270 lines
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270 lines
9.0 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>link</title>
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<basefont size="3"> <a name="link"></a> <a name="tag_03_333"></a><!-- link -->
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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_03_333_01"></a>NAME</h4>
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<blockquote>link - link to a file</blockquote>
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<h4><a name="tag_03_333_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br>
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<br>
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int link(const char *</tt><i>path1</i><tt>, const char *</tt><i>path2</i><tt>);<br>
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</tt></code></p>
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</blockquote>
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<h4><a name="tag_03_333_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>link</i>() function shall create a new link (directory entry) for the existing file, <i>path1</i>.</p>
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<p>The <i>path1</i> argument points to a pathname naming an existing file. The <i>path2</i> argument points to a pathname naming
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the new directory entry to be created. The <i>link</i>() function shall atomically create a new link for the existing file and the
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link count of the file shall be incremented by one.</p>
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<p>If <i>path1</i> names a directory, <i>link</i>() shall fail unless the process has appropriate privileges and the implementation
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supports using <i>link</i>() on directories.</p>
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<p>Upon successful completion, <i>link</i>() shall mark for update the <i>st_ctime</i> field of the file. Also, the <i>st_ctime</i>
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and <i>st_mtime</i> fields of the directory that contains the new entry shall be marked for update.</p>
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<p>If <i>link</i>() fails, no link shall be created and the link count of the file shall remain unchanged.</p>
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<p>The implementation may require that the calling process has permission to access the existing file.</p>
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</blockquote>
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<h4><a name="tag_03_333_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and <i>errno</i> set to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_333_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>link</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EACCES]</dt>
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<dd>A component of either path prefix denies search permission, or the requested link requires writing in a directory that denies
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write permission, or the calling process does not have permission to access the existing file and this is required by the
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implementation.</dd>
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<dt>[EEXIST]</dt>
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<dd>The <i>path2</i> argument resolves to an existing file or refers to a symbolic link.</dd>
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<dt>[ELOOP]</dt>
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<dd>A loop exists in symbolic links encountered during resolution of the <i>path1</i> or <i>path2</i> argument.</dd>
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<dt>[EMLINK]</dt>
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<dd>The number of links to the file named by <i>path1</i> would exceed {LINK_MAX}.</dd>
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<dt>[ENAMETOOLONG]</dt>
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<dd>
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The length of the <i>path1</i> or <i>path2</i> argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.</dd>
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<dt>[ENOENT]</dt>
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<dd>A component of either path prefix does not exist; the file named by <i>path1</i> does not exist; or <i>path1</i> or
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<i>path2</i> points to an empty string.</dd>
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<dt>[ENOSPC]</dt>
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<dd>The directory to contain the link cannot be extended.</dd>
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<dt>[ENOTDIR]</dt>
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<dd>A component of either path prefix is not a directory.</dd>
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<dt>[EPERM]</dt>
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<dd>The file named by <i>path1</i> is a directory and either the calling process does not have appropriate privileges or the
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implementation prohibits using <i>link</i>() on directories.</dd>
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<dt>[EROFS]</dt>
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<dd>The requested link requires writing in a directory on a read-only file system.</dd>
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<dt>[EXDEV]</dt>
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<dd>The link named by <i>path2</i> and the file named by <i>path1</i> are on different file systems and the implementation does not
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support links between file systems.</dd>
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<dt>[EXDEV]</dt>
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<dd><sup>[<a href="javascript:open_code('XSR')">XSR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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<i>path1</i> refers to a named STREAM. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
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</dl>
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<p>The <i>link</i>() function may fail if:</p>
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<dl compact>
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<dt>[ELOOP]</dt>
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<dd>More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the <i>path1</i> or <i>path2</i> argument.</dd>
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<dt>[ENAMETOOLONG]</dt>
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<dd>
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As a result of encountering a symbolic link in resolution of the <i>path1</i> or <i>path2</i> argument, the length of the
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substituted pathname string exceeded {PATH_MAX}.</dd>
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</dl>
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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_03_333_06"></a>EXAMPLES</h4>
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<blockquote>
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<h5><a name="tag_03_333_06_01"></a>Creating a Link to a File</h5>
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<p>The following example shows how to create a link to a file named <b>/home/cnd/mod1</b> by creating a new directory entry named
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<b>/modules/pass1</b>.</p>
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<pre>
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<tt>#include <unistd.h>
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<br>
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char *path1 = "/home/cnd/mod1";
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char *path2 = "/modules/pass1";
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int status;
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...
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status = link (path1, path2);
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</tt>
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</pre>
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<h5><a name="tag_03_333_06_02"></a>Creating a Link to a File Within a Program</h5>
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<p>In the following program example, the <i>link</i>() function links the <b>/etc/passwd</b> file (defined as <b>PASSWDFILE</b>) to
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a file named <b>/etc/opasswd</b> (defined as <b>SAVEFILE</b>), which is used to save the current password file. Then, after
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removing the current password file (defined as <b>PASSWDFILE</b>), the new password file is saved as the current password file
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using the <i>link</i>() function again.</p>
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<pre>
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<tt>#include <unistd.h>
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<br>
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#define LOCKFILE "/etc/ptmp"
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#define PASSWDFILE "/etc/passwd"
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#define SAVEFILE "/etc/opasswd"
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...
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/* Save current password file */
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link (PASSWDFILE, SAVEFILE);
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<br>
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/* Remove current password file. */
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unlink (PASSWDFILE);
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<br>
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/* Save new password file as current password file. */
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link (LOCKFILE,PASSWDFILE);
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_333_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>Some implementations do allow links between file systems.</p>
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</blockquote>
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<h4><a name="tag_03_333_08"></a>RATIONALE</h4>
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<blockquote>
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<p>Linking to a directory is restricted to the superuser in most historical implementations because this capability may produce
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loops in the file hierarchy or otherwise corrupt the file system. This volume of IEEE Std 1003.1-2001 continues that
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philosophy by prohibiting <i>link</i>() and <a href="../functions/unlink.html"><i>unlink</i>()</a> from doing this. Other functions
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could do it if the implementor designed such an extension.</p>
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<p>Some historical implementations allow linking of files on different file systems. Wording was added to explicitly allow this
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optional behavior.</p>
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<p>The exception for cross-file system links is intended to apply only to links that are programmatically indistinguishable from
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"hard" links.</p>
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</blockquote>
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<h4><a name="tag_03_333_09"></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_03_333_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="symlink.html"><i>symlink</i>()</a> , <a href="unlink.html"><i>unlink</i>()</a> , the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_333_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 1. Derived from Issue 1 of the SVID.</p>
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</blockquote>
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<h4><a name="tag_03_333_12"></a>Issue 6</h4>
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<blockquote>
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<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>The [ELOOP] mandatory error condition is added.</p>
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</li>
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<li>
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<p>A second [ENAMETOOLONG] is added as an optional error condition.</p>
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</li>
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</ul>
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<p>The following changes were made to align with the IEEE P1003.1a draft standard:</p>
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<ul>
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<li>
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<p>An explanation is added of the action when <i>path2</i> refers to a symbolic link.</p>
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</li>
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<li>
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<p>The [ELOOP] optional error condition is added.</p>
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</li>
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</ul>
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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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