Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
SYMLINK(7)	   FreeBSD 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
     dereferenced until	an object that is not a	symbolic link is found,	a sym-
     bolic 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 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
     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(2) 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 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
     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
     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
     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)

FreeBSD	6.0			March 31, 1994			   FreeBSD 6.0

NAME | SYMBOLIC LINK HANDLING | SEE ALSO

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=symlink&sektion=7&manpath=FreeBSD+6.0-RELEASE>

home | help