Linux ln command overview

This section provides basic information on the ln command and its usage.

ln (link) is one of the commands you can write in the Linux shell.

The most basic usage of the ln command allows you to create a link file (link-name) to your current file (file-name):

$ ln file-name link-name

There are a few additional syntax options you can use:

Syntax	Explanation
ln file-name link-name	Creates a hard link to the file with the name provided by the user.
ln file-name	Creates a hard link in the current directory. The link name will be the same as the file name.
ln file-name directory	Creates a hard link in the directory given by the user. The link name will be the same as the file name.

Note: By default, the ln command creates hard links. To create a symbolic link, use the -s argument.

Typically, ln does not replace existing files. If you would try to create a link with a link-name of an existing file, you will get an error:

ln: failed to create hard link 'link.txt': File exists

Use the -f (force) argument to replace an existing file.

Hard links overview

This section provides basic information on hard links.

Links are a method to refer to a file stored on the hard drive. They are a part of a file system that organizes directories and files.

A hard link is an exact copy of an existing file. It shares the same inode, so the information on the file and its attributes (such as type of a file, size, physical address, or ownership).

Note: Hard links do not use extra memory as they do not create a physical copy of the file. They serve as an additional way to access the original file stored in the inode.

Fig.1: Both files point to the same inode

The hard link points directly to the file's inode, not the file itself. Because of that, you can change the original file's contents or its location, and your hard link will still work. You can even remove the original file, and it will be accessible through the hard link. It will not become invalid.

Symbolic links overview

This section provides basic information on symbolic links.

A symbolic link is a special file version that points to another file. It can be compared to the shortcut, popular in Microsoft Windows systems. Unlike a hard link, a symbolic link does not contain the data in the target file. It points to another entry somewhere in the file system. If the file or directory that the symbolic link points to gets deleted, the link becomes invalid.

Fig.2: Symlink file points to the original file, which points to the inode

Practising link creation

This procedure provides exercises to practice hard and symbolic links creation.

Prerequisites:

• Access to a Linux shell

Procedure:

1. Set up example directories and files.
2. Create a hard link to an existing file.
3. Create a symbolic link to an existing file.

Result: Hard and symbolic links to the example files are created.

Setting up example directories and files

This procedure describes example directories and files creation.

Prerequisites:

• Access to a Linux shell

Procedure:

1. Create a new practice directory using the mkdir command.

   $ mkdir -p /tmp/practice/
   

   Tip: Create your practice files in the temporary (/tmp/) directory. The next time your PC boots up, all files and directories will be deleted.

2. Create the two example files using the touch command.

   $ touch file1.txt file2.txt
   

Result: A new directory and two example files are created.

Creating a hard link to an existing file

This procedure describes a hard link creation.

Prerequisites:

• Example directory and files are created

Procedure:

1. Go to the practice directory using the cd command.

   $ cd /tmp/practice
   

2. Create a hard link, within the current directory, to an example1.txt file using the ln command.

   $ ln example1.txt link1.txt
   

3. Optional: Check if the hard link was created correctly by checking the inode number, using the ln -i command.

   $ ls -i file1.txt link1.txt 
   

   Expected output: Files have the same inode number.

   522316 file1.txt  522316 link1.txt
   

Result: A new file with the same inode number is created.

Creating a symbolic link to an existing file

This procedure describes a symbolic link creation.

Prerequisites:

• Example directory and files are created

Procedure:

1. Go to the practice directory using the cd command.

   $ cd /tmp/practice
   

2. Create a symbolic link, within the current directory, to an example2.txt file using the `ln -s' command.

   $ ln -s example2.txt link2.txt
   

3. Optional: Check if the symbolic link was created correctly by checking the inode number using the readlink command.

   $ readlink link2.txt
   

   Expected output:

   file2.txt
   

Result: A new symlink file is created.

Glossary

inode

The inode is a database that uniquely describes a file with its attributes. Every newly created file receives a new inode number.

Command options

Command options (arguments) are a list of parameters that can control the command's behavior. Options can add additional actions to a command or change its behavior.

Shell

Shell is a command-line interface that directly exposes you to the operating system internals. It interprets your commands and passes the information on what to do with them to the operating system.