связать файлы (link files)
Пролог (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.
Имя (Name)
ln — link files
Синопсис (Synopsis)
ln [-fs] [-L|-P] source_file target_file
ln [-fs] [-L|-P] source_file... target_dir
Описание (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 POSIX.1‐2017, 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 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 the destination path 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.1‐2017, called using the destination path 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
equivalent to the symlink() function with source_file 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.
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.1‐2017 using
source_file as the path1 argument, and the destination path
as the path2 argument.
Параметры (Options)
The ln utility shall conform to the Base Definitions volume of
POSIX.1‐2017, 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
symbolic 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
symbolic 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.
Операнды (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.
Стандартный ввод (Stdin)
Not used.
Входные файлы (Input files)
None.
Переменные окружения (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 POSIX.1‐2017, 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.
Асинхронные события (Asynchronous events)
Default.
Стандартный вывод (Stdout)
Not used.
Стандартный вывод сообщений (Stderr)
The standard error shall be used only for diagnostic messages.
Выходные файлы (Output files)
None.
Расширенное описание (Extended description)
None.
Статус выхода (Exit)
The following exit values shall be returned:
0 All the specified files were linked successfully.
>0 An error occurred.
Последствия ошибок (Consequences of errors)
Default.
The following sections are informative.
Использование в приложениях (Application usage)
None.
Примеры (Examples)
None.
Обоснование (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.1‐2017 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 POSIX.1‐2017 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.1‐2017 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 applications
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 creating 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.
Будущие направления (Future directions)
None.
Смотри также (See also)
chmod(1p), find(1p), pax(1p), rm(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, link(3p),
unlink(3p)
