SYMLINK(7) BSD Miscellaneous Information Manual SYMLINK(7)NAMEsymlink — symbolic link handling
DESCRIPTION
Symbolic links are files that act as pointers to other files. To under‐
stand 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 filesys‐
tem 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 applica‐
tions 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:
1. Symbolic links used as file name arguments for system calls.
2. Symbolic links specified as command line arguments to utili‐
ties 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 file name arguments for system
calls.
Except as noted below, all system calls follow symbolic links. For exam‐
ple, if there were a symbolic link "slink" which pointed to a file named
"afile", the system call "open("slink" ...)" would return a file descrip‐
tor to the file "afile".
There are eight 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), rename(2), and unlink(2).
Because remove(3) is an alias for unlink(2), it also does not follow sym‐
bolic links.
The 4.4BSD system differs from historical 4BSD systems in that the system
call chown(2) has been changed to follow symbolic links.
If the filesystem is mounted with the symperm mount(8) option, the sym‐
bolic 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 lat‐
ter 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 exam‐
ple, "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 three 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 4.4BSD system differs from historical 4BSD systems in that the chown,
chgrp and file 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 sym‐
bolic 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 per‐
formed 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 sym‐
bolic 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 sym‐
bolic 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 refer‐
ences. 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 addi‐
tion, 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 com‐
mand 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 never
follows symbolic links unless 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:
@{var1}-@{var2}.@{var3}
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 emula‐
tion. 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
(equivalent to the output of “uname -m” or sysctl(3)
“hw.machine”).
@machine_arch Expands to the value of MACHINE_ARCH for the system
(equivalent to the output of “uname -p” or sysctl(3)
“hw.machine_arch”).
@osrelease Expands to the operating system release of the run‐
ning 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 ALSOchflags(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.
BSD June 2, 2011 BSD
