Links


Links

A link is a pointer to a file. Every time you create a file by using vim, touch, cp, or any other means, you are putting a pointer in a directory. This pointer associates a filename with a place on the disk. When you specify a filename in a command, you are indirectly pointing to the place on the disk that holds the information you want.

Sharing files can be useful when two or more people are working on the same project and need to share some information. You can make it easy for other users to access one of your files by creating additional links to the file.

To share a file with another user, first give the user permission to read from and write to the file (page ). You may also have to change the access permissions of the parent directory of the file to give the user read, write, or execute permission (page ). Once the permissions are appropriately set, the user can create a link to the file so that each of you can access the file from your separate directory hierarchies.

A link can also be useful to a single user with a large directory hierarchy. You can create links to cross-classify files in your directory hierarchy, using different classifications for different tasks. For example, if you have the file layout depicted in on page , a file named to_do might appear in each subdirectory of the correspond directorythat is, in personal, memos, and business. If you find it difficult to keep track of everything you need to do, you can create a separate directory named to_do in the correspond directory. You can then link each subdirectory's to-do list into that directory. For example, you could link the file named to_do in the memos directory to a file named memos in the to_do directory. This set of links is shown in .

Figure 6-13. Using links to cross-classify files

Although it may sound complicated, this technique keeps all your to-do lists conveniently in one place. The appropriate list is easily accessible in the task-related directory when you are busy composing letters, writing memos, or handling personal business.

Tip: About the discussion of hard links

Two kinds of links exist: hard links and symbolic (soft) links. Hard links are older and becoming outdated. The section on hard links is marked as optional; you can skip it, although it discusses inodes and gives you insight into the structure of the filesystem.

Optional: Hard Links

A hard link to a file appears as another file. If the file appears in the same directory as the linked-to file, the links must have different filenames because two files in the same directory cannot have the same name. You can create a hard link to a file only from within the filesystem that holds the file.

ln: Creates a Hard Link

The ln (link) utility (without the s or symbolic option) creates a hard link to an existing file using the following syntax:


ln existing-file new-link

The next command makes the link shown in by creating a new link named /home/alex/letter to an existing file named draft in Jenny's home directory: 

$ pwd
/home/jenny
$ ln draft /home/alex/letter

Figure 6-14. Two links to the same file: /home/alex/letter and /home/jenny/draft


The new link appears in the /home/alex directory with the filename letter. In practice, Alex may need to change the directory and file permissions so that Jenny will be able to access the file. Even though /home/alex/letter appears in Alex's directory, Jenny is the owner of the file because she created it.

The ln utility creates an additional pointer to an existing file but it does not make another copy of the file. Because there is only one file, the file status informationsuch as access permissions, owner, and the time the file was last modifiedis the same for all links; only the filenames differ. When Jenny modifies /home/jenny/draft, for example, Alex sees the changes in /home/alex/letter.

cp versus ln

The following commands verify that ln does not make an additional copy of a file. Create a file, use ln to make an additional link to the file, change the contents of the file through one link, and verify the change through the other link: 

$ cat file_a
This is file A.
$ ln file_a file_b
$ cat file_b
This is file A.
$ vim file_b
...
$ cat file_b
This is file B after the change.
$ cat file_a
This is file B after the change.

If you try the same experiment using cp instead of ln and change a copy of the file, the difference between the two utilities will become clearer. Once you change a copy of a file, the two files are different: 

$ cat file_c
This is file C.
$ cp file_c file_d
$ cat file_d
This is file C.
$ vim file_d
...
$ cat file_d
This is file D after the change.
$ cat file_c
This is file C.

ls and link counts

You can use ls with the l option, followed by the names of the files you want to compare, to confirm that the status information is the same for two links to the same file and is different for files that are not linked. In the following example, the 2 in the links field (just to the left of alex) shows there are two links to file_a and file_b: 

$ls -l file_a file_b file_c file_d
-rw-r--r-- 2 alex pubs 33  May 24 10:52 file_a
-rw-r--r-- 2 alex pubs 33  May 24 10:52 file_b
-rw-r--r-- 1 alex pubs 16  May 24 10:55 file_c
-rw-r--r-- 1 alex pubs 33  May 24 10:57 file_d

Although it is easy to guess which files are linked to one another in this example, ls does not explicitly tell you.

ls and inodes

Use ls with the i option to determine without a doubt which files are linked. The i option lists the inode (page ) number for each file. An inode is the control structure for a file. If the two filenames have the same inode number, they share the same control structure and are links to the same file. Conversely, when two filenames have different inode numbers, they are different files. The following example shows that file_a and file_b have the same inode number and that file_c and file_d have different inode numbers: 

$ ls -i file_a file_b file_c file_d
3534 file_a   3534 file_b   5800 file_c   7328 file_d

All links to a file are of equal value: The operating system cannot distinguish the order in which multiple links were created. When a file has two links, you can remove either one and still access the file through the remaining link. You can remove the link used to create the file, for example, and, as long as one link remains, still access the file through that link.


Symbolic Links

In addition to hard links, Linux supports symbolic links, also called soft links or symlinks. A hard link is a pointer to a file (the directory entry points to the inode), whereas a symbolic link is an indirect pointer to a file (the directory entry contains the pathname of the pointed-to filea pointer to the hard link to the file).

Advantages of symbolic links

Symbolic links were developed because of the limitations inherent in hard links. You cannot create a hard link to a directory, but you can create a symbolic link to a directory.

In many cases the Linux file hierarchy encompasses several filesystems. Because each filesystem keeps separate control information (that is, separate inode tables or filesystem structures) for the files it holds, it is not possible to create hard links between files in different filesystems. A symbolic link can point to any file, regardless of where it is located in the file structure, but a hard link to a file must be in the same filesystem as the other hard link(s) to the file. When you create links only among files in your home directory, you will not notice this limitation.

A major advantage of a symbolic link is that it can point to a nonexistent file. This ability is useful if you need a link to a file that is periodically removed and recreated. A hard link keeps pointing to a "removed" file, which the link keeps alive even after a new file is created. In contrast, a symbolic link always points to the newly created file and does not interfere when you delete the old file. For example, a symbolic link could point to a file that gets checked in and out under a source code control system, a .o file that is re-created by the C compiler each time you run make, or a log file that is repeatedly archived.

Although they are more general than hard links, symbolic links have some disadvantages. Whereas all hard links to a file have equal status, symbolic links do not have the same status as hard links. When a file has multiple hard links, it is analogous to a person having multiple full legal names, as many married women do. In contrast, symbolic links are analogous to nicknames. Anyone can have one or more nicknames, but these nicknames have a lesser status than legal names. The following sections describe some of the peculiarities of symbolic links.

ln: Creates a Symbolic Link

You use ln with the symbolic (or s) option to create a symbolic link. The following example creates a symbolic link /tmp/s3 to the file sum in Alex's home directory. When you use an ls l command to look at the symbolic link, ls displays the name of the link and the name of the file it points to. The first character of the listing is l (for link). 

$ ln --symbolic /home/alex/sum /tmp/s3
$ ls -l /home/alex/sum /tmp/s3
-rw-rw-r--  1 alex alex 38 Jun 12 09:51 /home/alex/sum
lrwxrwxrwx  1 alex alex 14 Jun 12 09:52 /tmp/s3 -> /home/alex/sum
$ cat /tmp/s3
This is sum.

The sizes and times of the last modifications of the two files are different. Unlike a hard link, a symbolic link to a file does not have the same status information as the file itself.

You can also use ln to create a symbolic link to a directory. When you use the symbolic option, ln does not care whether the file you are creating a link to is an ordinary file or a directory.

Tip: Use absolute pathnames with symbolic links

Symbolic links are literal and are not aware of directories. A link that points to a relative pathname, which includes simple filenames, assumes that the relative pathname is relative to the directory that the link was created in (not the directory the link was created from). In the following example, the link points to the file named sum in the /tmp directory. Because no such file exists, cat gives an error message: 

$ pwd
/home/alex
$ ln --symbolic sum /tmp/s4
$ ls -l sum /tmp/s4
lrwxrwxrwx  1 alex      alex    3 Jun 12 10:13 /tmp/s4 -> sum
-rw-rw-r--  1 alex      alex    38 Jun 12 09:51 sum
$ cat /tmp/s4
cat: /tmp/s4: No such file or directory


Optional: cd and Symbolic Links

When you use a symbolic link as an argument to cd to change directories, the results can be confusing, particularly if you did not realize that you were using a symbolic link.

If you use cd to change to a directory that is represented by a symbolic link, the pwd shell builtin lists the name of the symbolic link. The pwd utility (/bin/pwd) lists the name of the linked-to directory, not the link, regardless of how you got there. 

$ ln -s /home/alex/grades /tmp/grades.old
$ pwd
/home/alex
$ cd /tmp/grades.old
$ pwd
/tmp/grades.old
$ /bin/pwd
/home/alex/grades

When you change directories back to the parent, you end up in the directory holding the symbolic link: 

$ cd ..
$ pwd
/tmp
$ /bin/pwd
/tmp


rm: Removes a Link

When you create a file, there is one hard link to it. You can then delete the file or, using Linux terminology, remove the link with the rm utility. When you remove the last hard link to a file, you can no longer access the information stored there and the operating system releases the space the file occupied on the disk for subsequent use by other files. This space is released even if symbolic links to the file remain. When there is more than one hard link to a file, you can remove a hard link and still access the file from any remaining link. Unlike DOS and Windows, Linux does not provide an easy way to undelete a file once you have removed it. A skilled hacker, however, can sometimes piece the file together with time and effort.

When you remove all hard links to a file, you will not be able to access the file through a symbolic link. In the following example, cat reports that the file total does not exist because it is a symbolic link to a file that has been removed: 

$ ls -l sum
-rw-r--r-- 1 alex pubs 981  May 24 11:05 sum
$ ln -s sum total
$rm sum
$ cat total
cat: total: No such file or directory
$ ls -l total
lrwxrwxrwx 1 alex pubs 6  May 24 11:09 total -> sum

When you remove a file, be sure to remove all symbolic links to it. Remove a symbolic link in the same way you remove other files: 

$ rm total