mirror of https://github.com/mkerrisk/man-pages
489 lines
14 KiB
Groff
489 lines
14 KiB
Groff
.\" Copyright (c) 1992, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" %%%LICENSE_START(BSD_3_CLAUSE_UCB)
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\" %%%LICENSE_END
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.\"
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.\" @(#)symlink.7 8.3 (Berkeley) 3/31/94
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.\" $FreeBSD: src/bin/ln/symlink.7,v 1.30 2005/02/13 22:25:09 ru Exp $
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.\"
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.\" 2008-06-11, mtk, Taken from FreeBSD 6.2 and heavily edited for
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.\" specific Linux details, improved readability, and man-pages style.
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.\"
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.TH SYMLINK 7 2008-06-18 "Linux" "Linux Programmer's Manual"
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.SH NAME
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symlink \- symbolic link handling
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.SH SYMBOLIC LINK HANDLING
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Symbolic links are files that act as pointers to other files.
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To understand their behavior, you must first understand how hard links
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work.
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A hard link to a file is indistinguishable from the original file because
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it is a reference to the object underlying the original filename.
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(To be precise: each of the hard links to a file is a reference to
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the same
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.IR "i-node number" ,
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where an i-node number is an index into the i-node table,
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which contains metadata about all files on a file system.
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See
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.BR stat (2).)
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Changes to a file are independent of the name used to reference the file.
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Hard links may not refer to directories
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(to prevent the possibility of loops within the file system tree,
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which would confuse many programs)
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and may not refer to files on different file systems
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(because i-node numbers are not unique across file systems).
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A symbolic link is a special type of file whose contents are a string
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that is the pathname another file, the file to which the link refers.
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In other words, a symbolic link is a pointer to another name,
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and not to an underlying object.
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For this reason, symbolic links may refer to directories and may cross
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file system boundaries.
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There is no requirement that the pathname referred to by a symbolic link
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should exist.
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A symbolic link that refers to a pathname that does not exist is said
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to be a
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.IR "dangling link" .
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Because a symbolic link and its referenced object coexist in the file system
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name space, confusion can arise in distinguishing between the link itself
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and the referenced object.
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On historical systems,
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commands and system calls adopted their own link-following
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conventions in a somewhat ad-hoc fashion.
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Rules for a more uniform approach,
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as they are implemented on Linux and other systems,
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are outlined here.
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It is important that site-local applications also conform to these rules,
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so that the user interface can be as consistent as possible.
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.SS Symbolic link ownership, permissions, and timestamps
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The owner and group of an existing symbolic link can be changed
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using
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.BR lchown (2).
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The only time that the ownership of a symbolic link matters is
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when the link is being removed or renamed in a directory that
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has the sticky bit set (see
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.BR stat (2)).
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The last access and last modification timestamps
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of a symbolic link can be changed using
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.BR utimensat (2)
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or
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.BR lutimes (3).
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On Linux, the permissions of a symbolic link are not used
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in any operations; the permissions are always
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0777 (read, write, and execute for all user categories),
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.\" Linux does not currently implement an lchmod(2).
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and can't be changed.
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.\"
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.\" The
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.\" 4.4BSD
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.\" system differs from historical
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.\" 4BSD
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.\" systems in that the system call
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.\" .BR chown (2)
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.\" has been changed to follow symbolic links.
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.\" The
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.\" .BR lchown (2)
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.\" system call was added later when the limitations of the new
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.\" .BR chown (2)
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.\" became apparent.
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.SS Handling of symbolic links by system calls and commands
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Symbolic links are handled either by operating on the link itself,
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or by operating on the object referred to by the link.
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In the latter case,
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an application or system call is said to
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.I follow
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the link.
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Symbolic links may refer to other symbolic links,
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in which case the links are dereferenced until an object that is
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not a symbolic link is found,
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a symbolic link that refers to a file which does not exist is found,
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or a loop is detected.
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(Loop detection is done by placing an upper limit on the number of
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links that may be followed, and an error results if this limit is
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exceeded.)
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There are three separate areas that need to be discussed.
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They are as follows:
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.IP 1. 3
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Symbolic links used as filename arguments for system calls.
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.IP 2.
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Symbolic links specified as command-line arguments to utilities that
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are not traversing a file tree.
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.IP 3.
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Symbolic links encountered by utilities that are traversing a file tree
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(either specified on the command line or encountered as part of the
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file hierarchy walk).
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.SS System calls
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The first area is symbolic links used as filename arguments for
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system calls.
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Except as noted below, all system calls follow symbolic links.
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For example, if there were a symbolic link
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.I slink
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which pointed to a file named
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.IR afile ,
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the system call
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.I "open(""slink"" ...\&)"
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would return a file descriptor referring to the file
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.IR afile .
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Various system calls do not follow links, and operate
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on the symbolic link itself.
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They are:
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.BR lchown (2),
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.BR lgetxattr (2),
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.BR llistxattr (2),
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.BR lremovexattr (2),
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.BR lsetxattr (2),
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.BR lstat (2),
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.BR readlink (2),
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.BR rename (2),
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.BR rmdir (2),
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and
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.BR unlink (2).
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Certain other system calls optionally follow symbolic links.
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They are:
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.BR faccessat (2),
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.\" Maybe one day: .BR fchownat (2)
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.BR fchownat (2),
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.BR fstatat (2),
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.BR linkat (2),
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.BR open (2),
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.BR openat (2),
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and
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.BR utimensat (2);
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see their manual pages for details.
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Because
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.BR remove (3)
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is an alias for
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.BR unlink (2),
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that library function also does not follow symbolic links.
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When
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.BR rmdir (2)
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is applied to a symbolic link, it fails with the error
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.BR ENOTDIR .
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The
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.BR link (2)
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warrants special discussion.
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POSIX.1-2001 specifies that
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.BR link (2)
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should dereference
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.I oldpath
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if it is a symbolic link.
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However, Linux does not do this.
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(By default Solaris is the same,
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but the POSIX.1-2001 specified behavior can be obtained with
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suitable compiler options.)
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The upcoming POSIX.1 revision changes the specification to allow
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either behavior in an implementation.
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.SS Commands not traversing a file tree
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The second area is symbolic links, specified as command-line
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filename arguments, to commands which are not traversing a file tree.
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Except as noted below, commands follow symbolic links named as
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command-line arguments.
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For example, if there were a symbolic link
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.I slink
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which pointed to a file named
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.IR afile ,
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the command
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.I "cat slink"
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would display the contents of the file
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.IR afile .
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It is important to realize that this rule includes commands which may
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optionally traverse file trees, e.g., the command
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.I "chown file"
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is included in this rule, while the command
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.IR "chown\ \-R file" ,
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which performs a tree traversal, is not.
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(The latter is described in the third area, below.)
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If it is explicitly intended that the command operate on the symbolic
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link instead of following the symbolic link, e.g., it is desired that
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.I "chown slink"
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change the ownership of the file that
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.I slink
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is, whether it is a symbolic link or not, the
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.I \-h
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option should be used.
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In the above example,
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.I "chown root slink"
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would change the ownership of the file referred to by
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.IR slink ,
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while
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.I "chown\ \-h root slink"
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would change the ownership of
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.I slink
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itself.
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There are some exceptions to this rule:
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.IP * 2
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The
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.BR mv (1)
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and
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.BR rm (1)
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commands do not follow symbolic links named as arguments,
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but respectively attempt to rename and delete them.
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(Note, if the symbolic link references a file via a relative path,
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moving it to another directory may very well cause it to stop working,
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since the path may no longer be correct.)
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.IP *
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The
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.BR ls (1)
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command is also an exception to this rule.
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For compatibility with historic systems (when
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.BR ls (1)
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is not doing a tree walk, i.e., the
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.I \-R
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option is not specified),
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the
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.BR ls (1)
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command follows symbolic links named as arguments if the
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.I \-H
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or
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.I \-L
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option is specified,
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or if the
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.IR \-F ,
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.IR \-d ,
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or
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.I \-l
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options are not specified.
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(The
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.BR ls (1)
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command is the only command where the
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.I \-H
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and
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.I \-L
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options affect its behavior even though it is not doing a walk of
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a file tree.)
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.IP *
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The
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.BR file (1)
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command is also an exception to this rule.
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The
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.BR file (1)
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command does not follow symbolic links named as argument by default.
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The
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.BR file (1)
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command does follow symbolic links named as argument if the
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.I \-L
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option is specified.
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.\"
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.\"The 4.4BSD system differs from historical 4BSD systems in that the
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.\".BR chown (1)
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.\"and
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.\".BR chgrp (1)
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.\"commands follow symbolic links specified on the command line.
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.SS Commands traversing a file tree
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The following commands either optionally or always traverse file trees:
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.BR chgrp (1),
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.BR chmod (1),
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.BR chown (1),
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.BR cp (1),
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.BR du (1),
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.BR find (1),
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.BR ls (1),
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.BR pax (1),
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.BR rm (1),
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and
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.BR tar (1).
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It is important to realize that the following rules apply equally to
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symbolic links encountered during the file tree traversal and symbolic
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links listed as command-line arguments.
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The \fIfirst rule\fP applies to symbolic links that reference files other
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than directories.
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Operations that apply to symbolic links are performed on the links
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themselves, but otherwise the links are ignored.
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The command
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.I "rm\ \-r slink directory"
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will remove
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.IR slink ,
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as well as any symbolic links encountered in the tree traversal of
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.IR directory ,
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because symbolic links may be removed.
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In no case will
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.BR rm (1)
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affect the file referred to by
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.IR slink .
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The \fIsecond rule\fP applies to symbolic links that refer to directories.
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Symbolic links that refer to directories are never followed by default.
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This is often referred to as a "physical" walk, as opposed to a "logical"
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walk (where symbolic links the refer to directories are followed).
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Certain conventions are (should be) followed as consistently as
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possible by commands that perform file tree walks:
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.IP * 2
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A command can be made to follow
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any symbolic links named on the command line,
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regardless of the type of file they reference, by specifying the
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.I \-H
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(for "half-logical") flag.
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This flag is intended to make the command-line name space look
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like the logical name space.
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(Note, for commands that do not always do file tree traversals, the
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.I \-H
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flag will be ignored if the
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.I \-R
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flag is not also specified.)
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For example, the command
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.I "chown\ \-HR user slink"
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will traverse the file hierarchy rooted in the file pointed to by
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.IR slink .
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Note, the
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.I \-H
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is not the same as the previously discussed
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.I \-h
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flag.
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The
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.I \-H
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flag causes symbolic links specified on the command line to be
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dereferenced for the purposes of both the action to be performed
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and the tree walk, and it is as if the user had specified the
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name of the file to which the symbolic link pointed.
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.IP *
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A command can be made to
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follow any symbolic links named on the command line,
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as well as any symbolic links encountered during the traversal,
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regardless of the type of file they reference, by specifying the
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.I \-L
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(for "logical") flag.
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This flag is intended to make the entire name space look like
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the logical name space.
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(Note, for commands that do not always do file tree traversals, the
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.I \-L
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flag will be ignored if the
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.I \-R
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flag is not also specified.)
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For example, the command
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.I "chown\ \-LR user slink"
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will change the owner of the file referred to by
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.IR slink .
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If
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.I slink
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refers to a directory,
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.B chown
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will traverse the file hierarchy rooted in the directory that it
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references.
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In addition, if any symbolic links are encountered in any file tree that
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.B chown
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traverses, they will be treated in the same fashion as
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.IR slink .
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.IP *
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A command can be made to
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provide the default behavior by specifying the
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.I \-P
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(for "physical") flag.
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This flag is intended to make the entire name space look like the
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physical name space.
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.PP
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For commands that do not by default do file tree traversals, the
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.IR \-H ,
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.IR \-L ,
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and
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.I \-P
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flags are ignored if the
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.I \-R
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flag is not also specified.
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In addition, you may specify the
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.IR \-H ,
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.IR \-L ,
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and
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.I \-P
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options more than once;
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the last one specified determines the command's behavior.
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This is intended to permit you to alias commands to behave one way
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or the other, and then override that behavior on the command line.
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The
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.BR ls (1)
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and
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.BR rm (1)
|
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commands have exceptions to these rules:
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.IP * 2
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The
|
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.BR rm (1)
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command operates on the symbolic link, and not the file it references,
|
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and therefore never follows a symbolic link.
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The
|
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.BR rm (1)
|
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command does not support the
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.IR \-H ,
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.IR \-L ,
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or
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.I \-P
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options.
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.IP *
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To maintain compatibility with historic systems,
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the
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.BR ls (1)
|
|
command acts a little differently.
|
|
If you do not specify the
|
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.IR \-F ,
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.IR \-d
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or
|
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.I \-l
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options,
|
|
.BR ls (1)
|
|
will follow symbolic links specified on the command line.
|
|
If the
|
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.I \-L
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flag is specified,
|
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.BR ls (1)
|
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follows all symbolic links,
|
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regardless of their type,
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whether specified on the command line or encountered in the tree walk.
|
|
.SH SEE ALSO
|
|
.BR chgrp (1),
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.BR chmod (1),
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.BR find (1),
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.BR ln (1),
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.BR ls (1),
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.BR mv (1),
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.BR rm (1),
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.BR lchown (2),
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.BR link (2),
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|
.BR lstat (2),
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.BR readlink (2),
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|
.BR rename (2),
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|
.BR symlink (2),
|
|
.BR unlink (2),
|
|
.BR utimensat (2),
|
|
.BR lutimes (3),
|
|
.BR path_resolution (7)
|