SYMLINK(7) | Miscellaneous Information Manual | SYMLINK(7) |
symlink
—
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:
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)).
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.
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.
To illustrate the pattern matching rules, assume that “@foo” is a valid magic string:
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:
netbsd
. Other valid emulations are:
aout
, aoutm68k
,
freebsd
, linux
,
linux32
, m68k4k
,
netbsd32
, sunos
,
sunos32
, ultrix
,
vax1k
.GENERIC
.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
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.)March 25, 2019 | NetBSD 9.4 |