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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>ln</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="ln"></a> <a name="tag_04_75"></a><!-- ln --> <!--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_75_01"></a>NAME</h4><blockquote>ln - link files</blockquote><h4><a name="tag_04_75_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>ln</tt> <b>[</b><tt>-fs</tt><b>]</b> <i>source_file target_file</i><tt><br><br>ln</tt> <b>[</b><tt>-fs</tt><b>]</b> <i>source_file</i> <tt>...</tt> <i>target_dir</i><tt><br></tt></code></p></blockquote><h4><a name="tag_04_75_03"></a>DESCRIPTION</h4><blockquote><p>In the first synopsis form, the <i>ln</i> utility shall create a new directory entry (link) at the destination path specified bythe <i>target_file</i> operand. If the <b>-s</b> option is specified, a symbolic link shall be created for the file specified bythe <i>source_file</i> operand. This first synopsis form shall be assumed when the final operand does not name an existingdirectory; if more than two operands are specified and the final is not an existing directory, an error shall result.</p><p>In the second synopsis form, the <i>ln</i> utility shall create a new directory entry (link), or if the <b>-s</b> option isspecified a symbolic link, for each file specified by a <i>source_file</i> operand, at a destination path in the existing directorynamed by <i>target_dir</i>.</p><p>If the last operand specifies an existing file of a type not specified by the System Interfaces volume ofIEEE Std 1003.1-2001, the behavior is implementation-defined.</p><p>The corresponding destination path for each <i>source_file</i> shall be the concatenation of the target directory pathname, aslash character, and the last pathname component of the <i>source_file</i>. The second synopsis form shall be assumed when thefinal operand names an existing directory.</p><p>For each <i>source_file</i>:</p><ol><li><p>If the destination path exists:</p><ol type="a"><li><p>If the <b>-f</b> option is not specified, <i>ln</i> shall write a diagnostic message to standard error, do nothing more with thecurrent <i>source_file</i>, and go on to any remaining <i>source_files</i>.</p></li><li><p>Actions shall be performed equivalent to the <a href="../functions/unlink.html"><i>unlink</i>()</a> function defined in theSystem Interfaces volume of IEEE Std 1003.1-2001, called using <i>destination</i> as the <i>path</i> argument. If thisfails for any reason, <i>ln</i> shall write a diagnostic message to standard error, do nothing more with the current<i>source_file</i>, and go on to any remaining <i>source_files</i>.</p></li></ol></li><li><p>If the <b>-s</b> option is specified, <i>ln</i> shall create a symbolic link named by the destination path and containing as itspathname <i>source_file</i>. The <i>ln</i> utility shall do nothing more with <i>source_file</i> and shall go on to any remainingfiles.</p></li><li><p>If <i>source_file</i> is a symbolic link, actions shall be performed equivalent to the <a href="../functions/link.html"><i>link</i>()</a> function using the object that <i>source_file</i> references as the <i>path1</i>argument and the destination path as the <i>path2</i> argument. The <i>ln</i> utility shall do nothing more with <i>source_file</i>and shall go on to any remaining files.</p></li><li><p>Actions shall be performed equivalent to the <a href="../functions/link.html"><i>link</i>()</a> function defined in the SystemInterfaces volume of IEEE Std 1003.1-2001 using <i>source_file</i> as the <i>path1</i> argument, and the destination pathas the <i>path2</i> argument.</p></li></ol></blockquote><h4><a name="tag_04_75_04"></a>OPTIONS</h4><blockquote><p>The <i>ln</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 option shall be supported:</p><dl compact><dt><b>-f</b></dt><dd>Force existing destination pathnames to be removed to allow the link.</dd><dt><b>-s</b></dt><dd>Create symbolic links instead of hard links.</dd></dl></blockquote><h4><a name="tag_04_75_05"></a>OPERANDS</h4><blockquote><p>The following operands shall be supported:</p><dl compact><dt><i>source_file</i></dt><dd>A pathname of a file to be linked. If the <b>-s</b> option is specified, no restrictions on the type of file or on itsexistence shall be made. If the <b>-s</b> option is not specified, whether a directory can be linked isimplementation-defined.</dd><dt><i>target_file</i></dt><dd>The pathname of the new directory entry to be created.</dd><dt><i>target_dir</i></dt><dd>A pathname of an existing directory in which the new directory entries are created.</dd></dl></blockquote><h4><a name="tag_04_75_06"></a>STDIN</h4><blockquote><p>Not used.</p></blockquote><h4><a name="tag_04_75_07"></a>INPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_08"></a>ENVIRONMENT VARIABLES</h4><blockquote><p>The following environment variables shall affect the execution of <i>ln</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).</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_75_09"></a>ASYNCHRONOUS EVENTS</h4><blockquote><p>Default.</p></blockquote><h4><a name="tag_04_75_10"></a>STDOUT</h4><blockquote><p>Not used.</p></blockquote><h4><a name="tag_04_75_11"></a>STDERR</h4><blockquote><p>The standard error shall be used only for diagnostic messages.</p></blockquote><h4><a name="tag_04_75_12"></a>OUTPUT FILES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_13"></a>EXTENDED DESCRIPTION</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_14"></a>EXIT STATUS</h4><blockquote><p>The following exit values shall be returned:</p><dl compact><dt> 0</dt><dd>All the specified files were linked successfully.</dd><dt>>0</dt><dd>An error occurred.</dd></dl></blockquote><h4><a name="tag_04_75_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_75_16"></a>APPLICATION USAGE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_17"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_18"></a>RATIONALE</h4><blockquote><p>Some historic versions of <i>ln</i> (including the one specified by the SVID) unlink the destination file, if it exists, bydefault. If the mode does not permit writing, these versions prompt for confirmation before attempting the unlink. In theseversions the <b>-f</b> option causes <i>ln</i> not to attempt to prompt for confirmation.</p><p>This allows <i>ln</i> to succeed in creating links when the target file already exists, even if the file itself is not writable(although the directory must be). Early proposals specified this functionality.</p><p>This volume of IEEE Std 1003.1-2001 does not allow the <i>ln</i> utility to unlink existing destination paths bydefault for the following reasons:</p><ul><li><p>The <i>ln</i> utility has historically been used to provide locking for shell applications, a usage that is incompatible with<i>ln</i> unlinking the destination path by default. There was no corresponding technical advantage to adding thisfunctionality.</p></li><li><p>This functionality gave <i>ln</i> the ability to destroy the link structure of files, which changes the historical behavior of<i>ln</i>.</p></li><li><p>This functionality is easily replicated with a combination of <a href="../utilities/rm.html"><i>rm</i></a> and <i>ln</i>.</p></li><li><p>It is not historical practice in many systems; BSD and BSD-derived systems do not support this behavior. Unfortunately,whichever behavior is selected can cause scripts written expecting the other behavior to fail.</p></li><li><p>It is preferable that <i>ln</i> perform in the same manner as the <a href="../functions/link.html"><i>link</i>()</a> function,which does not permit the target to exist already.</p></li></ul><p>This volume of IEEE Std 1003.1-2001 retains the <b>-f</b> option to provide support for shell scripts depending on theSVID semantics. It seems likely that shell scripts would not be written to handle prompting by <i>ln</i> and would therefore havespecified the <b>-f</b> option.</p><p>The <b>-f</b> option is an undocumented feature of many historical versions of the <i>ln</i> utility, allowing linking todirectories. These versions require modification.</p><p>Early proposals of this volume of IEEE Std 1003.1-2001 also required a <b>-i</b> option, which behaved like the<b>-i</b> options in <a href="../utilities/cp.html"><i>cp</i></a> and <a href="../utilities/mv.html"><i>mv</i></a>, prompting forconfirmation before unlinking existing files. This was not historical practice for the <i>ln</i> utility and has been omitted.</p></blockquote><h4><a name="tag_04_75_19"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_04_75_20"></a>SEE ALSO</h4><blockquote><p><a href="chmod.html"><i>chmod</i>()</a> , <a href="find.html"><i>find</i></a> , <a href="pax.html"><i>pax</i></a> , <a href="rm.html"><i>rm</i></a> , the System Interfaces volume of IEEE Std 1003.1-2001, <a href="../functions/link.html"><i>link</i>()</a>, <a href="../functions/unlink.html"><i>unlink</i>()</a></p></blockquote><h4><a name="tag_04_75_21"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 2.</p></blockquote><h4><a name="tag_04_75_22"></a>Issue 6</h4><blockquote><p>The <i>ln</i> utility is updated to include symbolic link processing as defined in the IEEE P1003.2b draft standard.</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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