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Introducing UNIX and Linux

More on shells

Simple arithmetic
      Arithmetic expansion
            Operators for arithmetic expansion
      The 'expr' command
Pattern matching
            Examples of patterns
      The case statement
Entering and leaving the shell
More about scripts with options
Symbolic links
Setting up terminals
Conventions used in UNIX file systems

Symbolic links

Recall that a file is represented by a name and by an inode, and that a single inode can have several names. We use a link to create an extra name for an inode using the command ln, so

ln fileA fileB

will cause fileA and fileB to be two names for the same file. If you delete one of them, the other continues to exist, and the file only disappears when both are removed. They share the same inode. These hard links can only be used within a single filesystem. Hard links can also only be used on ordinary files, and not on directories. If you try, for instance,

ln / rootdirectory

you will get an error message. There is another type of link referred to as a symbolic link, or soft link which can get around these problems.

A hard link is an entry in a directory associating a filename with an inode. A soft link is an entry in a directory associating a filename with another filename. This is an important distinction - hard links are names for inodes, soft links are names for other filenames. To create a soft link, use ln with option -s ('symbolic'). Consider:

ln -s fileA fileB

which will create a symbolic link, called fileB to a file fileA which should already exist. Examining your files with ls -l would give something like

lrw-r--r--  1 chris ugrads 122 May 21 18:40 fileB -> fileA

indicating that fileB is a symbolic link (l in column 1), and that it points to (->) fileA. Whenever you use fileB UNIX will assume you want to access fileA and treat fileB accordingly. If fileA does not exist, and you try to access fileB you will get an error message telling you fileB does not exist. You can make a symbolic link to any file, provided that the file already exists. The advantage of symbolic links is that you do not have to worry about the filesystems the system's storage is divided into. There is a danger, though: if the file pointed to by a symbolic link is deleted, the link remains in place. Try:

ln -s fileA fileB
rm fileA
cat fileB
cat: fileB: No such file or directory

Thus you must be careful when deleting files which are pointed to by symbolic links.

Worked example

In your home directory create a symbolic link called systmp which is linked to /tmp.
Solution: Use ln -s, as just described. You cannot use a hard link, since /tmp will (almost certainly) be on a different filesystem.

ln -s /tmp $HOME/systmp

Now try the following to confirm it works:

ls $HOME/systmp
ls /tmp

Copyright © 2002 Mike Joy, Stephen Jarvis and Michael Luck