LN(1P) POSIX Programmer's Manual LN(1P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEln — link files
SYNOPSISln [−fs] [−L|−P] source_file target_file
ln [−fs] [−L|−P] 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 POSIX.1‐2008, the behavior is imple‐
mentation-defined.
The corresponding destination path for each source_file shall be the
concatenation of the target directory pathname, a <slash> character if
the target directory pathname did not end in a <slash>, 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 and was created by a previous step,
it is unspecified whether 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; or will continue processing
the current source_file. 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. If destination names the same directory entry as the current
source_file 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.
c. Actions shall be performed equivalent to the unlink() function
defined in the System Interfaces volume of POSIX.1‐2008, 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, actions shall be performed equiva‐
lent to the symlink() function with source_file as the path1 argu‐
ment 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.
3. If source_file is a symbolic link:
a. If the −P option is in effect, actions shall be performed
equivalent to the linkat() function with source_file as the
path1 argument, the destination path as the path2 argument,
AT_FDCWD as the fd1 and fd2 arguments, and zero as the flag
argument.
b. If the −L option is in effect, actions shall be performed
equivalent to the linkat() function with source_file as the
path1 argument, the destination path as the path2 argument,
AT_FDCWD as the fd1 and fd2 arguments, and AT_SYMLINK_FOLLOW as
the flag 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 POSIX.1‐2008 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
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
−f Force existing destination pathnames to be removed to allow
the link.
−L For each source_file operand that names a file of type sym‐
bolic link, create a (hard) link to the file referenced by
the symbolic link.
−P For each source_file operand that names a file of type sym‐
bolic link, create a (hard) link to the symbolic link itself.
−s Create symbolic links instead of hard links. If the −s option
is specified, the −L and −P options shall be silently
ignored.
Specifying more than one of the mutually-exclusive options −L and −P
shall not be considered an error. The last option specified shall
determine the behavior of the utility (unless the −s option causes it
to be ignored).
If the −s option is not specified and neither a −L nor a −P option is
specified, it is implementation-defined which of the −L and −P options
will be used as the default.
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 direc‐
tory 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 vari‐
ables that are unset or null. (See the Base Definitions vol‐
ume of POSIX.1‐2008, 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
The CONSEQUENCES OF ERRORS section does not require ln −f a b to remove
b if a subsequent link operation would fail.
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 POSIX.1‐2008 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 struc‐
ture 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 POSIX.1‐2008 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 POSIX.1‐2008 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.
The −L and −P options allow for implementing both common behaviors of
the ln utility. Earlier versions of this standard did not specify these
options and required the behavior now described for the −L option. Many
systems by default or as an alternative provided a non-conforming ln
utility with the behavior now described for the −P option. Since appli‐
cations could not rely on ln following links in practice, the −L and −P
options were added to specify the desired behavior for the application.
The −L and −P options are ignored when −s is specified in order to
allow an alias to be created to alter the default behavior when creat‐
ing hard links (for example, alias ln='ln −L'). They serve no purpose
when −s is specified, since source_file is then just a string to be
used as the contents of the created symbolic link and need not exist as
a file.
The specification ensures that ln a a with or without the −f option
will not unlink the file a. Earlier versions of this standard were
unclear in this case.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod, find, pax, rm
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, link(), unlink()COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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IEEE/The Open Group 2013 LN(1P)
