Hard Link vs Soft link

http://linuxgazette.net/105/pitcher.html

Q: Can someone give me a simple explanation of the difference between a soft link and a hard link? The documentation I've read mention these links but make no strong explanations of their meaning and how/when to use them. Thanks!

A: OK, I'll give it a try...

Unix files consist of two parts: the data part and the filename part.

The data part is associated with something called an 'inode'. The inode carries the map of where the data is, the file permissions, etc. for the data.

                               .---------------> ! data ! ! data ! etc
                              /                  +------+ !------+
        ! permbits, etc ! data addresses !
        +------------inode---------------+
 

The filename part carries a name and an associated inode number.

                         .--------------> ! permbits, etc ! addresses !
                        /                 +---------inode-------------+
        ! filename ! inode # !
        +--------------------+
 

More than one filename can reference the same inode number; these files are said to be 'hard linked' together.

        ! filename ! inode # !
        +--------------------+
                        \
                         >--------------> ! permbits, etc ! addresses !
                        /                 +---------inode-------------+
         ! othername ! inode # !
        +---------------------+

On the other hand, there's a special file type whose data part carries a path to another file. Since it is a special file, the OS recognizes the data as a path, and redirects opens, reads, and writes so that, instead of accessing the data within the special file, they access the data in the file named by the data in the special file. This special file is called a 'soft link' or a 'symbolic link' (aka a 'symlink').

        ! filename ! inode # !
        +--------------------+
                        \
                         .-------> ! permbits, etc ! addresses !
                                   +---------inode-------------+
                                                       /
                                                     /
                                                    /
    .----------------------------------------------'
    ( 
    '-->  !"/path/to/some/other/file"! 
          +---------data-------------+
                  /                      }
    .~ ~ ~ ~ ~ ~ ~                       }-- (redirected at open() time)
    (                                     }
    '~~> ! filename ! inode # !
         +--------------------+
                         \
                          '------------> ! permbits, etc ! addresses !
                                          +---------inode-------------+
                                                            /
                                                           /
     .----------------------------------------------------'
     (
     '->  ! data !  ! data ! etc.
          +------+  +------+ 

Now, the filename part of the file is stored in a special file of its own along with the filename parts of other files; this special file is called a directory. The directory, as a file, is just an array of filename parts of other files.

When a directory is built, it is initially populated with the filename parts of two special files: the '.' and '..' files. The filename part for the '.' file is populated with the inode# of the directory file in which the entry has been made; '.' is a hardlink to the file that implements the current directory.

The filename part for the '..' file is populated with the inode# of the directory file that contains the filename part of the current directory file. '..' is a hardlink to the file that implements the immediate parent of the current directory.

The 'ln' command knows how to build hardlinks and softlinks; the 'mkdir' command knows how to build directories (the OS takes care of the above hardlinks).

There are restrictions on what can be hardlinked (both links must reside on the same filesystem, the source file must exist, etc.) that are not applicable to softlinks (source and target can be on seperate file systems, source does not have to exist, etc.). OTOH, softlinks have other restrictions not shared by hardlinks (additional I/O necessary to complete file access, additional storage taken up by softlink file's data, etc.)

In other words, there's tradeoffs with each.

Now, let's demonstrate some of this...