SYMLINK(7)          NetBSD Miscellaneous Information Manual         SYMLINK(7)

NAME
     symlink -- 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
     Magic symlinks can be enabled by setting ``vfs.generic.magiclinks'' with
     sysctl(8).  When magic symlinks are enabled ``magic'' patterns in sym-
     links 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 emulation.

     @kernel_ident  Expands to the name of the config(1) file used to generate
                    the running kernel.

     @machine       Expands to the value of MACHINE for the system (equivalent
                    to the output of ``uname -m'').

     @machine_arch  Expands to the value of MACHINE_ARCH for the system
                    (equivalent to the output of ``uname -p'').

     @osrelease     Expands to the operating system release of the running
                    kernel (equivalent to the output of ``uname -r'').

     @ostype        Expands to the operating system type of the running kernel
                    (equivalent to the output of ``uname -s'').  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.

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.

NetBSD 5.0_STABLE              December 4, 2007              NetBSD 5.0_STABLE
