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NAME
ln - link files
SYNOPSIS
ln [-fs] source_file target_file
ln [-fs] source_file ... target_dir
DESCRIPTION
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 is implementation-defined.
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:
1. If the destination path exists:
a. 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 source_files.
b. 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.
2. 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.
3. 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
files.
4. 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 argument.
OPTIONS
The ln utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2,
Utility Syntax Guidelines.
The following option shall be supported:
-f Force existing destination pathnames to be removed to allow the link.
-s Create symbolic links instead of hard links.
OPERANDS
The following operands shall be supported:
source_file
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.
target_file
The pathname of the new directory entry to be created.
target_dir
A pathname of an existing directory in which the new directory entries are created.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of ln:
LANG 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.)
LC_ALL If set to a non-empty string value, override the values of all the other internationalization
variables.
LC_CTYPE
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).
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of diagnostic messages
written to standard error.
NLSPATH
Determine the location of message catalogs for the processing of LC_MESSAGES .
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 All the specified files were linked successfully.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
None.
EXAMPLES
None.
RATIONALE
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 reasons:
* 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 ln.
* 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 already.
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 modification.
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 omitted.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod() , find , pax , rm , the System Interfaces volume of IEEE Std 1003.1-2001, link(), unlink()
COPYRIGHT
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 .
IEEE/The Open Group 2003 LN(P)