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SYMLINK(7)           BSD Miscellaneous Information Manual           SYMLINK(7)

NAME
     symlink -- symbolic link handling

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.

     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.

     Symbolic links are handled either by operating on the link itself, or by operating on the object refer-enced referenced
     enced by the link.  In the latter case, an application or system call is said to ``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.)

     There are three separate areas that need to be discussed.  They are as follows:

           1.   Symbolic links used as file name arguments for system calls.
           2.   Symbolic links specified as command line arguments to utilities that are not traversing a
                file tree.
           3.   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).

   System calls.
     The first area is symbolic links used as file name arguments for system calls.

     Except as noted below, all system calls follow symbolic links.  For example, if there were a symbolic
     link ``slink'' which pointed to a file named ``afile'', the system call ``open("slink" ...)'' would
     return a file descriptor to the file ``afile''.

     There are nine system calls that do not follow links, and which operate on the symbolic link itself.
     They are: lchflags(2), lchmod(2), lchown(2), lstat(2), lutimes(3), readlink(2), rename(2), rmdir(2),
     and unlink(2).  Because remove(3) is an alias for unlink(2), it also does not follow symbolic links.
     When rmdir(2) is applied to a symbolic link, it fails with the error ENOTDIR.

     The owner and group of an existing symbolic link can be changed by means of the 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 lchflags(2), lchmod(2), lchown(2), and lutimes(3) system calls, respectively.
     Of these, only the flags are used by the system; the access permissions and ownership are ignored.

     The 4.4BSD system differs from historical 4BSD systems in that the system call chown(2) has been
     changed to follow symbolic links.  The lchown(2) system call was added later when the limitations of
     the new chown(2) became apparent.

   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.

     Except as noted below, commands follow symbolic links named as command line arguments.  For example, if
     there were a symbolic link ``slink'' which pointed to a file named ``afile'', the command ``cat slink''
     would display the contents of the file ``afile''.

     It is important to realize that this rule includes commands which may optionally traverse file trees,
     e.g. the command ``chown file'' is included in this rule, while the command ``chown -R file'' is not.
     (The latter is described in the third area, below.)

     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 ``chown slink'' change the ownership of the file that ``slink''
     is, whether it is a symbolic link or not, the -h option should be used.  In the above example, ``chown
     root slink'' would change the ownership of the file referenced by ``slink'', while ``chown -h root
     slink'' would change the ownership of ``slink'' itself.

     There are four exceptions to this rule.  The mv(1) and 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.)

     The ls(1) command is also an exception to this rule.  For compatibility with historic systems (when ls
     is not doing a tree walk, i.e., the -R option is not specified), the ls command follows symbolic links
     named as arguments if the -H or -L option is specified, or if the -F, -d or -l options are not speci-fied. specified.
     fied.  (The ls command is the only command where the -H and -L options affect its behavior even though
     it is not doing a walk of a file tree.)

     The file(1) command is also an exception to this rule.  The file(1) command does not follow symbolic
     links named as argument by default.  The file(1) command does follow symbolic links named as argument
     if -L option is specified.

     The 4.4BSD system differs from historical 4BSD systems in that the chown and chgrp commands follow sym-bolic symbolic
     bolic links specified on the command line.

   Commands traversing a file tree.
     The following commands either optionally or always traverse file trees: chflags(1), chgrp(1), chmod(1),
     cp(1), du(1), find(1), ls(1), pax(1), rm(1), tar(1) and chown(8).

     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.

     The first rule applies to symbolic links that reference files that are not of type directory.  Opera-tions Operations
     tions that apply to symbolic links are performed on the links themselves, but otherwise the links are
     ignored.

     The command ``rm -r slink directory'' will remove ``slink'', as well as any symbolic links encountered
     in the tree traversal of ``directory'', because symbolic links may be removed.  In no case will rm
     affect the file which ``slink'' references in any way.

     The second rule applies to symbolic links that reference files of type directory.  Symbolic links which
     reference files of type directory are never ``followed'' by default.  This is often referred to as a
     ``physical'' walk, as opposed to a ``logical'' walk (where symbolic links referencing directories are
     followed).

     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 -H (for
     ``half-logical'') flag.  This flag is intended to make the command line name space look like the logi-cal logical
     cal name space.  (Note, for commands that do not always do file tree traversals, the -H flag will be
     ignored if the -R flag is not also specified.)

     For example, the command ``chown -HR user slink'' will traverse the file hierarchy rooted in the file
     pointed to by ``slink''.  Note, the -H is not the same as the previously discussed -h flag.  The -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.

     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 -L (for ``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 -L flag will be ignored if the -R flag is not also specified.)

     For example, the command ``chown -LR user slink'' will change the owner of the file referenced by
     ``slink''.  If ``slink'' references a directory, 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
     chown traverses, they will be treated in the same fashion as ``slink''.

     As consistently as possible, you can specify the default behavior by specifying the -P (for
     ``physical'') flag.  This flag is intended to make the entire name space look like the physical name
     space.

     For commands that do not by default do file tree traversals, the -H, -L and -P flags are ignored if the
     -R flag is not also specified.  In addition, you may specify the -H, -L and -P options more than once;
     the last one specified determines the command's behavior.  This is intended to permit you to alias com-mands commands
     mands to behave one way or the other, and then override that behavior on the command line.

     The ls(1) and rm(1) commands have exceptions to these rules.  The rm command operates on the symbolic
     link, and not the file it references, and therefore never follows a symbolic link.  The rm command does
     not support the -H, -L or -P options.

     To maintain compatibility with historic systems, the ls command acts a little differently.  If you do
     not specify the -F, -d or -l options, ls will follow symbolic links specified on the command line.  If
     the -L flag is specified, ls follows all symbolic links, regardless of their type, whether specified on
     the command line or encountered in the tree walk.

SEE ALSO
     chflags(1), chgrp(1), chmod(1), cp(1), du(1), find(1), ln(1), ls(1), mv(1), pax(1), rm(1), tar(1),
     lchflags(2), lchmod(2), lchown(2), lstat(2), lutimes(3), readlink(2), rename(2), symlink(2), unlink(2),
     fts(3), remove(3), chown(8)

BSD                             March 31, 1994                             BSD

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