man ln - Man page for ln
June 1, 2007 – 1:41 amLN
Section: POSIX Programmer’s Manual (P)
Updated: 2003
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NAME
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
INPUT FILES
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
STDOUT
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
EXTENDED DESCRIPTION
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
EXAMPLES
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
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 .
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- OPTIONS
- OPERANDS
- STDIN
- INPUT FILES
- ENVIRONMENT VARIABLES
- ASYNCHRONOUS EVENTS
- STDOUT
- STDERR
- OUTPUT FILES
- EXTENDED DESCRIPTION
- EXIT STATUS
- CONSEQUENCES OF ERRORS
- APPLICATION USAGE
- EXAMPLES
- RATIONALE
- FUTURE DIRECTIONS
- SEE ALSO
- COPYRIGHT
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