hard links and Symbolic Links

摘自《Linux programming by example》(作者：Arnold Robbins)

Hard links

When a file is created with open()or creat(), the system finds an unused inode and assigns it to the new file. It
creates the directory entry for the file, with the file's name and inode number in it. The -ioption to lsshows the
inode number:
$ echo hello, world > message  Create new file
$ ls -il message  Show inode number too

228786 -rw-r--r-- 1 arnold devel 13 May 4 15:43 message

Since directory entries associate filenames with inodes, it is possible for one file to have multiple names. Each
directory entry referring to the same inode is called a link, or hard link, to the file. Links are created with the ln

command. The usage is 'ln oldfile newfile':

$ ln message msg  Create a link
$ cat msg  Show contents of new name
hello, world
$ ls -il msg message  Show inode numbers
228786 -rw-r--r-- 2 arnold devel 13 May 4 15:43 message

228786 -rw-r--r-- 2 arnold devel 13 May 4 15:43 msg

The output shows that the inode numbers of the two files are the same, and the third field in the long output is now
2. This field is the link count, which reports how many links (directory entries referring to the inode) the file has.
It cannot be emphasized enough: Hard links all refer to the same file. If you change one, you have changed the

others:

$ echo "Hi, how ya doin' ?" > msg  Change file by new name
$ cat message  Show contents by old name
Hi, how ya doin' ?
$ ls -il message msg  Show info. Size changed
228786 -rw-r--r-- 2 arnold devel 19 May 4 15:51 message
228786 -rw-r--r-- 2 arnold devel 19 May 4 15:51 msg

Although we've created two links to the same file in a single directory, hard links are not restricted to being in the

same directory; they can be in any other directory on the same filesystem.

Additionally, you can create a link to a file you don't own as long as you have write permission in the directory in
which you're creating the link. (Such a file retains all the attributes of the original file: the owner, permissions, and
so on. This is because it isthe original file; it has only acquired an additional name.) User-level code cannot create

a hard link to a directory.

Once a link is removed, creating a new file by the same name as the original file creates a new file:
$ rm message  Remove old name
$ echo "What's happenin?" > message  Reuse the name
$ ls -il msg message  Show information
228794 -rw-r--r-- 1 arnold devel 17 May 4 15:58 message
228786 -rw-r--r-- 1 arnold devel 19 May 4 15:51 msg

Notice that the link counts for both files are now equal to 1.

Symbolic Links

We started the chapter with a discussion of partitions, filesystems, and inodes. We also saw that directory entries
associate names with inode numbers. Because directory entries contain no other information, hard links are
restricted to files within the same filesystem. This has to be; there is no way to distinguish inode 2341on one
filesystem from inode 2341on another filesystem. Here is what happens when we try:
$ mount  Show filesystems in use
/dev/hda2 on / type ext3 (rw)
/dev/hda5 on /d type ext3 (rw)
...
$ ls -li /tmp/message  Earlier example was on filesystem for /
228786 -rw-r--r-- 2 arnold devel 19 May 4 15:51 /tmp/message
$ cat /tmp/message
Hi, how ya doin' ?
$ /bin/pwd  Current directory is on a different filesystem
/d/home/arnold
$ ln /tmp/message .  Attempt the link

ln: creating hard link `./message' to `/tmp/message': Invalid cross-device link

A symbolic link (also referred to as a soft
link) is a special kind of file (just as a directory is a special kind of file). The contents of the file are the pathname
of the file being "pointed to." All modern Unix systems, including Linux, provide symbolic links; indeed they are

now part of POSIX.

Symbolic links have their disadvantages:

• They take up extra disk space, requiring a separate inode and data block. Hard links take up only a

directory slot.

• They add overhead. The kernel has to work harder to resolve a pathname containing symbolic links.
• They can introduce "loops." Consider the following:

$ rm -f a b  Make sure 'a' and 'b' don't exist
$ ln -s a b  Symlink old file 'a' to new file 'b'
$ ln -s b a  Symlink old file 'b' to new file 'a'
$ cat a  What happens?
cat: a: Too many levels of symbolic links
The kernel has to be able to detect this case and produce an error message.

• They are easy to break. If you move the pointed-to file to a different location or rename it, the symbolic link

is no longer valid. This can't happen with a hard link.