| 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 |