287 lines
8.7 KiB
HTML
287 lines
8.7 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>read</title>
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<basefont size="3"> <a name="read"></a> <a name="tag_04_117"></a><!-- read -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_04_117_01"></a>NAME</h4>
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<blockquote>read - read a line from standard input</blockquote>
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<h4><a name="tag_04_117_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>read</tt> <b>[</b><tt>-r</tt><b>]</b> <i>var</i><tt>...</tt></code></p>
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</blockquote>
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<h4><a name="tag_04_117_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>read</i> utility shall read a single line from standard input.</p>
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<p>By default, unless the <b>-r</b> option is specified, backslash ( <tt>'\'</tt> ) shall act as an escape character, as described
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in <a href="xcu_chap02.html#tag_02_02_01"><i>Escape Character (Backslash)</i></a> . If standard input is a terminal device and the
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invoking shell is interactive, <i>read</i> shall prompt for a continuation line when:</p>
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<ul>
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<li>
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<p>The shell reads an input line ending with a backslash, unless the <b>-r</b> option is specified.</p>
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</li>
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<li>
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<p>A here-document is not terminated after a <newline> is entered.</p>
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</li>
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</ul>
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<p>The line shall be split into fields as in the shell (see <a href="xcu_chap02.html#tag_02_06_05"><i>Field Splitting</i></a> );
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the first field shall be assigned to the first variable <i>var</i>, the second field to the second variable <i>var</i>, and so on.
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If there are fewer <i>var</i> operands specified than there are fields, the leftover fields and their intervening separators shall
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be assigned to the last <i>var</i>. If there are fewer fields than <i>var</i>s, the remaining <i>var</i>s shall be set to empty
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strings.</p>
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<p>The setting of variables specified by the <i>var</i> operands shall affect the current shell execution environment; see <a href=
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"xcu_chap02.html#tag_02_12"><i>Shell Execution Environment</i></a> . If it is called in a subshell or separate utility execution
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environment, such as one of the following:</p>
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<pre>
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<tt>(read foo)
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nohup read ...
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find . -exec read ... \;
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</tt>
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</pre>
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<p>it shall not affect the shell variables in the caller's environment.</p>
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</blockquote>
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<h4><a name="tag_04_117_04"></a>OPTIONS</h4>
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<blockquote>
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<p>The <i>read</i> utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/xbd_chap12.html#tag_12_02">Section 12.2, Utility Syntax Guidelines</a>.</p>
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<p>The following option is supported:</p>
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<dl compact>
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<dt><b>-r</b></dt>
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<dd>Do not treat a backslash character in any special way. Consider each backslash to be part of the input line.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_117_05"></a>OPERANDS</h4>
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<blockquote>
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<p>The following operand shall be supported:</p>
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<dl compact>
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<dt><i>var</i></dt>
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<dd>The name of an existing or nonexisting shell variable.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_117_06"></a>STDIN</h4>
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<blockquote>
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<p>The standard input shall be a text file.</p>
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</blockquote>
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<h4><a name="tag_04_117_07"></a>INPUT FILES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_117_08"></a>ENVIRONMENT VARIABLES</h4>
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<blockquote>
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<p>The following environment variables shall affect the execution of <i>read</i>:</p>
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<dl compact>
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<dt><i>IFS</i></dt>
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<dd>Determine the internal field separators used to delimit fields; see <a href="xcu_chap02.html#tag_02_05_03"><i>Shell
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Variables</i></a> .</dd>
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<dt><i>LANG</i></dt>
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<dd>Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap08.html#tag_08_02">Section 8.2, Internationalization Variables</a> for
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the precedence of internationalization variables used to determine the values of locale categories.)</dd>
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<dt><i>LC_ALL</i></dt>
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<dd>If set to a non-empty string value, override the values of all the other internationalization variables.</dd>
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<dt><i>LC_CTYPE</i></dt>
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<dd>Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments).</dd>
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<dt><i>LC_MESSAGES</i></dt>
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<dd>Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard
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error.</dd>
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<dt><i>NLSPATH</i></dt>
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<dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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Determine the location of message catalogs for the processing of <i>LC_MESSAGES .</i> <img src="../images/opt-end.gif" alt=
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"[Option End]" border="0"></dd>
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<dt><i>PS2</i></dt>
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<dd>Provide the prompt string that an interactive shell shall write to standard error when a line ending with a backslash is read
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and the <b>-r</b> option was not specified, or if a here-document is not terminated after a <newline> is entered.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_117_09"></a>ASYNCHRONOUS EVENTS</h4>
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<blockquote>
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<p>Default.</p>
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</blockquote>
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<h4><a name="tag_04_117_10"></a>STDOUT</h4>
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<blockquote>
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<p>Not used.</p>
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</blockquote>
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<h4><a name="tag_04_117_11"></a>STDERR</h4>
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<blockquote>
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<p>The standard error shall be used for diagnostic messages and prompts for continued input.</p>
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</blockquote>
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<h4><a name="tag_04_117_12"></a>OUTPUT FILES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_117_13"></a>EXTENDED DESCRIPTION</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_117_14"></a>EXIT STATUS</h4>
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<blockquote>
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<p>The following exit values shall be returned:</p>
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<dl compact>
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<dt> 0</dt>
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<dd>Successful completion.</dd>
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<dt>>0</dt>
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<dd>End-of-file was detected or an error occurred.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_117_15"></a>CONSEQUENCES OF ERRORS</h4>
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<blockquote>
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<p>Default.</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_04_117_16"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <b>-r</b> option is included to enable <i>read</i> to subsume the purpose of the <i>line</i> utility, which is not included
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in IEEE Std 1003.1-2001.</p>
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<p>The results are undefined if an end-of-file is detected following a backslash at the end of a line when <b>-r</b> is not
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specified.</p>
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</blockquote>
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<h4><a name="tag_04_117_17"></a>EXAMPLES</h4>
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<blockquote>
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<p>The following command:</p>
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<pre>
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<tt>while read -r xx yy
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do
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printf "%s %s\n" "$yy" "$xx"
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done <</tt> <i>input_file</i>
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</pre>
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<p>prints a file with the first field of each line moved to the end of the line.</p>
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</blockquote>
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<h4><a name="tag_04_117_18"></a>RATIONALE</h4>
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<blockquote>
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<p>The <i>read</i> utility historically has been a shell built-in. It was separated off into its own utility to take advantage of
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the richer description of functionality introduced by this volume of IEEE Std 1003.1-2001.</p>
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<p>Since <i>read</i> affects the current shell execution environment, it is generally provided as a shell regular built-in. If it
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is called in a subshell or separate utility execution environment, such as one of the following:</p>
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<pre>
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<tt>(read foo)
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nohup read ...
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find . -exec read ... \;
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</tt>
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</pre>
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<p>it does not affect the shell variables in the environment of the caller.</p>
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</blockquote>
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<h4><a name="tag_04_117_19"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_04_117_20"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="xcu_chap02.html#tag_02"><i>Shell Command Language</i></a></p>
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</blockquote>
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<h4><a name="tag_04_117_21"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 2.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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