>expr 1 + (2 * 3)<td align=left>expr 1 + \( 2 \* 3 \)</table><p></ul><p>In many cases, the arithmetic and string features providedas part of the shell command language are easierto use than their equivalents in<i>expr</i>.Newly written scripts should avoid<i>expr</i>in favour of the new features within the shell.See<xref href=parms><a href="chap2.html#tag_001_005">Parameters and Variables</a></xref>and<xref href=aritexp><a href="chap2.html#tag_001_006_004">Arithmetic Expansion</a></xref>.<p>The following command:<pre><code>a=$(expr $a + 1)</code></pre>adds 1 to the variable<i>a</i>.<p>The following command, for<b>$a</b>equal to either<b>/usr/abc/file</b>or just<b>file:</b><pre><code>expr $a : '.*/\(.*\)' \| $a</code></pre>returns the last segment of a pathname (that is,<b>file</b>).Applications should avoid the character "/" used alone as an argument:<i>expr</i>may interpret it as the division operator.<p>The following command:<pre><code>expr "//$a" : '.*/\(.*\)'</code></pre>is a better representation of the previous example.The addition of the//characters eliminates any ambiguity about the division operatorand 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<b>$a</b>expand into multiple arguments.<p>The following command:<pre><code>expr "$VAR" : '.*'</code></pre>returns the number of characters in<i>VAR</i>.</blockquote><h4><a name = "tag_001_014_641"> </a>FUTURE DIRECTIONS</h4><blockquote>The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about parts of this interface definition to the IEEE PASC Shell and Utilities Working Groupwhich is identifying the corrections.A future revision of this specification will align withIEEE Std. 1003.2b when finalised.</blockquote><h4><a name = "tag_001_014_642"> </a>SEE ALSO</h4><blockquote><xref href=aritexp><a href="chap2.html#tag_001_006_004">Arithmetic Expansion</a></xref>.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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