symlink - 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
inode
number, where an inode number is an index into the inode table, which
contains metadata about all files on a filesystem. 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 filesystem tree, which would confuse many programs) and may not refer to
files on different filesystems (because inode numbers are not unique across
filesystems).
A symbolic link is a special type of file whose contents are a string that is
the pathname of another file, the file to which the link refers. (The contents
of a symbolic link can be read using
readlink(2).) 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
filesystem 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 filesystem 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.
There is a special class of symbolic-link-like objects known as "magic
links", which can be found in certain pseudofilesystems such as
proc(5) (examples include
/proc/[pid]/exe and
/proc/[pid]/fd/*). Unlike normal symbolic links, magic links are not
resolved through pathname-expansion, but instead act as direct references to
the kernel's own representation of a file handle. As such, these magic links
allow users to access files which cannot be referenced with normal paths (such
as unlinked files still referenced by a running program ).
Because they can bypass ordinary
mount_namespaces(7)-based restrictions,
magic links have been used as attack vectors in various exploits.
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 an ordinary 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.
However, magic links do not follow this rule. They can have a non-0777 mode,
though this mode is not currently used in any permission checks.
Using the combination of the
O_PATH and
O_NOFOLLOW flags to
open(2) yields a file descriptor that can be passed as the
dirfd
argument in system calls such as
fstatat(2),
fchownat(2),
fchmodat(2),
linkat(2), and
readlinkat(2), in order to
operate on the symbolic link itself (rather than the file to which it refers).
By default (i.e., if the
AT_SYMLINK_FOLLOW flag is not specified), if
name_to_handle_at(2) is applied to a symbolic link, it yields a handle
for the symbolic link (rather than the file to which it refers). One can then
obtain a file descriptor for the symbolic link (rather than the file to which
it refers) by specifying the
O_PATH flag in a subsequent call to
open_by_handle_at(2). Again, that file descriptor can be used in the
aforementioned system calls to operate on the symbolic link itself.
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:
- •
- Symbolic links used as filename arguments for system
calls.
- •
- Symbolic links specified as command-line arguments to
utilities that are not traversing a file tree.
- •
- 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).
Before describing the treatment of symbolic links by system calls and commands,
we require some terminology. Given a pathname of the form
a/b/c, the
part preceding the final slash (i.e.,
a/b) is called the
dirname
component, and the part following the final slash (i.e.,
c) is called
the
basename component.
The first area is symbolic links used as filename arguments for system calls.
The treatment of symbolic links within a pathname passed to a system call is as
follows:
- (1)
- Within the dirname component of a pathname, symbolic links
are always followed in nearly every system call. (This is also true for
commands.) The one exception is openat2(2), which provides flags
that can be used to explicitly prevent following of symbolic links in the
dirname component.
- (2)
- Except as noted below, all system calls follow symbolic
links in the basename component of a pathname. 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 in the basename component of a
pathname, 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 in the basename
component of a pathname. They are:
faccessat(2),
fchownat(2),
fstatat(2),
linkat(2),
name_to_handle_at(2),
open(2),
openat(2),
open_by_handle_at(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.
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.) POSIX.1-2008 changed the specification to allow either behavior in
an implementation.
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; for example, 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—for example, it is desired that
chown slink change the ownership of the file that
slink is,
whether it is a symbolic link or not—then 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—that is, -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.
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 that 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.
chgrp(1),
chmod(1),
find(1),
ln(1),
ls(1),
mv(1),
namei(1),
rm(1),
lchown(2),
link(2),
lstat(2),
readlink(2),
rename(2),
symlink(2),
unlink(2),
utimensat(2),
lutimes(3),
path_resolution(7)