Linux/Basic commands/ln

ln is a standard Unix command used to create links (link) to files.

Link files

Links allow more than one file name to refer to the same file, elsewhere.

There are two types of links, both of which are created by ln:

symbolic links, which refer to a symbolic path indicating the abstract location of another file, and
hard links, which refer to the specific location of physical data.

These links behave differently when the source of the link (what is being linked to) is moved or removed. Symbolic links are not updated (they merely contain a string which is the pathname of its target); hard links always refer to the source, even if moved or removed.

Specification

The Single Unix Specification (SUS) specifies the behaviour that a link or links (either symbolic or hard) will be created where specified that will link to the target file (or directory) specified.

More precisely, ln can be invoked in one of two ways: two arguments -- first an argument specifying the source file then the target, or multiple (greater than two) arguments, specifying firstly a number of source files, then a directory in which all the links are to be created. In the latter invocation, the names of the links will be that of the source file. This invocation will be assumed if the last argument is a directory. If invoked in the latter form, ln's behaviour is not specified (it is implementation-defined).

ln is specified to use behaviour identical to that of the standard unlink() and link() functions.

Usage

The SUS mandates for ln two options: -f will force removal of existing files to allow the link to be created; and -s will create symbolic links. Therefore, ln with no options creates a hard link, ln -f forces a hard link, ln -s creates a symbolic link, and ln -fs forces a symbolic link.

Other Unix and Unix-like operating systems may add extra options. GNU ln adds options such as -b to back up files before creating links, -v to print out the name of each file before links are created, and others. BSD ln adds -h, preventing ln from descending into targets whose symlinks point to directories

ln file_name link_name

would have the effect of creating a hard link called link_name that points to the same data as the existing file file_name.

Symbolic link creation and deletion:

To begin with, enter the directory called "test":

alex@ubuntu:~/playground/test$ ls -ali
total 8
 969797 drwxr-xr-x 2 alex alex 4096 Dec  9 09:10 .
1036602 drwxr-xr-x 3 alex alex 4096 Dec  9 09:10 ..
alex@ubuntu:~/playground/test$

There is nothing in it. Let's create a small text file called data.txt:

alex@ubuntu:~/playground/test$ echo "some data" > data.txt
alex@ubuntu:~/playground/test$ ls -ali
total 12
 969797 drwxr-xr-x 2 alex alex 4096 Dec  9 09:11 .
1036602 drwxr-xr-x 3 alex alex 4096 Dec  9 09:10 ..
 969768 -rw-r--r-- 1 alex alex   10 Dec  9 09:11 data.txt
alex@ubuntu:~/playground/test$

Notice that the reference count of the directory (total 8) just increased (total 12). The new file data.txt is stored in inode 969768. Let's create a symbolic (soft) link to data.txt. We will name our symbolic link slink.txt:

alex@ubuntu:~/playground/test$ ln -s data.txt slink.txt
alex@ubuntu:~/playground/test$ ls -ali
total 12
 969797 drwxr-xr-x 2 alex alex 4096 Dec  9 09:11 .
1036602 drwxr-xr-x 3 alex alex 4096 Dec  9 09:10 ..
 969768 -rw-r--r-- 1 alex alex   10 Dec  9 09:11 data.txt
 969817 lrwxrwxrwx 1 alex alex    8 Dec  9 09:11 slink.txt -> data.txt
alex@ubuntu:~/playground/test$

The reference count of the directory has not changed (total 12). Our symbolic (soft) link is stored in a different inode than the text file (969817). The information stored in data.txt is accessible through the slink.txt:

alex@ubuntu:~/playground/test$ file slink.txt
slink.txt: symbolic link to `data.txt'
alex@ubuntu:~/playground/test$ cat slink.txt
some data
alex@ubuntu:~/playground/test$

If we delete the text file data.txt, slink.txt becomes a broken link and our data is lost.

alex@ubuntu:~/playground/test$ rm data.txt
alex@ubuntu:~/playground/test$ ls -ali
total 8
 969797 drwxr-xr-x 2 alex alex 4096 Dec  9 09:36 .
1036602 drwxr-xr-x 3 alex alex 4096 Dec  9 09:32 ..
 969817 lrwxrwxrwx 1 alex alex    8 Dec  9 09:11 slink.txt -> data.txt
alex@ubuntu:~/playground/test$ file slink.txt
slink.txt: broken symbolic link to `data.txt'
alex@ubuntu:~/playground/test$ cat slink.txt
cat: slink.txt: No such file or directory
alex@ubuntu:~/playground/test$

If slink.txt was a hard link, our data would still be accessible through slink.txt. Also, if you delete the soft link itself, the data would still be there:

$ ls -ali

    63500 drwxr-xr-x   2 sc69876  support      96 Aug 29 18:14 .
    43038 drwxr-xr-x  12 sc69876  support    1024 Aug 29 18:13 ..
   104690 -rw-r--r--   1 sc69876  support      10 Aug 29 18:13 data.txt
   104825 lrwxrwxrwx   1 sc69876  support       8 Aug 29 18:14 slink.txt -> data.txt

$ rm slink.txt

$ ls -ali

    63500 drwxr-xr-x   2 sc69876  support      96 Aug 29 18:15 .
    43038 drwxr-xr-x  12 sc69876  support    1024 Aug 29 18:13 ..
   104690 -rw-r--r--   1 sc69876  support      10 Aug 29 18:13 data.txt

External links

ln -- specification from the Single Unix Specification
Simple guide to ln

Manual pages

ln -- manual page from GNU coreutils
ln -- manual page from OpenBSD

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