symlink.7

早期freebsd实现

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.\"	@(#)symlink.7	8.3 (Berkeley) 3/31/94
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.Dd March 31, 1994
.Dt SYMLINK 7
.Os BSD 4
.Sh NAME
.Nm symlink
.Nd symbolic link handling
.Sh 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 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.
.Pp
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.
.Pp
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
.Dq 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.)
.Pp
There are three separate areas that need to be discussed.
They are as follows:
.sp
.Bl -enum -compact -offset indent
.It
Symbolic links used as file name arguments for system calls.
.It
Symbolic links specified as command line arguments to utilities that
are not traversing a file tree.
.It
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).
.El
.Ss System calls.
The first area is symbolic links used as file name arguments for
system calls.
.Pp
Except as noted below, all system calls follow symbolic links.
For example, if there were a symbolic link
.Dq Li slink
which pointed to a file named
.Dq Li afile ,
the system call
.Dq Li open("slink" ...)
would return a file descriptor to the file
.Dq afile .
.Pp
There are four system calls that do not follow links, and which operate
on the symbolic link itself.
They are:
.Xr lstat 2 ,
.Xr readlink 2 ,
.Xr rename 2 ,
and
.Xr unlink 2 .
Because
.Xr remove 3
is an alias for
.Xr unlink 2 ,
it also does not follow symbolic links.
.Pp
Unlike other filesystem objects, symbolic links do not have an owner,
group, permissions, access and modification times, etc.
The only attributes returned from an
.Xr lstat 2
that refer to the symbolic link itself are the file type (S_IFLNK),
size, blocks, and link count (always 1).
The other attributes are filled in from the directory that contains
the link.
For portability reasons, you should be aware that other implementations
(including historic implementations of 4BSD), implement symbolic links
such that they have the same attributes as any other file.
.Pp
The
.Bx 4.4
system differs from historical 4BSD systems in that the system call
.Xr chown 2
has been changed to follow symbolic links.
.Ss 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.
.Pp
Except as noted below, commands follow symbolic links named as command
line arguments.
For example, if there were a symbolic link 
.Dq Li slink
which pointed to a file named
.Dq Li afile ,
the command
.Dq Li cat slink
would display the contents of the file
.Dq Li afile .
.Pp
It is important to realize that this rule includes commands which may
optionally traverse file trees, e.g. the command
.Dq Li "chown file"
is included in this rule, while the command
.Dq Li "chown -R file"
is not.
(The latter is described in the third area, below.)
.Pp
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
.Dq Li "file slink"
display the type of file that
.Dq Li slink
is, whether it is a symbolic link or not, the
.Fl h
option should be used.
In the above example,
.Dq Li "file slink"
would report the type of the file referenced by
.Dq Li slink ,
while
.Dq Li "file -h slink"
would report that
.Dq Li slink
was a symbolic link.
.Pp
There are three exceptions to this rule.
The
.Xr mv 1
and
.Xr 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.)
.Pp
The
.Xr ls 1
command is also an exception to this rule.
For compatibility with historic systems (when
.Nm ls
is not doing a tree walk, i.e. the
.Fl R
option is not specified),
the
.Nm ls
command follows symbolic links named as arguments if the
.Fl L
option is specified,
or if the
.Fl F ,
.Fl d
or
.Fl l
options are not specified.
(If the
.Fl L
option is specified,
.Nm ls
always follows symbolic links.
.Nm Ls
is the only command where the
.Fl L
option affects its behavior even though it is not doing a walk of
a file tree.)
.Pp
The
.Bx 4.4
system differs from historical 4BSD systems in that the
.Nm chown ,
.Nm chgrp
and
.Nm file
commands follow symbolic links specified on the command line.
.Ss Commands traversing a file tree.
The following commands either optionally or always traverse file trees:
.Xr chflags 1 ,
.Xr chgrp 1 ,
.Xr chmod 1 ,
.Xr cp 1 ,
.Xr du 1 ,
.Xr find 1 ,
.Xr ls 1 ,
.Xr pax 1 ,
.Xr rm 1 ,
.Xr tar 1
and
.Xr chown 8 .
.Pp
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.
.Pp
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.
.Pp
For example, the command
.Dq Li "chown -R user slink directory"
will ignore
.Dq Li slink ,
because symbolic links in this system do not have owners.
Any symbolic links encountered during the tree traversal will also be
ignored.
The command
.Dq Li "rm -r slink directory"
will remove
.Dq Li slink ,
as well as any symbolic links encountered in the tree traversal of
.Dq Li directory ,
because symbolic links may be removed.
In no case will either
.Nm chown
or
.Nm rm
affect the file which
.Dq Li slink
references in any way.
.Pp
The second rule applies to symbolic links that reference files of type
directory.
Symbolic links which reference files of type directory are never
.Dq followed
by default.
This is often referred to as a
.Dq physical
walk, as opposed to a
.Dq logical
walk (where symbolic links referencing directories are followed).
.Pp
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
.Fl H 
(for
.Dq 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
.Fl H
flag will be ignored if the
.Fl R
flag is not also specified.)
.Pp
For example, the command
.Dq Li "chown -HR user slink"
will traverse the file hierarchy rooted in the file pointed to by
.Dq Li slink .
Note, the
.Fl H
is not the same as the previously discussed
.Fl h
flag.
The
.Fl 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.
.Pp
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
.Fl L
(for
.Dq 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
.Fl L
flag will be ignored if the
.Fl R
flag is not also specified.)
.Pp
For example, the command
.Dq Li "chown -LR user slink"
will change the owner of the file referenced by
.Dq Li slink .
If
.Dq Li slink
references a directory,
.Nm 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
.Nm chown
traverses, they will be treated in the same fashion as
.Dq Li slink .
.Pp
As consistently as possible, you can specify the default behavior by
specifying the
.Fl P
(for
.Dq physical )
flag.
This flag is intended to make the entire name space look like the
physical name space.
.Pp
For commands that do not by default do file tree traversals, the
.Fl H ,
.Fl L
and
.Fl P
flags are ignored if the
.Fl R
flag is not also specified.
In addition, you may specify the
.Fl H ,
.Fl L
and
.Fl 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.
.Pp
The
.Xr ls 1
and
.Xr rm 1
commands have exceptions to these rules.
The
.Nm rm
command operates on the symbolic link, and not the file it references,
and therefore never follows a symbolic link.
The
.Nm rm
command does not support the
.Fl H ,
.Fl L
or
.Fl P
options.
.Pp
To maintain compatibility with historic systems,
the
.Nm ls
command never follows symbolic links unless the
.Fl L
flag is specified.
If the
.Fl L
flag is specified,
.Nm ls
follows all symbolic links,
regardless of their type,
whether specified on the command line or encountered in the tree walk.
The
.Nm ls
command does not support the
.Fl H
or
.Fl P
options.
.Sh SEE ALSO
.Xr chflags 1 ,
.Xr chgrp 1 ,
.Xr chmod 1 ,
.Xr cp 1 ,
.Xr du 1 ,
.Xr find 1 ,
.Xr ln 1 ,
.Xr ls 1 ,
.Xr mv 1 ,
.Xr pax 1 ,
.Xr rm 1 ,
.Xr tar 1 ,
.Xr lstat 2 ,
.Xr readlink 2 ,
.Xr rename 2 ,
.Xr unlink 2 ,
.Xr fts 3 ,
.Xr remove 3 ,
.Xr chown 8