NAME
symlink —
symbolic link handling
DESCRIPTION
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 file name. Changes to a file
are independent of the name used to reference the file. Hard links may not
refer to directories and may not reference files on different file systems.
A symbolic link contains the name of the file to which it is linked, i.e. it is
a pointer to another name, and not to an underlying object. For this reason,
symbolic links may reference directories and may span file systems.
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. Historically, commands and system calls have adopted their
own link following conventions in a somewhat ad-hoc fashion. Rules for more a
uniform approach, as they are implemented in this system, are outlined here.
It is important that local applications conform to these rules, too, so that
the user interface can be as consistent as possible.
Symbolic links are handled either by operating on the link itself, or by
operating on the object referenced by the link. In the latter case, an
application or system call is said to “follow” the link.
Symbolic links may reference other symbolic links, in which case the links are
dereferenced until an object that is not a symbolic link is found, a symbolic
link which references a file which doesn't 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 file
name 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).
System calls
The first area is symbolic links used as file name 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 to the file “afile”.
There are eleven system calls that do not follow links, and which operate on the
symbolic link itself. They are:
lchflags(2),
lchmod(2),
lchown(2),
lstat(2),
lutimes(2),
readlink(2),
readlinkat(2),
rename(2),
renameat(2),
unlinkat(2). and
unlink(2). Because
remove(3) is an alias for
unlink(2), it also does not
follow symbolic links. When
rmdir(2) or
unlinkat(2) with the
AT_REMOVEDIR
flag is applied to a symbolic link, it
fails with the error
ENOTDIR
.
The
linkat(2) system call does not
follow symbolic links unless given the
AT_SYMLINK_FOLLOW
flag.
The following system calls follow symbolic links unless given the
AT_SYMLINK_NOFOLLOW
flag:
fchmodat(2),
fchownat(2),
fstatat(2), and
utimensat(2).
The owner and group of an existing symbolic link can be changed by means of the
lchown(2) system call. The
flags, access permissions, owner/group and modification time of an existing
symbolic link can be changed by means of the
lchflags(2),
lchmod(2),
lchown(2), and
lutimes(2) system calls,
respectively. Of these, only the flags and ownership are used by the system;
the access permissions are ignored.
The
4.4BSD system differs from historical
4BSD systems in that the system call
chown(2) has been changed to
follow symbolic links. The
lchown(2) system call was added
later when the limitations of the new
chown(2) became apparent.
If the filesystem is mounted with the
symperm
mount(8) option, the symbolic
link file permission bits have the following effects:
The
readlink(2) system call
requires read permissions on the symbolic link.
System calls that follow symbolic links will fail without execute/search
permissions on all the symbolic links followed.
The write, sticky, set-user-ID-on-execution and set-group-ID-on-execution
symbolic link mode bits have no effect on any system calls (including
execve(2)).
Commands not traversing
a file tree
The second area is symbolic links, specified as command line file name
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
” 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
“
file slink
” display the type of file that
“
slink
” is, whether it is a symbolic link
or not, the
-h option should be used. In the above example,
“
file slink
” would report the type of the
file referenced by “
slink
”, while
“
file -h slink
” would report that
“
slink
” was a symbolic link.
There are five 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 is not doing a tree walk, i.e. the
-R
option is not specified), the
ls command follows symbolic
links named as arguments if the
-L option is specified, or
if the
-F,
-d, or
-l
options are not specified. (If the
-L option is specified,
ls always follows symbolic links.
ls is
the only command where the
-L option affects its behavior
even though it is not doing a walk of a file tree).
The
file(1) and
stat(1) commands are also
exceptions to this rule. These commands do not follow symbolic links named as
argument by default, but do follow symbolic links named as argument if the
-L option is specified.
The
4.4BSD system differs from historical
4BSD systems in that the
chown and
chgrp commands follow symbolic links specified on the
command line.
Commands traversing a file
tree
The following commands either optionally or always traverse file trees:
chflags(1),
chgrp(1),
chmod(1),
cp(1),
du(1),
find(1),
ls(1),
pax(1),
rm(1),
tar(1), and
chown(8).
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 that are not of
type directory. Operations that apply to symbolic links are performed on the
links themselves, but otherwise the links are ignored.
For example, the command “
chown -R user slink
directory
” will ignore
“
slink
”, because the
-h
flag must be used to change owners of symbolic links. Any symbolic links
encountered during the tree traversal will also be 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 either
chown or
rm affect the file which
“
slink
” references in any way.
The second rule applies to symbolic links that reference files of type
directory. Symbolic links which reference files of type directory are never
“followed” by default. This is often referred to as a
“physical” walk, as opposed to a “logical” walk (where
symbolic links referencing directories are followed).
As consistently as possible, you can make commands doing a file tree walk 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 both for the purposes of 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.
As consistently as possible, you can make commands doing a file tree walk 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 referenced by
“
slink
”. If
“
slink
” references 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
”.
As consistently as possible, you can specify 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 command operates on the symbolic link,
and not the file it references, and therefore never follows a symbolic link.
The
rm command does not support the
-H,
-L, or
-P options.
To maintain compatibility with historic systems, the
ls
command acts a little differently. If you do not specify the
-F,
-d, or
-l options,
ls will follow symbolic links specified on the command line.
If the
-L flag is specified. If the
-L
flag is specified,
ls follows all symbolic links, regardless
of their type, whether specified on the command line or encountered in the
tree walk. The
ls command does not support the
-H or
-P options.
Magic symlinks
So-called “magic symlinks” can be enabled by setting the
“vfs.generic.magiclinks” variable with
sysctl(8). When magic symlinks
are enabled “magic” patterns in symlinks are expanded. Those
patterns begin with “@” (an at-sign), and end at the end of the
pathname component (i.e. at the next “/”, or at the end of the
symbolic link if there are no more slashes).
To illustrate the pattern matching rules, assume that “@foo” is a
valid magic string:
- @foo
- would be matched
- @foo/bar
- would be matched
- bar@foo
- would be matched
- @foobar
- would not be matched
Magic strings may also be delimited with ‘{’ and ‘}’
characters, allowing for more complex patterns in symbolic links such as:
The following patterns are supported:
-
-
- @domainname
- Expands to the machine's domain name, as set by
setdomainname(3).
-
-
- @hostname
- Expands to the machine's host name, as set by
sethostname(3).
-
-
- @emul
- Expands to the name of the current process's emulation.
Defaults to
netbsd
. Other valid emulations are:
aout
, aoutm68k
,
darwin
, freebsd
,
ibcs2
, linux
,
linux32
, m68k4k
,
netbsd32
, osf1
,
sunos
, sunos32
,
svr4
, svr4_32
,
ultrix
, vax1k
.
-
-
- @kernel_ident
- Expands to the name of the
config(1) file used to
generate the running kernel. For example
GENERIC
.
-
-
- @machine
- Expands to the value of
MACHINE
for
the system. For native binaries, this is equivalent to the output of
“uname -m” or
sysctl(3)
“hw.machine”. (For non-native binaries, the values returned by
uname and sysctl typically vary to match the emulation environment.)
-
-
- @machine_arch
- Expands to the value of
MACHINE_ARCH
for the system. For native binaries,
this is equivalent to the output of “uname -p” or
sysctl(3)
“hw.machine_arch”. (For non-native binaries, the values
returned by uname and sysctl typically vary to match the emulation
environment.)
-
-
- @osrelease
- Expands to the operating system release of the running
kernel (equivalent to the output of “uname -r” or
sysctl(3)
“kern.osrelease”).
-
-
- @ostype
- Expands to the operating system type of the running kernel
(equivalent to the output of “uname -s” or
sysctl(3)
“kern.ostype”). This will always be “NetBSD” on
NetBSD systems.
-
-
- @ruid
- Expands to the real user-id of the process.
-
-
- @uid
- Expands to the effective user-id of the process.
-
-
- @rgid
- Expands to the real group-id of the process.
-
-
- @gid
- Expands to the effective group-id of the process.
SEE ALSO
chflags(1),
chgrp(1),
chmod(1),
cp(1),
du(1),
find(1),
ln(1),
ls(1),
mv(1),
pax(1),
rm(1),
tar(1),
uname(1),
chown(2),
execve(2),
lchflags(2),
lchmod(2),
lchown(2),
lstat(2),
lutimes(2),
mount(2),
readlink(2),
rename(2),
symlink(2),
unlink(2),
fts(3),
remove(3),
chown(8),
mount(8)
HISTORY
Magic symlinks appeared in
NetBSD 4.0.