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 distinguishing 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 implemented 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), openat(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 ENOTDIR. 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 behavior 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 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, which performs
a tree traversal, 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
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 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)
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 traversal 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 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 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 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(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), symlink(2), unlink(2),
utimensat(2), lutimes(3), path_resolution(7)
COLOPHON
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