Книга: Fedora™ Unleashed, 2008 edition

Creating Links Between Files with ln

Creating Links Between Files with ln

Linux enables you to create links between files that look and work like normal files for the most part. Moreover, it allows you to make two types of links, known as hard links and symbolic links (symlinks). The difference between the two is crucial, although it might not be obvious at first!

Each filename on your system points to what is known as an inode, which is the absolute location of a file. Linux allows you to point more than one filename to a given inode, and the result is a hard link — two filenames pointing to exactly the same file. Each of these files shares the same contents and attributes. So, if you edit one, the other changes because they are both the same file.

On the other hand, a symlink — sometimes called a soft link — is a redirect to the real file. When a program tries to read from a symlink, it automatically is redirected to what the symlink is pointing at. The fact that symlinks are really just dumb pointers has two advantages: You can link to something that does not exist (and create it later if you want), and you can link to directories.

Both types of links have their uses. Creating a hard link is a great way to back up a file on the same disk. For example, if you delete the file in one location, it still exists untouched in the other location. Symlinks are popular because they allow a file to appear to be in a different location; you could store your website in /var/www/live and an under-construction holding page in /var/www/construction. Then you could have Apache point to a symlink /var/www/html that is redirected to either the live or construction directory depending on what you need.

Both types of link are created with the ln command. By default, the ln command creates hard links, but you can create symlinks by passing it the -s parameter. The syntax is ln [-s] <something> <somewhere>, for example:

$ ln -s myfile.txt mylink

That command creates the symlink mylink that points to myfile.txt. Remove the -s to create a hard link. You can verify that your link has been created by running ls -l. Your symlink should look something like this:

lrwxrwxrwx 1 paul paul 5 2007-11-12 12:39 mylink -> myfile.txt

Note how the file properties start with l (lowercase L) for link and how ls -l also prints where the link is going. Symlinks are always very small in size; the previous link is 5 bytes in size. If you created a hard link, it should look like this:

-rw-rw-r 2 paul paul 341 2007-11-12 12:39 mylink

This time the file has normal attributes, but the second number is 2 rather than 1. That number is how many hard links point to this file, which is why it is 2 now. The file size is also the same as that of the previous filename because it is the file, as opposed to just being a pointer.

Symlinks are used extensively in Linux. Programs that have been superseded, such as sh, now point to their replacements (in this case, bash), and library versioning is accomplished through symlinks. For example, applications that link against zlib load /usr/lib/libz.so. Internally, however, that is just a symlink that points to the actual zlib library: /usr/lib/libz.so.1.2.1.2. This enables multiple versions of libraries to be installed without applications needing to worry about the version name.

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