There are two concepts of ‘link’ in Unix, usually called
hard link and soft link. A hard link is just a name for a file.
(And a file can have several names. It is deleted from disk only
when the last name is removed. The number of names is given by
ls(1).
There is no such thing as an ‘original’ name: all names have the
same status. Usually, but not necessarily, all names of a file
are found in the filesystem that also contains its data.)
A soft link (or symbolic link, or symlink) is an entirely different
animal: it is a small special file that contains a pathname.
Thus, soft links can point at files on different filesystems
(possibly NFS mounted from different machines), and need not point
to actually existing files.
When accessed (with the
open(2)
or
stat(2)
system calls), a reference to a symlink is replaced by the operating
system kernel with a reference to the file named by the pathname.
(However, with
rm(1)
and
unlink(2)
the link itself is removed, not the file it points to.
There are special system calls
lstat(2)
and
readlink(2)
that read the status of a symlink and the filename it points to.
For various other system calls there is some uncertainty
and variation between operating systems as to whether
the operation acts on the symlink itself, or on the file pointed to.)
ln makes links between files. By default, it makes hard links;
with the
-s option, it makes symbolic (or ‘soft’) links.
If only one file is given, it links that file into
the current directory, that is, creates a link to that file
in the current directory, with name equal to (the last component of)
the name of that file. (This is a GNU extension.)
Otherwise, if the last argument names an existing directory,
ln will create links to each mentioned
source file in that directory, with a name equal to (the last component of)
the name of that
source file. (But see the description of the
--no-dereference option below.)
Otherwise, if only two files are given, it creates a link named
dest to the file
source. It is an error if the last argument is not a directory and
more than two files are given.
By default,
ln does not remove existing files or existing symbolic links.
(Thus, it can be used for locking purposes: it will succeed only if
dest did not exist already.)
But it can be forced to do so with the option -f.
On existing implementations, if it is at all possible to make a hard link
to a directory, this may be done by the superuser only. POSIX forbids
the system call
link(2)
and the utility
ln to make hard links to directories (but does not forbid
hard links to cross filesystem boundaries).
Allow the superuser to make hard links to directories.
-f, --force
Remove existing destination files.
-i, --interactive
Prompt whether to remove existing destination files.
-n, --no-dereference
When given an explicit destination that is a symlink to a
directory, treat that destination as if it were a normal file.
When the destination is an actual directory (not a symlink to one),
there is no ambiguity. The link is created in that directory.
But when the specified destination is a symlink to a directory,
there are two ways to treat the user’s request.
ln can treat the destination just as it would a normal directory and
create the link in it. On the other hand, the destination can be
viewed as a non-directory — as the symlink itself. In that case,
ln must delete or backup that symlink before creating the new link.
The default is to treat a destination that is a symlink to a directory
just like a directory.
-s, --symbolic
Make symbolic links instead of hard links. This option merely
produces an error message on systems that do not support symbolic links.
The GNU versions of programs like
cp,
mv,
ln,
install and
patch will make a backup of files about to be overwritten, changed or destroyed
if that is desired. That backup files are desired is indicated by
the -b option. How they should be named is specified by the -V option.
In case the name of the backup file is given by the name of the file
extended by a suffix, this suffix is specified by the -S option.
-b, --backup
Make backups of files that are about to be overwritten or removed.
-S SUFFIX, --suffix=SUFFIX
Append
SUFFIX to each backup file made.
If this option is not specified, the value of the
SIMPLE_BACKUP_SUFFIX environment variable is used. And if
SIMPLE_BACKUP_SUFFIX is not set, the default is ‘~’.
-V METHOD, --version-control=METHOD
Specify how backup files are named. The
METHOD argument can be ‘numbered’ (or ‘t’), ‘existing’ (or ‘nil’), or ‘never’ (or
‘simple’).
If this option is not specified, the value of the
VERSION_CONTROL environment variable is used. And if
VERSION_CONTROL is not set, the default backup type is ‘existing’.
This option corresponds to the Emacs variable ‘version-control’.
The valid
METHODs are (unique abbreviations are accepted):
t, numbered
Always make numbered backups.
nil, existing
Make numbered backups of files that already have them, simple
backups of the others.
POSIX.2.
However, POSIX.2 (1996) does not discuss soft links.
Soft links were introduced by BSD, and do not occur in System V release 3
(and older) systems.
