diff options
Diffstat (limited to 'file_cmds/ln/symlink.7')
| -rw-r--r-- | file_cmds/ln/symlink.7 | 457 |
1 files changed, 457 insertions, 0 deletions
diff --git a/file_cmds/ln/symlink.7 b/file_cmds/ln/symlink.7 new file mode 100644 index 0000000..7c6c757 --- /dev/null +++ b/file_cmds/ln/symlink.7 @@ -0,0 +1,457 @@ +.\"- +.\" Copyright (c) 1992, 1993, 1994 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)symlink.7 8.3 (Berkeley) 3/31/94 +.\" $FreeBSD: src/bin/ln/symlink.7,v 1.30 2005/02/13 22:25:09 ru Exp $ +.\" +.Dd March 31, 1994 +.Dt SYMLINK 7 +.Os +.Sh NAME +.Nm symlink +.Nd symbolic link handling +.Sh SYMBOLIC LINK HANDLING +Symbolic links are files that act as pointers to other files. +To understand their behavior, you must first understand how hard links +work. +A hard link to a file is indistinguishable from the original file because +it is a reference to the object underlying the original file name. +Changes to a file are independent of the name used to reference the +file. +Hard links may not refer to directories and may not reference files +on different file systems. +A symbolic link contains the name of the file to which it is linked, +i.e., it is a pointer to another name, and not to an underlying object. +For this reason, symbolic links may reference directories and may span +file systems. +.Pp +Because a symbolic link and its referenced object coexist in the file system +name space, confusion can arise in distinguishing between the link itself +and the referenced object. +Historically, commands and system calls have adopted their own link +following conventions in a somewhat ad-hoc fashion. +Rules for more a uniform approach, as they are implemented in this system, +are outlined here. +It is important that local applications conform to these rules, too, +so that the user interface can be as consistent as possible. +.Pp +Symbolic links are handled either by operating on the link itself, +or by operating on the object referenced by the link. +In the latter case, +an application or system call is said to +.Dq follow +the link. +Symbolic links may reference other symbolic links, +in which case the links are dereferenced until an object that is +not a symbolic link is found, +a symbolic link which references a file which does not exist is found, +or a loop is detected. +(Loop detection is done by placing an upper limit on the number of +links that may be followed, and an error results if this limit is +exceeded.) +.Pp +There are three separate areas that need to be discussed. +They are as follows: +.Pp +.Bl -enum -compact -offset indent +.It +Symbolic links used as file name arguments for system calls. +.It +Symbolic links specified as command line arguments to utilities that +are not traversing a file tree. +.It +Symbolic links encountered by utilities that are traversing a file tree +(either specified on the command line or encountered as part of the +file hierarchy walk). +.El +.Ss System calls. +The first area is symbolic links used as file name arguments for +system calls. +.Pp +Except as noted below, all system calls follow symbolic links. +For example, if there were a symbolic link +.Dq Li slink +which pointed to a file named +.Dq Li afile , +the system call +.Dq Li open("slink" ...\&) +would return a file descriptor to the file +.Dq afile . +.Pp +There are nine system calls that do not follow links, and which operate +on the symbolic link itself. +They are: +.Xr lchflags 2 , +.Xr lchmod 2 , +.Xr lchown 2 , +.Xr lstat 2 , +.Xr lutimes 3 , +.Xr readlink 2 , +.Xr rename 2 , +.Xr rmdir 2 , +and +.Xr unlink 2 . +Because +.Xr remove 3 +is an alias for +.Xr unlink 2 , +it also does not follow symbolic links. +When +.Xr rmdir 2 +is applied to a symbolic link, it fails with the error +.Er ENOTDIR . +.Pp +The owner and group of an existing symbolic link can be changed by +means of the +.Xr lchown 2 +system call. +The flags, access permissions, owner/group and modification time of +an existing symbolic link can be changed by means of the +.Xr lchflags 2 , +.Xr lchmod 2 , +.Xr lchown 2 , +and +.Xr lutimes 3 +system calls, respectively. +Of these, only the flags are used by the system; +the access permissions and ownership are ignored. +.Pp +The +.Bx 4.4 +system differs from historical +.Bx 4 +systems in that the system call +.Xr chown 2 +has been changed to follow symbolic links. +The +.Xr lchown 2 +system call was added later when the limitations of the new +.Xr chown 2 +became apparent. +.Ss Commands not traversing a file tree. +The second area is symbolic links, specified as command line file +name arguments, to commands which are not traversing a file tree. +.Pp +Except as noted below, commands follow symbolic links named as command +line arguments. +For example, if there were a symbolic link +.Dq Li slink +which pointed to a file named +.Dq Li afile , +the command +.Dq Li cat slink +would display the contents of the file +.Dq Li afile . +.Pp +It is important to realize that this rule includes commands which may +optionally traverse file trees, e.g.\& the command +.Dq Li "chown file" +is included in this rule, while the command +.Dq Li "chown -R file" +is not. +(The latter is described in the third area, below.) +.Pp +If it is explicitly intended that the command operate on the symbolic +link instead of following the symbolic link, e.g., it is desired that +.Dq Li "chown slink" +change the ownership of the file that +.Dq Li slink +is, whether it is a symbolic link or not, the +.Fl h +option should be used. +In the above example, +.Dq Li "chown root slink" +would change the ownership of the file referenced by +.Dq Li slink , +while +.Dq Li "chown -h root slink" +would change the ownership of +.Dq Li slink +itself. +.Pp +There are four exceptions to this rule. +The +.Xr mv 1 +and +.Xr rm 1 +commands do not follow symbolic links named as arguments, +but respectively attempt to rename and delete them. +(Note, if the symbolic link references a file via a relative path, +moving it to another directory may very well cause it to stop working, +since the path may no longer be correct.) +.Pp +The +.Xr ls 1 +command is also an exception to this rule. +For compatibility with historic systems (when +.Nm ls +is not doing a tree walk, i.e., the +.Fl R +option is not specified), +the +.Nm ls +command follows symbolic links named as arguments if the +.Fl H +or +.Fl L +option is specified, +or if the +.Fl F , +.Fl d +or +.Fl l +options are not specified. +(The +.Nm ls +command is the only command where the +.Fl H +and +.Fl L +options affect its behavior even though it is not doing a walk of +a file tree.) +.Pp +The +.Xr file 1 +command is also an exception to this rule. +The +.Xr file 1 +command does not follow symbolic links named as argument by default. +The +.Xr file 1 +command does follow symbolic links named as argument if +.Fl L +option is specified. +.Pp +The +.Bx 4.4 +system differs from historical +.Bx 4 +systems in that the +.Nm chown +and +.Nm chgrp +commands follow symbolic links specified on the command line. +.Ss Commands traversing a file tree. +The following commands either optionally or always traverse file trees: +.Xr chflags 1 , +.Xr chgrp 1 , +.Xr chmod 1 , +.Xr cp 1 , +.Xr du 1 , +.Xr find 1 , +.Xr ls 1 , +.Xr pax 1 , +.Xr rm 1 , +.Xr tar 1 +and +.Xr chown 8 . +.Pp +It is important to realize that the following rules apply equally to +symbolic links encountered during the file tree traversal and symbolic +links listed as command line arguments. +.Pp +The first rule applies to symbolic links that reference files that are +not of type directory. +Operations that apply to symbolic links are performed on the links +themselves, but otherwise the links are ignored. +.Pp +The command +.Dq Li "rm -r slink directory" +will remove +.Dq Li slink , +as well as any symbolic links encountered in the tree traversal of +.Dq Li directory , +because symbolic links may be removed. +In no case will +.Nm rm +affect the file which +.Dq Li slink +references in any way. +.Pp +The second rule applies to symbolic links that reference files of type +directory. +Symbolic links which reference files of type directory are never +.Dq followed +by default. +This is often referred to as a +.Dq physical +walk, as opposed to a +.Dq logical +walk (where symbolic links referencing directories are followed). +.Pp +As consistently as possible, you can make commands doing a file tree +walk follow any symbolic links named on the command line, regardless +of the type of file they reference, by specifying the +.Fl H +(for +.Dq half\-logical ) +flag. +This flag is intended to make the command line name space look +like the logical name space. +(Note, for commands that do not always do file tree traversals, the +.Fl H +flag will be ignored if the +.Fl R +flag is not also specified.) +.Pp +For example, the command +.Dq Li "chown -HR user slink" +will traverse the file hierarchy rooted in the file pointed to by +.Dq Li slink . +Note, the +.Fl H +is not the same as the previously discussed +.Fl h +flag. +The +.Fl H +flag causes symbolic links specified on the command line to be +dereferenced both for the purposes of the action to be performed +and the tree walk, and it is as if the user had specified the +name of the file to which the symbolic link pointed. +.Pp +As consistently as possible, you can make commands doing a file tree +walk follow any symbolic links named on the command line, as well as +any symbolic links encountered during the traversal, regardless of +the type of file they reference, by specifying the +.Fl L +(for +.Dq logical ) +flag. +This flag is intended to make the entire name space look like +the logical name space. +(Note, for commands that do not always do file tree traversals, the +.Fl L +flag will be ignored if the +.Fl R +flag is not also specified.) +.Pp +For example, the command +.Dq Li "chown -LR user slink" +will change the owner of the file referenced by +.Dq Li slink . +If +.Dq Li slink +references a directory, +.Nm chown +will traverse the file hierarchy rooted in the directory that it +references. +In addition, if any symbolic links are encountered in any file tree that +.Nm chown +traverses, they will be treated in the same fashion as +.Dq Li slink . +.Pp +As consistently as possible, you can specify the default behavior by +specifying the +.Fl P +(for +.Dq physical ) +flag. +This flag is intended to make the entire name space look like the +physical name space. +.Pp +For commands that do not by default do file tree traversals, the +.Fl H , +.Fl L +and +.Fl P +flags are ignored if the +.Fl R +flag is not also specified. +In addition, you may specify the +.Fl H , +.Fl L +and +.Fl P +options more than once; the last one specified determines the +command's behavior. +This is intended to permit you to alias commands to behave one way +or the other, and then override that behavior on the command line. +.Pp +The +.Xr ls 1 +and +.Xr rm 1 +commands have exceptions to these rules. +The +.Nm rm +command operates on the symbolic link, and not the file it references, +and therefore never follows a symbolic link. +The +.Nm rm +command does not support the +.Fl H , +.Fl L +or +.Fl P +options. +.Pp +To maintain compatibility with historic systems, +the +.Nm ls +command acts a little differently. +If you do not specify the +.Fl F , +.Fl d +or +.Fl l +options, +.Nm ls +will follow symbolic links specified on the command line. +If the +.Fl L +flag is specified, +.Nm ls +follows all symbolic links, +regardless of their type, +whether specified on the command line or encountered in the tree walk. +.Sh SEE ALSO +.Xr chflags 1 , +.Xr chgrp 1 , +.Xr chmod 1 , +.Xr cp 1 , +.Xr du 1 , +.Xr find 1 , +.Xr ln 1 , +.Xr ls 1 , +.Xr mv 1 , +.Xr pax 1 , +.Xr rm 1 , +.Xr tar 1 , +.Xr lchflags 2 , +.Xr lchmod 2 , +.Xr lchown 2 , +.Xr lstat 2 , +.Xr lutimes 3 , +.Xr readlink 2 , +.Xr rename 2 , +.Xr symlink 2 , +.Xr unlink 2 , +.Xr fts 3 , +.Xr remove 3 , +.Xr chown 8 |