Editing Ln (Unix)

{{Short description|Shell command for creating a link file}}
{{Lowercase title}}
{{Infobox software
| name                   = ln
| logo                   = 
| screenshot             = Ln-example-command.gif
| screenshot size        = 
| caption                = Example usage of {{code|ln}} utility
| author                 = [[AT&T Bell Laboratories]]
| developer              = Various [[open-source software|open-source]] and [[commercial software|commercial]] developers
| released               = {{Start date and age|1971|11|3}}
| latest release version = 
| latest release date    = 
| operating system       = [[Unix]], [[Unix-like]], [[IBM i]]
| platform               = [[Cross-platform]]
| genre                  = [[Command (computing)|Command]]
| license                = [[coreutils]]: [[GPLv3+]]
| website                = 
}}
'''{{code|ln}}''' is a [[shell (computing)|shell]] [[command (computing)|command]] for creating a link file to an existing [[computer file|file]] or [[computer directory|directory]].<ref name=UNIXKP>{{cite book|last1=Kernighan|first1=Brian W.|last2=Pike|first2=Rob|title=The Unix programming environment.|date=1984|publisher=Prentice-Hall|location=Englewood Cliffs, N.J.|isbn=013937681X|page=[https://archive.org/details/unixprogramminge0000kern/page/59 59]|url-access=registration|url=https://archive.org/details/unixprogramminge0000kern/page/59}}</ref> By default, the command creates a [[hard link]], but with the {{code|-s}} [[Command-line interface|command line]] option, it creates a [[symbolic link]].<ref>{{cite web|title=ln - The Open Group Base Specifications Issue 7|url=http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ln.html|website=pubs.opengroup.org|publisher=The IEEE and The Open Group|access-date=7 August 2015}}</ref> Most systems disallow a hard link to a directory since such links could disrupt the structure of a [[file system]] and interfere with the operation of other utilities.<ref>{{cite web|title=Why are hard links not allowed to directories in UNIX/Linux?|url=http://unix.stackexchange.com/questions/22394/why-hard-links-not-allowed-to-directories-in-unix-linux?rq=1|website=unix.stackexchange.com|access-date=7 August 2015}}</ref> The command can create a symbolic link to non-existent file.<ref name=GNULnman />

The command appeared in Issue 2 of the [[X/Open]] Portability Guidelines.<ref name=SUS/> The version in [[GNU Core Utilities]] was written by Mike Parker and David MacKenzie.<ref>{{man|1|ln|ManKier}}</ref>
The command is available in [[Windows]] via [[UnxUtils]]<ref>{{Cite web|url=http://unxutils.sourceforge.net/|title=Native Win32 ports of some GNU utilities|website=unxutils.sourceforge.net}}</ref> and has been ported to [[IBM i]].<ref>{{cite web |title=IBM System i Version 7.2 Programming Qshell |language=en |author=IBM |website=[[IBM]] |author-link=IBM |url=https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahz/rzahzpdf.pdf?view=kc |access-date=2020-09-05 }}</ref>

==Links==
A link allows more than one [[file path|path]] to refer to the same file. 

A hard link is a directory entry that refers to a file's [[inode]] (an internal [[Reference (computer science)|reference]]). A file can have multiple hard links {{endash}} each referring to the same [[inode]].<ref name=libc /> Creating a hard link does not copy the contents of the file; but merely causes another name to be associated with the same content. When a hard link is created for a file, a [[Reference counting#File systems|counter]] in its inode is incremented, and removing a hard link results in decrementing the counter. A file is not deleted (to allow reusing its storage space) until its reference count reaches zero.<ref name=UNIXKP /><ref name=IBMInode>{{cite web|title=Speaking UNIX: It is all about the inode|url=http://www.ibm.com/developerworks/aix/library/au-speakingunix14/|website=ibm.com|publisher=IBM developerWorks|access-date=7 August 2015}}</ref> A hard link can only refer to an object on the same [[file system]].<ref>{{cite book|last1=Bovet|first1=Daniel P.|last2=Cesati|first2=Marco|title=Understanding the Linux Kernel|date=2005|publisher=Safari Tech Books Online|location=Boston, MA|isbn=0596005652|page=14|edition=3rd}}</ref>

A symbolic link refers to another file by its path (a symbolic reference).<ref name=GNULnman>{{cite web|title=GNU Coreutils: ln invocation|url=https://www.gnu.org/software/coreutils/manual/html_node/ln-invocation.html|website=gnu.org|publisher=Free Software Foundation, Inc.|access-date=7 August 2015}}</ref> When encountered during pathname resolution, modify the [http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13 pathname resolution] to be taken to the location which the symbolic link contains. The content of a symbolic link is the target path, which can also be examined via the {{code|readlink}} command.<ref>{{cite web|title=readlink(1) - Linux man page|url=http://linux.die.net/man/1/readlink|website=linux.die.net|access-date=7 August 2015|ref=ReadlinkMan}}</ref> A symbolic link may specify a path that does not refer to an existing file. Resolving such a symbolic link will fail until a file is created at its specified path. Also, a symbolic link to an existing file will fail to resolve if the file is moved.<ref name=libc>{{cite book|last1=Loosemore|first1=Sandra|last2=Stallman|first2=Richard M.|last3=McGrath|first3=Roland|last4=Oram|first4=Andrew|last5=Drepper|first5=Ulrich|title=The GNU C Library Reference Manual|publisher=Free Software Foundation, Inc|page=392|url=https://www.gnu.org/software/libc/manual/pdf/libc.pdf|access-date=7 August 2015}}</ref>

==Use ==
Per the [[Single Unix Specification]], the {{code|ln}} command is specified in the Shell and Utilities (XCU) document.<ref name=SUSOverview>{{cite web|title=The Single UNIX Specification, Version 4 - Overview|url=http://www.unix.org/version4/overview.html|website=unix.org|publisher=unix.org|access-date=7 August 2015}}</ref> [[POSIX]] includes a similar specification.<ref name=SUS>{{man|1|ln|SUS}}</ref>

If neither target file nor target directory are specified, links are created in the current [[working directory]].

===Options===

The specification indicates command line options that must be supported:

* {{code|-f}} (force) [[clobbering|Clobbers]] an existing file if the output path specifies an existing file
* {{code|-L}} If a source operand specifies a symbolic link, create a hard link to the link's target file
* {{code|-P}} If a source operand specifies a symbolic link, create a hard link to the symbolic link file
* {{code|-s}} Create symbolic links instead of hard links; causes {{code|-L}} and {{code|-P}} to be silently ignored

If more than one of the mutually-exclusive options {{code|-L}} and {{code|-P}} is specified, the last option specified determines the resulting behavior.

If the {{code|-s}} option is not specified and neither a {{code|-L}} nor a {{code|-P}} option is specified, the implementation defines which of the {{code|-L}} and {{code|-P}} options is used by default.

===Single file invocation===
The command creates a new link to the path indicated by {{code|source}}; stored at the path indicated by {{code|target}}. The syntax is as follows:

<syntaxhighlight lang="terminal">
ln [-fs] [-L|-P] source target
</syntaxhighlight>

===Multiple file invocation===
The command creates a new link to each file specified by the {{code|source_#}} operands; stored in an existing directory indicated by {{code|target_dir}}.

<syntaxhighlight lang="terminal">
ln [-fs] [-L|-P] source_1 source_2 ... target_dir
</syntaxhighlight>

==Examples==

The following creates a hard link to file '''foo''' called '''bar'''.

 ln foo bar

The following creates a symbolic link to file '''foo''' called '''bar'''.

 ln -s foo bar

==See also==
* [[List of POSIX commands]]
* [[NTFS junction point]]

==References==
{{Reflist|30em}}

==External links==
{{Wikibooks|Guide to Unix|Commands/File_System_Utilities#ln| File System Utilities - ln}}
* {{man|1|ln|die.net}}
* {{man|1|ln|FreeBSD}}
* {{man|1|ln|NetBSD}}
* {{man|1|ln|OpenBSD}}
* {{man|1|ln|Darwin}}

{{Unix commands}}
{{Core Utilities commands}}

[[Category:Standard Unix programs]]
[[Category:Unix SUS2008 utilities]]
[[Category:IBM i Qshell commands]]