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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 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 derefer-
     enced 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 num-
     ber 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 thirteen	system calls that do not follow	links, and which oper-
     ate on the	symbolic link itself.  They are: lchflags(2), lchmod(2),
     lchown(2),	lpathconf(2), lstat(2),	lutimes(2), readlink(2),
     readlinkat(2), rename(2), renameat(2), rmdir(2), unlink(2), and
     unlinkat(2).  Because remove(3) is	an alias for unlink(2),	it also	does
     not follow	symbolic links.	 When rmdir(2) or unlinkat(2) with the
     AT_REMOVEDIR flag is applied to a symbolic	link, it fails with the	error
     ENOTDIR.

     The linkat(2) system call does not	follow symbolic	links unless given the
     AT_SYMLINK_FOLLOW flag.

     The following system calls	follow symbolic	links unless given the
     AT_SYMLINK_NOFOLLOW flag: chflagsat(2), fchmodat(2), fchownat(2),
     fstatat(2)	and utimensat(2).

     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(2) system
     calls, respectively.  Of these, only the flags and	ownership are used by
     the system; the access permissions	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) be-
     came 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 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
     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	refer-
     enced by "slink", while "chown -h root slink" would change	the ownership
     of	"slink"	itself.

     There are five 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 -H or -L option is specified, or if the -F, -d or	-l op-
     tions 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) and stat(1) commands are also exceptions to this rule.	 These
     commands do not follow symbolic links named as argument by	default, but
     do	follow symbolic	links named as argument	if the -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
     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.

     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 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 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(2), readlink(2),	rename(2), symlink(2), unlink(2),
     fts(3), remove(3),	chown(8)

BSD			       February	16, 2015			   BSD

NAME | SYMBOLIC LINK HANDLING | SEE ALSO

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