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symlink(7)

NAME

     symlink -- symbolic link handling

SYMBOLIC LINK HANDLING

     Symbolic links are files that act as pointers to other files.  To under-
     stand 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 direc-
     tories 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 applica-
     tions 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 referenced 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 sym-
     bolic link which references a file which doesn't 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 utili-
		ties 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 exam-
     ple, 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(2), readlink(2), rename(2), rmdir(2), and
     unlink(2).  Because remove(3) is an alias for unlink(2), it also does not
     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 specified.  (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) com-
     mand 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 symbolic 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 sym-
     bolic links encountered during the file tree traversal and symbolic links
     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 logical 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 sym-
     bolic 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 direc-
     tory, 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 addi-
     tion, 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 commands 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 com-
     mand 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
     fts(3), remove(3), chown(8)

FreeBSD 5.4			March 31, 1994			   FreeBSD 5.4

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