262 lines
8.2 KiB
HTML
262 lines
8.2 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<basefont size="3"> <a name="head"></a> <a name="tag_04_65"></a><!-- head -->
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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_65_01"></a>NAME</h4>
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<blockquote>head - copy the first part of files</blockquote>
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<h4><a name="tag_04_65_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>head</tt> <b>[</b><tt>-n</tt> <i>number</i><b>][</b><i>file</i><tt>...</tt><b>]</b></code></p>
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</blockquote>
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<h4><a name="tag_04_65_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>head</i> utility shall copy its input files to the standard output, ending the output for each file at a designated
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point.</p>
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<p>Copying shall end at the point in each input file indicated by the <b>-n</b> <i>number</i> option. The option-argument
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<i>number</i> shall be counted in units of lines.</p>
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</blockquote>
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<h4><a name="tag_04_65_04"></a>OPTIONS</h4>
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<blockquote>
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<p>The <i>head</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 shall be supported:</p>
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<dl compact>
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<dt><b>-n </b> <i>number</i></dt>
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<dd>The first <i>number</i> lines of each input file shall be copied to standard output. The application shall ensure that the
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<i>number</i> option-argument is a positive decimal integer.</dd>
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</dl>
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<p>When a file contains less than <i>number</i> lines, it shall be copied to standard output in its entirety. This shall not be an
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error.</p>
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<p>If no options are specified, <i>head</i> shall act as if <b>-n 10</b> had been specified.</p>
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</blockquote>
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<h4><a name="tag_04_65_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>file</i></dt>
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<dd>A pathname of an input file. If no <i>file</i> operands are specified, the standard input shall be used.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_65_06"></a>STDIN</h4>
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<blockquote>
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<p>The standard input shall be used only if no <i>file</i> operands are specified. See the INPUT FILES section.</p>
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</blockquote>
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<h4><a name="tag_04_65_07"></a>INPUT FILES</h4>
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<blockquote>
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<p>Input files shall be text files, but the line length is not restricted to {LINE_MAX} bytes.</p>
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</blockquote>
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<h4><a name="tag_04_65_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>head</i>:</p>
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<dl compact>
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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 and input files).</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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</dl>
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</blockquote>
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<h4><a name="tag_04_65_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_65_10"></a>STDOUT</h4>
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<blockquote>
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<p>The standard output shall contain designated portions of the input files.</p>
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<p>If multiple <i>file</i> operands are specified, <i>head</i> shall precede the output for each with the header:</p>
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<pre>
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<tt>"\n==> %s <==\n", <</tt><i>pathname</i><tt>>
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</tt>
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</pre>
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<p>except that the first header written shall not include the initial <newline>.</p>
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</blockquote>
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<h4><a name="tag_04_65_11"></a>STDERR</h4>
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<blockquote>
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<p>The standard error shall be used only for diagnostic messages.</p>
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</blockquote>
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<h4><a name="tag_04_65_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_65_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_65_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>An error occurred.</dd>
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</dl>
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</blockquote>
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<h4><a name="tag_04_65_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_65_16"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The obsolescent <b>-</b> <i>number</i> form is withdrawn in this version. Applications should use the <b>-n</b> <i>number</i>
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option.</p>
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</blockquote>
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<h4><a name="tag_04_65_17"></a>EXAMPLES</h4>
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<blockquote>
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<p>To write the first ten lines of all files (except those with a leading period) in the directory:</p>
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<pre>
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<tt>head *
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_04_65_18"></a>RATIONALE</h4>
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<blockquote>
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<p>Although it is possible to simulate <i>head</i> with <a href="../utilities/sed.html"><i>sed</i></a> 10q for a single file, the
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standard developers decided that the popularity of <i>head</i> on historical BSD systems warranted its inclusion alongside <a href=
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"../utilities/tail.html"><i>tail</i></a>.</p>
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<p>This standard version of <i>head</i> follows the Utility Syntax Guidelines. The <b>-n</b> option was added to this new interface
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so that <i>head</i> and <a href="../utilities/tail.html"><i>tail</i></a> would be more logically related.</p>
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<p>There is no <b>-c</b> option (as there is in <a href="../utilities/tail.html"><i>tail</i></a>) because it is not historical
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practice and because other utilities in this volume of IEEE Std 1003.1-2001 provide similar functionality.</p>
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</blockquote>
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<h4><a name="tag_04_65_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_65_20"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="sed.html"><i>sed</i></a> , <a href="tail.html"><i>tail</i></a></p>
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</blockquote>
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<h4><a name="tag_04_65_21"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 4.</p>
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</blockquote>
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<h4><a name="tag_04_65_22"></a>Issue 6</h4>
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<blockquote>
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<p>The obsolescent <b>-number</b> form is withdrawn.</p>
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<p>The normative text is reworded to avoid use of the term "must" for application requirements.</p>
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<p>The DESCRIPTION is updated to clarify that when a file contains less than the number of lines requested, the entire file is
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copied to standard output.</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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<hr size="2" noshade>
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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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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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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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