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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>tail</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="tail"></a> <a name="tag_04_137"></a><!-- tail --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_04_137_01"></a>NAME</h4><blockquote>tail - copy the last part of a file</blockquote><h4><a name="tag_04_137_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>tail</tt> <b>[</b><tt>-f</tt><b>][</b> <tt>-c</tt> <i>number</i><tt>| -n</tt> <i>number</i><b>][</b><i>file</i><b>]</b></code></p></blockquote><h4><a name="tag_04_137_03"></a>DESCRIPTION</h4><blockquote><p>The <i>tail</i> utility shall copy its input file to the standard output beginning at a designated place.</p><p>Copying shall begin at the point in the file indicated by the <b>-c</b> <i>number</i> or <b>-n</b> <i>number</i> options. Theoption-argument <i>number</i> shall be counted in units of lines or bytes, according to the options <b>-n</b> and <b>-c</b>. Bothline and byte counts start from 1.</p><p>Tails relative to the end of the file may be saved in an internal buffer, and thus may be limited in length. Such a buffer, ifany, shall be no smaller than {LINE_MAX}*10 bytes.</p></blockquote><h4><a name="tag_04_137_04"></a>OPTIONS</h4><blockquote><p>The <i>tail</i> utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap12.html#tag_12_02">Section 12.2, Utility Syntax Guidelines</a>.</p><p>The following options shall be supported:</p><dl compact><dt><b>-c </b> <i>number</i></dt><dd>The application shall ensure that the <i>number</i> option-argument is a decimal integer whose sign affects the location in thefile, measured in bytes, to begin the copying: <center><table border="1" cellpadding="3" align="center"><tr valign="top"><th align="center"><p class="tent"><b>Sign</b></p></th><th align="center"><p class="tent"><b>Copying Starts</b></p></th></tr><tr valign="top"><td align="center"><p class="tent">+</p></td><td align="left"><p class="tent">Relative to the beginning of the file.</p></td></tr><tr valign="top"><td align="center"><p class="tent">-</p></td><td align="left"><p class="tent">Relative to the end of the file.</p></td></tr><tr valign="top"><td align="center"><p class="tent"><i>none</i></p></td><td align="left"><p class="tent">Relative to the end of the file.</p></td></tr></table></center><p>The origin for counting shall be 1; that is, <b>-c</b> +1 represents the first byte of the file, <b>-c</b> -1 the last.</p></dd><dt><b>-f</b></dt><dd>If the input file is a regular file or if the <i>file</i> operand specifies a FIFO, do not terminate after the last line of theinput file has been copied, but read and copy further bytes from the input file when they become available. If no <i>file</i>operand is specified and standard input is a pipe, the <b>-f</b> option shall be ignored. If the input file is not a FIFO, pipe, orregular file, it is unspecified whether or not the <b>-f</b> option shall be ignored.</dd><dt><b>-n </b> <i>number</i></dt><dd>This option shall be equivalent to <b>-c</b> <i>number</i>, except the starting location in the file shall be measured in linesinstead of bytes. The origin for counting shall be 1; that is, <b>-n</b> +1 represents the first line of the file, <b>-n</b> -1 thelast.</dd></dl><p>If neither <b>-c</b> nor <b>-n</b> is specified, <b>-n</b> 10 shall be assumed.</p></blockquote><h4><a name="tag_04_137_05"></a>OPERANDS</h4><blockquote><p>The following operand shall be supported:</p><dl compact><dt><i>file</i></dt><dd>A pathname of an input file. If no <i>file</i> operands are specified, the standard input shall be used.</dd></dl></blockquote><h4><a name="tag_04_137_06"></a>STDIN</h4><blockquote><p>The standard input shall be used only if no <i>file</i> operands are specified. See the INPUT FILES section.</p></blockquote><h4><a name="tag_04_137_07"></a>INPUT FILES</h4><blockquote><p>If the <b>-c</b> option is specified, the input file can contain arbitrary data; otherwise, the input file shall be a textfile.</p></blockquote><h4><a name="tag_04_137_08"></a>ENVIRONMENT VARIABLES</h4><blockquote><p>The following environment variables shall affect the execution of <i>tail</i>:</p><dl compact><dt><i>LANG</i></dt><dd>Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap08.html#tag_08_02">Section 8.2, Internationalization Variables</a> forthe precedence of internationalization variables used to determine the values of locale categories.)</dd><dt><i>LC_ALL</i></dt><dd>If set to a non-empty string value, override the values of all the other internationalization variables.</dd><dt><i>LC_CTYPE</i></dt><dd>Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte asopposed to multi-byte characters in arguments and input files).</dd><dt><i>LC_MESSAGES</i></dt><dd>Determine the locale that should be used to affect the format and contents of diagnostic messages written to standarderror.</dd><dt><i>NLSPATH</i></dt><dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">Determine the location of message catalogs for the processing of <i>LC_MESSAGES .</i> <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd></dl></blockquote><h4><a name="tag_04_137_09"></a>ASYNCHRONOUS EVENTS</h4><blockquote><p>Default.</p></blockquote><h4><a name="tag_04_137_10"></a>STDOUT</h4><blockquote><p>The designated portion of the input file shall be written to standard output.</p></blockquote><h4><a name="tag_04_137_11"></a>STDERR</h4><blockquote><p>The standard error shall be used only for diagnostic messages.</p></blockquote><h4><a name="tag_04_137_12"></a>OUTPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_137_13"></a>EXTENDED DESCRIPTION</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_137_14"></a>EXIT STATUS</h4><blockquote><p>The following exit values shall be returned:</p><dl compact><dt> 0</dt><dd>Successful completion.</dd><dt>>0</dt><dd>An error occurred.</dd></dl></blockquote><h4><a name="tag_04_137_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_137_16"></a>APPLICATION USAGE</h4><blockquote><p>The <b>-c</b> option should be used with caution when the input is a text file containing multi-byte characters; it may produceoutput that does not start on a character boundary.</p><p>Although the input file to <i>tail</i> can be any type, the results might not be what would be expected on some characterspecial device files or on file types not described by the System Interfaces volume of IEEE Std 1003.1-2001. Since thisvolume of IEEE Std 1003.1-2001 does not specify the block size used when doing input, <i>tail</i> need not read all ofthe data from devices that only perform block transfers.</p></blockquote><h4><a name="tag_04_137_17"></a>EXAMPLES</h4><blockquote><p>The <b>-f</b> option can be used to monitor the growth of a file that is being written by some other process. For example, thecommand:</p><pre><tt>tail -f fred</tt></pre><p>prints the last ten lines of the file <b>fred</b>, followed by any lines that are appended to <b>fred</b> between the time<i>tail</i> is initiated and killed. As another example, the command:</p><pre><tt>tail -f -c 15 fred</tt></pre><p>prints the last 15 bytes of the file <b>fred</b>, followed by any bytes that are appended to <b>fred</b> between the time<i>tail</i> is initiated and killed.</p></blockquote><h4><a name="tag_04_137_18"></a>RATIONALE</h4><blockquote><p>This version of <i>tail</i> was created to allow conformance to the Utility Syntax Guidelines. The historical <b>-b</b> optionwas omitted because of the general non-portability of block-sized units of text. The <b>-c</b> option historically meant"characters", but this volume of IEEE Std 1003.1-2001 indicates that it means "bytes". This was selected to allowreasonable implementations when multi-byte characters are possible; it was not named <b>-b</b> to avoid confusion with thehistorical <b>-b</b>.</p><p>The origin of counting both lines and bytes is 1, matching all widespread historical implementations.</p><p>The restriction on the internal buffer is a compromise between the historical System V implementation of 4096 bytes and the BSD32768 bytes.</p><p>The <b>-f</b> option has been implemented as a loop that sleeps for 1 second and copies any bytes that are available. This issufficient, but if more efficient methods of determining when new data are available are developed, implementations are encouragedto use them.</p><p>Historical documentation indicates that <i>tail</i> ignores the <b>-f</b> option if the input file is a pipe (pipe and FIFO onsystems that support FIFOs). On BSD-based systems, this has been true; on System V-based systems, this was true when input wastaken from standard input, but it did not ignore the <b>-f</b> flag if a FIFO was named as the <i>file</i> operand. Since the<b>-f</b> option is not useful on pipes and all historical implementations ignore <b>-f</b> if no <i>file</i> operand is specifiedand standard input is a pipe, this volume of IEEE Std 1003.1-2001 requires this behavior. However, since the <b>-f</b>option is useful on a FIFO, this volume of IEEE Std 1003.1-2001 also requires that if standard input is a FIFO or a FIFOis named, the <b>-f</b> option shall not be ignored. Although historical behavior does not ignore the <b>-f</b> option for otherfile types, this is unspecified so that implementations are allowed to ignore the <b>-f</b> option if it is known that the filecannot be extended.</p><p>This was changed to the current form based on comments noting that <b>-c</b> was almost never used without specifying a numberand that there was no need to specify <b>-l</b> if <b>-n</b> <i>number</i> was given.</p></blockquote><h4><a name="tag_04_137_19"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_137_20"></a>SEE ALSO</h4><blockquote><p><a href="head.html"><i>head</i></a></p></blockquote><h4><a name="tag_04_137_21"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 2.</p></blockquote><h4><a name="tag_04_137_22"></a>Issue 6</h4><blockquote><p>The obsolescent SYNOPSIS lines and associated text are removed.</p><p>The normative text is reworded to avoid use of the term "must" for application requirements.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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