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NetBSD 6.1.5 - man page for symlink (netbsd section 7)

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

NAME
     symlink -- symbolic link handling

DESCRIPTION
     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 refer-
     ence 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 ref-
     erence directories and may span file systems.

     Because a symbolic link and its referenced object coexist in the filesystem name space, con-
     fusion can arise in distinguishing between the link itself and the referenced object.  His-
     torically, 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 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 symbolic 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 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 eight 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), and unlink(2).  Because remove(3) is an alias for unlink(2), it also
     does not follow symbolic links.

     The 4.4BSD system differs from historical 4BSD systems in that the system call chown(2) has
     been changed to follow symbolic links.

     If the filesystem is mounted with the symperm mount(8) option, the symbolic link file per-
     mission bits have the following effects:

     The readlink(2) system call requires read permissions on the symbolic link.

     System calls that follow symbolic links will fail without execute/search permissions on all
     the symbolic links followed.

     The write, sticky, set-user-ID-on-execution and set-group-ID-on-execution symbolic link mode
     bits have no effect on any system calls (including execve(2)).

   Commands not traversing a file tree
     The second area is symbolic links, specified as command line file name arguments, to com-
     mands 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 fol-
     lowing the symbolic link, e.g., it is desired that "file slink" display the type of file
     that "slink" is, whether it is a symbolic link or not, the -h option should be used.  In the
     above example, "file slink" would report the type of the file referenced by "slink", while
     "file -h slink" would report that "slink" was a symbolic link.

     There are three exceptions to this rule.  The mv(1) and rm(1) commands do not follow sym-
     bolic 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 sys-
     tems (when ls is not doing a tree walk, i.e.  the -R option is not specified), the ls com-
     mand follows symbolic links named as arguments if the -L option is specified, or if the -F,
     -d or -l options are not specified.  (If the -L option is specified, ls always follows sym-
     bolic links.  ls is the only command where the -L option affects its behavior even though it
     is not doing a walk of a file tree).

     The 4.4BSD system differs from historical 4BSD systems in that the chown, chgrp and file
     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 symbolic links encoun-
     tered 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 direc-
     tory.  Operations that apply to symbolic links are performed on the links themselves, but
     otherwise the links are ignored.

     For example, the command "chown -R user slink directory" will ignore "slink", because the -h
     flag must be used to change owners of symbolic links.  Any symbolic links encountered during
     the tree traversal will also be 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 either chown or 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 sym-
     bolic 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 sym-
     bolic 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 encoun-
     tered 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 physi-
     cal 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 commands to behave one way or the other, and then over-
     ride 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 never follows symbolic links
     unless the -L flag is specified.  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.  The ls command does not support the -H or -P options.

   Magic symlinks
     So-called ``magic symlinks'' can be enabled by setting the ``vfs.generic.magiclinks'' vari-
     able with sysctl(8).  When magic symlinks are enabled ``magic'' patterns in symlinks are
     expanded.	Those patterns begin with ``@'' (an at-sign), and end at the end of the pathname
     component (i.e. at the next ``/'', or at the end of the symbolic link if there are no more
     slashes).

     To illustrate the pattern matching rules, assume that ``@foo'' is a valid magic string:

	   @foo 	  would be matched
	   @foo/bar	  would be matched
	   bar@foo	  would be matched
	   @foobar	  would not be matched

     Magic strings may also be delimited with '{' and '}' characters, allowing for more complex
     patterns in symbolic links such as:

	   @{var1}-@{var2}.@{var3}

     The following patterns are supported:

	   @domainname	  Expands to the machine's domain name, as set by setdomainname(3).

	   @hostname	  Expands to the machine's host name, as set by sethostname(3).

	   @emul	  Expands to the name of the current process's emulation.  Defaults to
			  netbsd.  Other valid emulations are: aout, aoutm68k, darwin, freebsd,
			  ibcs2, linux, linux32, m68k4k, netbsd32, osf1, sunos, sunos32, svr4,
			  svr4_32, ultrix, vax1k.

	   @kernel_ident  Expands to the name of the config(1) file used to generate the running
			  kernel.  For example GENERIC.

	   @machine	  Expands to the value of MACHINE for the system (equivalent to the
			  output of ``uname -m'' or sysctl(3) ``hw.machine'').

	   @machine_arch  Expands to the value of MACHINE_ARCH for the system (equivalent to the
			  output of ``uname -p'' or sysctl(3) ``hw.machine_arch'').

	   @osrelease	  Expands to the operating system release of the running kernel
			  (equivalent to the output of ``uname -r'' or sysctl(3)
			  ``kern.osrelease'').

	   @ostype	  Expands to the operating system type of the running kernel (equivalent
			  to the output of ``uname -s'' or sysctl(3) ``kern.ostype'').	This will
			  always be ``NetBSD'' on NetBSD systems.

	   @ruid	  Expands to the real user-id of the process.

	   @uid 	  Expands to the effective user-id of the process.

	   @rgid	  Expands to the real group-id of the process.

	   @gid 	  Expands to the effective group-id of the process.

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), uname(1), chown(2), execve(2), lchflags(2), lchmod(2), lchown(2), lstat(2),
     lutimes(2), mount(2), readlink(2), rename(2), symlink(2), unlink(2), fts(3), remove(3),
     chown(8), mount(8)

HISTORY
     Magic symlinks appeared in NetBSD 4.0.

BSD					   June 2, 2011 				      BSD


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