|
NAME | DESCRIPTION | SEE ALSO | COLOPHON |
SYMLINK(7) Linux Programmer's Manual SYMLINK(7)
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.
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. (Note that there are
some "magic" symbolic links in the /proc directory tree—for example,
the /proc/[pid]/fd/* files—that have different permissions.)
Obtaining a file descriptor that refers to a symbolic link
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.
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),
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.
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; 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—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.
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 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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 SYMLINK(7)
Pages that refer to this page: namei(1), tar(1), access(2), chmod(2), chown(2), getxattr(2), intro(2), link(2), listxattr(2), open(2), readlink(2), removexattr(2), rename(2), setxattr(2), stat(2), statx(2), symlink(2), unlink(2), utimensat(2), futimes(3), remove(3), inode(7), path_resolution(7)