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SYMLINK(7) Linux Programmer's Manual SYMLINK(7)
NAME
symlink - symbolic link handling
SYMBOLIC LINK HANDLING
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 filename. (To be precise: each of the hard links to a file is a reference to the same i-node number, where an
i-node number is an index into the i-node table, which contains metadata about all files on a file system. See stat(2).)
Changes to a file are independent of the name used to reference the file. Hard links may not refer to directories (to
prevent the possibility of loops within the file system tree, which would confuse many programs) and may not refer to
files on different file systems (because i-node numbers are not unique across file systems).
A symbolic link is a special type of file whose contents are a string that is the pathname another file, the file to
which the link refers. In other words, a symbolic link is a pointer to another name, and not to an underlying object.
For this reason, symbolic links may refer to directories and may cross file system boundaries.
There is no requirement that the pathname referred to by a symbolic link should exist. A symbolic link that refers to a
pathname that does not exist is said to be a dangling link.
Because a symbolic link and its referenced object coexist in the file system name space, confusion can arise in distin-
guishing between the link itself and the referenced object. On historical systems, commands and system calls adopted
their own link-following conventions in a somewhat ad-hoc fashion. Rules for a more uniform approach, as they are imple-
mented on Linux and other systems, are outlined here. It is important that site-local applications also conform to these
rules, so that the user interface can be as consistent as possible.
Symbolic link ownership, permissions, and timestamps
The owner and group of an existing symbolic link can be changed using lchown(2). The only time that the ownership of a
symbolic link matters is when the link is being removed or renamed in a directory that has the sticky bit set (see
stat(2)).
The last access and last modification timestamps of a symbolic link can be changed using utimensat(2) or lutimes(3).
On Linux, the permissions of a symbolic link are not used in any operations; the permissions are always 0777 (read,
write, and execute for all user categories), and can't be changed.
Handling of symbolic links by system calls and commands
Symbolic links are handled either by operating on the link itself, or by operating on the object referred to by the link.
In the latter case, an application or system call is said to follow the link. Symbolic links may refer to other symbolic
links, in which case the links are dereferenced until an object that is not a symbolic link is found, a symbolic link
that refers to 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 filename 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 filename 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 referring to the file
afile.
Various system calls do not follow links, and operate on the symbolic link itself. They are: lchown(2), lgetxattr(2),
llistxattr(2), lremovexattr(2), lsetxattr(2), lstat(2), readlink(2), rename(2), rmdir(2), and unlink(2). Certain other
system calls optionally follow symbolic links. They are: faccessat(2), fchownat(2), fstatat(2), linkat(2), open(2), ope-
nat(2), and utimensat(2); see their manual pages for details. Because remove(3) is an alias for unlink(2), that library
function also does not follow symbolic links. When rmdir(2) is applied to a symbolic link, it fails with the error ENOT-
DIR. The link(2) warrants special discussion. POSIX.1-2001 specifies that link(2) should dereference oldpath if it is a
symbolic link. However, Linux does not do this. (By default Solaris is the same, but the POSIX.1-2001 specified behav-
ior can be obtained with suitable compiler options.) The upcoming POSIX.1 revision changes the specification to allow
either behavior in an implementation.
Commands not traversing a file tree
The second area is symbolic links, specified as command-line filename 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 sym-
bolic 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, which performs a tree traversal, is not. (The lat-
ter 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 referred to by
slink, while chown -h root slink would change the ownership of slink itself.
There are some 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(1) is not doing
a tree walk, i.e., the -R option is not specified), the ls(1) 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(1) command is the only com-
mand 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) command does not follow symbolic links named as
argument by default. The file(1) command does follow symbolic links named as argument if the -L option is specified.
Commands traversing a file tree
The following commands either optionally or always traverse file trees: chgrp(1), chmod(1), chown(1), cp(1), du(1),
find(1), ls(1), pax(1), rm(1), and tar(1).
It is important to realize that the following rules apply equally to symbolic links encountered during the file tree tra-
versal and symbolic links listed as command-line arguments.
The first rule applies to symbolic links that reference files other than directories. Operations that apply to symbolic
links are performed 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(1) affect the file referred to by slink.
The second rule applies to symbolic links that refer to directories. Symbolic links that refer to directories are never
followed by default. This is often referred to as a "physical" walk, as opposed to a "logical" walk (where symbolic
links the refer to directories are followed).
Certain conventions are (should be) followed as consistently as possible by commands that perform file tree walks:
* A command can be made to follow any symbolic links named on the command line, regardless of the type of file they ref-
erence, 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 for the purposes of both 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.
* A command can be made to follow any symbolic 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 referred to by slink. If slink refers
to a directory, 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.
* A command can be made to provide 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 addition, you may specify the -H, -L, and -P options more than once; the last one specified deter-
mines 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(1) command operates on the symbolic link, and not the file it references, and therefore never follows a symbolic
link. The rm(1) command does not support the -H, -L, or -P options.
* To maintain compatibility with historic systems, the ls(1) command acts a little differently. If you do not specify
the -F, -d or -l options, ls(1) will follow symbolic links specified on the command line. If the -L flag is specified,
ls(1) follows all symbolic links, regardless of their type, whether specified on the command line or encountered in the
tree walk.
SEE ALSO
chgrp(1), chmod(1), find(1), ln(1), ls(1), mv(1), rm(1), lchown(2), link(2), lstat(2), readlink(2), rename(2), sym-
link(2), unlink(2), utimensat(2), lutimes(3), path_resolution(7)
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about
reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-06-18 SYMLINK(7)
