Ln (Unix)
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ln is a shell command for creating a link file to an existing file or directory.[1] By default, the command creates a hard link, but with the -s command line option, it creates a symbolic link.[2] 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.[3] The command can create a symbolic link to non-existent file.[4]
The command appeared in Issue 2 of the X/Open Portability Guidelines.[5] The version in GNU Core Utilities was written by Mike Parker and David MacKenzie.[6] The command is available in Windows via UnxUtils[7] and has been ported to IBM i.[8]
Links
A link allows more than one path to refer to the same file.
A hard link is a directory entry that refers to a file's inode (an internal reference). A file can have multiple hard links Template:Endash each referring to the same inode.[9] 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 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.[1][10] A hard link can only refer to an object on the same file system.[11]
A symbolic link refers to another file by its path (a symbolic reference).[4] When encountered during pathname resolution, modify the 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 readlink command.[12] 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.[9]
Use
Per the Single Unix Specification, the ln command is specified in the Shell and Utilities (XCU) document.[13] POSIX includes a similar specification.[5]
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:
-f(force) Clobbers an existing file if the output path specifies an existing file-LIf a source operand specifies a symbolic link, create a hard link to the link's target file-PIf a source operand specifies a symbolic link, create a hard link to the symbolic link file-sCreate symbolic links instead of hard links; causes-Land-Pto be silently ignored
If more than one of the mutually-exclusive options -L and -P is specified, the last option specified determines the resulting behavior.
If the -s option is not specified and neither a -L nor a -P option is specified, the implementation defines which of the -L and -P options is used by default.
Single file invocation
The command creates a new link to the path indicated by source; stored at the path indicated by target. The syntax is as follows:
ln [-fs] [-L|-P] source target
Multiple file invocation
The command creates a new link to each file specified by the source_# operands; stored in an existing directory indicated by target_dir.
ln [-fs] [-L|-P] source_1 source_2 ... target_dir
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
References
External links
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