LN(P) POSIX Programmer's Manual LN(P)NAMEln - link files
SYNOPSISln [-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 oper‐
and. 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 direc‐
tory 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 direc‐
tory.
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 diag‐
nostic message to standard error, do nothing more with the cur‐
rent 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 speci‐
fied, 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 Vari‐
ables 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 techni‐
cal 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 histori‐
cal practice for the ln utility and has been omitted.
FUTURE DIRECTIONS
None.
SEE ALSOchmod() , 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)
