symlink - symbolic link handling
Symbolic links are files that act as pointers to other
files. To understand
their behavior, it is necessary to 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 a 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 a more 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, with an error resulting 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 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 [Toc] [Back]
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 at least five system calls that do not follow
links, and which
operate on the symbolic link itself. They are: lchown(2),
lstat(2),
readlink(2), rename(2), and unlink(2). Because remove(3) is
an alias for
unlink(2), it also does not follow symbolic links.
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.
Commands not traversing a file tree [Toc] [Back]
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 commandline
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 owner file'' is
included in this rule, while the command ``chown -R owner
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
``chown owner slink'' change the ownership of ``slink'', not
of what it
points to -- the -h option should be used. In the above example, ``chown
owner slink'' would change the owner of ``afile'' to ``owner'', while
``chown -h owner slink'' would change the ownership of
``slink''.
There are several 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 that 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. The -L option affects its behavior even
though it is not
doing a walk of a file tree.)
The file(1) command behaves as ls(1) in that the -L option
makes it follow
a symbolic link. By default, ``file slink'' will report
that
``slink'' is a symbolic link. This behavior is different
from file(1) on
some other systems, where the -h convention is followed.
The 4.4BSD system differs from historical 4BSD systems in
that the
chown(8), chgrp(1), and file(1) commands follow symbolic
links specified
on the command line (unless the -h option is used).
Commands traversing a file tree [Toc] [Back]
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 option was not given. 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(8) or
rm(1) follow
the symlink to affect the file which ``slink'' references.
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, it is possible to 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''. 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, it is possible to 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, it is possible to 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,
the -H, -L, and -P options may be specified more than
once; the
last one specified determines the command's behavior. This
is intended
to permit aliasing 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 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.
chflags(1), chgrp(1), chmod(1), cp(1), du(1), find(1),
ln(1), ls(1),
mv(1), pax(1), rm(1), tar(1), lchown(2), lstat(2), readlink(2),
rename(2), symlink(2), unlink(2), fts(3), remove(3),
chown(8)
OpenBSD 3.6 January 25, 1997
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