In the first synopsis form, the ln utility shall create a new directory
entry (link) at the destination path specified by the target_file
operand. If the -s option is specified, a symbolic link shall be
created for the file specified by the source_file operand. This first
synopsis form shall be assumed when the final operand does not name an
existing directory; if more than two operands are specified and the final is
not an existing directory, an error shall result.
In the second synopsis form, the ln utility shall create a new directory
entry (link), or if the -s option is specified a symbolic link, for
each file specified by a source_file operand, at a destination path in
the existing directory named by target_dir.
If the last operand specifies an existing file of a type not specified by the
System Interfaces volume of IEEE Std 1003.1-2001, the behavior
The corresponding destination path for each source_file shall be the
concatenation of the target directory pathname, a slash character, and the
last pathname component of the source_file. The second synopsis form
shall be assumed when the final operand names an existing directory.
For each source_file:
If the destination path exists:
If the -f option is not specified, ln shall
write a diagnostic message to standard error, do nothing more with the
current source_file, and go on to any remaining
Actions shall be performed equivalent to the
unlink() function defined in the System Interfaces volume of
IEEE Std 1003.1-2001, called using destination as the
path argument. If this fails for any reason, ln shall write
a diagnostic message to standard error, do nothing more with the current
source_file, and go on to any remaining source_files.
If the -s option is specified, ln shall
create a symbolic link named by the destination path and containing as its
pathname source_file. The ln utility shall do nothing more
with source_file and shall go on to any remaining files.
If source_file is a symbolic link, actions shall be
performed equivalent to the link() function using the object that
source_file references as the path1 argument and the
destination path as the path2 argument. The ln utility shall
do nothing more with source_file and shall go on to any remaining
Actions shall be performed equivalent to the link()
function defined in the System Interfaces volume of
IEEE Std 1003.1-2001 using source_file as the
path1 argument, and the destination path as the path2
A pathname of a file to be linked. If the -s option
is specified, no restrictions on the type of file or on its existence
shall be made. If the -s option is not specified, whether a
directory can be linked is implementation-defined.
The pathname of the new directory entry to be created.
A pathname of an existing directory in which the new
directory entries are created.
The following environment variables shall affect the execution of ln:
Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization
Variables for the precedence of internationalization variables used to
determine the values of locale categories.)
If set to a non-empty string value, override the values of
all the other internationalization variables.
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments).
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the
processing of LC_MESSAGES.
Some historic versions of ln (including the one specified by the SVID)
unlink the destination file, if it exists, by default. If the mode does not
permit writing, these versions prompt for confirmation before attempting the
unlink. In these versions the -f option causes ln not to attempt
to prompt for confirmation.
This allows ln to succeed in creating links when the target file already
exists, even if the file itself is not writable (although the directory must
be). Early proposals specified this functionality.
This volume of IEEE Std 1003.1-2001 does not allow the ln
utility to unlink existing destination paths by default for the following
The ln utility has historically been used to provide
locking for shell applications, a usage that is incompatible with
ln unlinking the destination path by default. There was no
corresponding technical advantage to adding this functionality.
This functionality gave ln the ability to destroy
the link structure of files, which changes the historical behavior of
This functionality is easily replicated with a combination
of rm and ln.
It is not historical practice in many systems; BSD and
BSD-derived systems do not support this behavior. Unfortunately, whichever
behavior is selected can cause scripts written expecting the other
behavior to fail.
It is preferable that ln perform in the same manner
as the link() function, which does not permit the target to exist
This volume of IEEE Std 1003.1-2001 retains the -f option
to provide support for shell scripts depending on the SVID semantics. It seems
likely that shell scripts would not be written to handle prompting by
ln and would therefore have specified the -f option.
The -f option is an undocumented feature of many historical versions of
the ln utility, allowing linking to directories. These versions require
Early proposals of this volume of IEEE Std 1003.1-2001 also
required a -i option, which behaved like the -i options in
cp and mv, prompting for confirmation before unlinking existing
files. This was not historical practice for the ln utility and has been
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html