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¶
This page is part of release 3.44 of the Linux
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description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.