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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 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 directories and may span	file
     systems.

     Because a symbolic	link and its referenced	object coexist in the filesys-
     tem 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 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 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 descrip-
     tor 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 sym-
     bolic 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	sym-
     bolic link	file permission	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 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 op-
     tionally 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 de-
     scribed 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
     "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 exam-
     ple, "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	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 op-
     tion is not specified), the ls command 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
     symbolic 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 sym-
     bolic 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.  Operations	that apply to symbolic links are per-
     formed 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	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 sym-
     bolic 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 directory, chown
     will traverse the file hierarchy rooted in	the directory that it refer-
     ences.  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 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" variable 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 emula-
			  tion.	 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 run-
			  ning 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

NAME | DESCRIPTION | SEE ALSO | HISTORY

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