Here are two types of links available in Linux — Soft Link
and Hard Link.
Linux ln
command is used to create either soft or hard links.
This
article explains how to create soft link, how to create hard link, and various
link tips and tricks with 10 practical examples.
$ ls -l
total 4
lrwxrwxrwx 1 chris chris 10 2010-09-17 23:40 file1 ->
sample.txt
-rw-r--r-- 1 chris chris 22 2010-09-17 23:36 sample.txt
The 1st
character in each and every line of the ls command output indicates
one of the following file types. If the 1st character is l (lower case L), then
it is a link file.
§ - regular
file
§ l link
file
§ d directory
§ p pipe
§ c character
special device
§ b block
special device
1. What
is Soft Link and Hard Link?
Soft Link
Linux OS
recognizes the data part of this special file as a reference to another file
path. The data in the original file can be accessed through the special file,
which is called as Soft Link.
To create
a soft link, do the following (ln command with -s option):
$ ln -s /full/path/of/original/file /full/path/of/soft/link/file
Hard Link
With Hard
Link, more than one file name reference the same inode number. Once you create
a directory, you would see the hidden directories “.” and “..” . In this, “.”
directory is hard linked to the current directory and the “..” is hard linked
to the parent directory.
When you
use link files, it helps us to reduce the disk space by having single copy of
the original file and ease the administration tasks as the modification in
original file reflects in other places.
To create
a hard link, do the following (ln command with no option):
$ ln /full/path/of/original/file /full/path/of/hard/link/file
2. Create
Symbolic Link for File or Directory
Create a symbolic link for
a File
The
following examples creates a symbolic link library.so under /home/chris/lib,
based on the library.so located under /home/chris/src/ directory.
$ cd /home/chris/lib
$ ln -s /home/chris/src/library.so library.so
$ ls -l library.so
lrwxrwxrwx 1 chris
chris 21 2010-09-18 07:23
library.so -> /home/chris/src/library.so
Create a symbolic link for
a Directory
Just like
file, you can create symbolic link for directories as shown below.
$ mkdir /home/chris/obj
$ cd tmp
$ ln -s /home/chris/obj objects
$ ls -l objects
lrwxrwxrwx 1 chris chris
6 2010-09-19 16:48 objects -> /home/chris/obj
Note: The
inode of the original file/directory and the soft link should not be identical.
3. Create
Hard Link for Files
The inode
number for the hard linked files would be same. The hard link for files can be
created as follows,
$ ln src_original.txt dst_link.txt
$ ls -i dst_link.txt
253564 dst_link.txt
$ ls -i src_original.txt
253564 src_original.txt
Note: Unix
/ Linux will not allow any user (even root) to create hard link for a
directory.
4. Create
Links Across Different Partitions
When you
want to create the link across partitions, you are allowed to create only the
symbolic links. Creating hard link across partitions is not allowed, as Unix
can’t create/maintain same inode numbers across partitions.
You would
see the “Invalid cross-device link” error when you are trying
to create a hard link file across partitions.
# mount /dev/sda5 /mnt
# cd /mnt
# ls
main.c Makefile
# ln Makefile /tmp/Makefile
ln: creating hard link `/tmp/Makefile' to `Makefile': Invalid
cross-device link
And the
symbolic link can be created in the same way as we did in the above.
5. Backup
the Target Files If it Already Exists
When you
create a new link (if another file exist already with the same name as the new
link name), you can instruct ln command to take a backup of the original file
before creating the new link using the –backup option as shown below.
$ ls
ex1.c ex2.c
$ ln --backup -s ex1.c ex2.c
$ ls -lrt
total 8
-rw-r--r-- 1 chris chris 20 2010-09-19 16:57 ex1.c
-rw-r--r-- 1 chris chris 20 2010-09-19 16:57 ex2.c~
lrwxrwxrwx 1 chris chris 5
2010-09-19 17:02 ex2.c -> ex1.c
Note: If
you don’t want the backup and overwrite the existing file then use -f option.
6. Create
Link Using “No-Deference” ln Command Option
While
creating a new soft link, normally OS would de-reference the destination path
before it creates the new soft link.
Sometimes
you might not want ln command to create the new link, if the destination path
is already a symbolic link that is pointing to a directory.
Following
examples shows a normal way of creating soft link inside a directory.
$ cd ~
$ mkdir example
$ ln -s /etc/passwd example
$ cd example/
$ ls -l
total 0
lrwxrwxrwx 1 root root 16 2010-09-19 17:24 passwd ->
/etc/passwd
In case
the “example” directory in the above code-snippet is a symbolic link pointing
to some other directory (for example second-dir), the ln command shown will
still create the link under second-dir. If you don’t want that to happen, use
ln -n option as shown below.
$ cd ~
$ rm -rf example
$ mkdir second-dir
$ ln -s second-dir example
$ ln -n -s /etc/passwd example
ln: creating symbolic link `example': File exists
Note: In
the above example, if you don’t use the -n option, the link will be created
under ~/second-dir directory.
7. Create
Link for Multiple Files at the Same Time
In the
following example, there are two directories — first-dir and second-dir. The
directory first-dir contains couple of C program files. If you want to create
soft links for these files in second-dir, you’ll typically do it one by one.
Instead, you can create soft list for multiple files together using -t option
as shown below.
$ ls
first-dir second-dir
$ ls first-dir
ex1.c ex2.c
$ cd second-dir
$ ln -s ../first-dir/*.c -t .
$ ls -l
total 0
lrwxrwxrwx 1 chris chris 14 2010-09-19 15:20 ex1.c ->
../first-dir/ex1.c
lrwxrwxrwx 1 chris chris 14 2010-09-19 15:20 ex2.c ->
../first-dir/ex2.c
Keep in
mind that whenever you are creating link files with -t option, it is better to
go into target directory and perform the link creation process. Otherwise, you
would face the broken link files as shown below.
$ cd first-dir
$ ln -s *.c /home/chris/second-dir
$ cd /home/chris/second-dir
$ ls -l
total 0
lrwxrwxrwx 1 chris chris 5 2010-09-19 15:26 ex1.c -> ex1.c
lrwxrwxrwx 1 chris chris 5 2010-09-19 15:26 ex2.c -> ex2.c
Instead,
you might also use actual path for source files to create the link properly.
8.
Removing the Original File When a Soft Link is pointing to it
When the
original file referred by a soft-link is deleted, the soft link will be broken
as shown below.
$ ln -s file.txt /tmp/link
$ ls -l /tmp/link
lrwxrwxrwx 1 chris chris 9 2010-09-19 15:38 /tmp/link ->
file1.txt
$ rm file.txt
$ ls -l /tmp/link
lrwxrwxrwx 1 chris chris 9 2010-09-19 15:38 /tmp/link -> file1.txt
9. Links
Help You to Increase the Partition Size Virtually
Let us
assume that you have two partitions – 5GB and 20GB. The first partition does
not have too much free space available in it. If a program located on the first
partition needs more space (For example, for it’s log file), you can use some
of the space from the second partition by creating a link for the log files as
shown below.
Consider
that partition1 is mounted on /, and partition2 is mounted to /mnt/. Let us
assume that the logs that are located on partition1 is running out of space,
and you’ve decided to move them to partition2. You can achieve this as shown
below.
$ mkdir /mnt/logs
$ cd /logs
$ mv * /mnt/logs
$ cd /; rmdir logs
$ ln -s /mnt/logs logs
10.
Removing the Hard Linked Files
When you
delete a file that is hard linked, you would be still able to access the
content of the file until you have the last file which is hard linked to it, as
shown in the example below.
Create a
sample file.
$ vim src_original.txt
Created this file to test the hard link.
Create a
hard link to the sample file.
$ ln src_original.txt dst_link.txt
Delete
the original file.
$ rm src_original.txt
You can
still access the original file content by using the hard link you created.
$ cat dst_link.txt
Created this file to test the hard link.
