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In this article, learn to create and manage hard and symbolic links. Learn to:
- Create hard or soft links
- Identify links and know their type
- Understand the difference between copying and linking files
- Use links for system administration tasks
This article helps you prepare for Objective 104.6 in Topic 104 of the Linux Professional Institute's Junior Level Administration (LPIC-1) exam 101. The objective has a weight of 2.
To get the most from the articles in this series, you should have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this article. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here. In particular, much of the output we show is highly dependent on the packages that are already installed on our systems. Your own output may be quite different, but you should be able to recognize the important commonalities.
On a storage device, a file or directory is contained in a collection of blocks. Information about a file is contained in an inode, which records information such as the owner, when the file was last accessed, how large it is, whether it is a directory or not, and who can read from or write to it. The inode number is also known as the file serial number and is unique within a particular filesystem. A directory entry contains a name for a file or directory and a pointer to the inode where the information about the file or directory is stored.
A link is simply an additional directory entry for a file or directory, allowing two or more names for the same thing. A hard link is a directory entry that points to an inode, while a soft link or symbolic link is a directory entry that points to an inode that provides the name of another directory entry. The exact mechanism for storing the second name may depend on both the file system and the length of the name. Symbolic links are also called symlinks.
You can create hard links only for files and not for directories. The exception is the special directory entries in a directory for the directory itself and for its parent (. and ..), which are hard links that maintain the count of the number of subdirectories. Because hard links point to an inode, and inodes are only unique within a particular file system, hard links cannot cross file systems. If a file has multiple hard links, the file is deleted only when the last link pointing to the inode is deleted and the link count goes to 0.
Soft links, or symlinks, merely point to another file or directory by name rather than by inode. Soft links can cross file system boundaries. Deleting a soft link does not delete the target file or directory, and deleting the target file or directory does not automatically remove any soft links.
First let's look at how to create hard and soft links. Later in this article, we'll look at ways to identify and use the links we create here.
You use the ln command to create additional hard
links to an existing file (but not to a directory, even though the system
sets up . and .. as hard links).
Listing 1 shows how to create a directory containing two files and a subdirectory with two hard links to file1, one in the same directory and one in the subdirectory. We have added a word to file1, and then another word to file3 and displayed the contents of the link in the subdirectory to show that all do indeed point to the same data.
Listing 1. Creating hard links
ian@attic4:~$ mkdir -p lpi104-6/subdir ian@attic4:~$ touch lpi104-6/file1 ian@attic4:~$ touch lpi104-6/file2 ian@attic4:~$ ln lpi104-6/file1 lpi104-6/file3 ian@attic4:~$ ln lpi104-6/file1 lpi104-6/subdir/file3sub ian@attic4:~$ echo "something" > lpi104-6/file1 ian@attic4:~$ echo "else" >> lpi104-6/file3 ian@attic4:~$ cat lpi104-6/subdir/file3sub something else |
You will get an error if you attempt to create hard links that cross file systems or that are for directories. Listing 2 shows that my home and research directories are on different file systems and that an attempt to create a hard link across these fails, as does an attempt to create a hard link to the lpi104-6 directory.
Listing 2. Failures with hard link creation
ian@attic4:~$ df . research Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda7 71205436 9355052 58233352 14% / /dev/sdb3 137856204 27688208 103165264 22% /home/ian/ian-research ian@attic4:~$ ln lpi104-6/file1 research/lpi104-6/file3 ln: creating hard link `research/lpi104-6/file3' => `lpi104-6/file1': No such file or dir ectory ian@attic4:~$ ln lpi104-6 lpidir104-6 ln: `lpi104-6': hard link not allowed for directory |
You use the ln command with the
-s option to create soft links. Soft links use
file or directory names, which may be relative or absolute. If you are
using relative names, you will usually want the current working directory
to be the directory where you are creating the link; otherwise, the link
you create will be relative to another point in the file system. Listing 3
shows you two ways to create a soft link for the file1 that we just
created, and also how to create soft links instead of the two hard links
that failed in Listing 2.
Listing 3. Creating soft links
ian@attic4:~$ # Create symlink using absolute paths ian@attic4:~$ ln -s ~/lpi104-6/file1 ~/lpi104-6/file4 ian@attic4:~$ # Create symlink using relative paths ian@attic4:~$ cd lpi104-6/ ian@attic4:~/lpi104-6$ ln -s file1 file5 ian@attic4:~/lpi104-6$ cd .. ian@attic4:~$ # Create symlink across file systems ian@attic4:~$ mkdir ~ian/research/lpi104-6 ian@attic4:~$ ln -s ~/lpi104-6/file1 ~ian/research/lpi104-6/file4 ian@attic4:~$ # Create symlink for directory ian@attic4:~$ ln -s lpi104-6 lpidir104-6 |
As before, you can use any of the links or the target file name to reference the file or directory. Listing 4 shows some examples.
Listing 4. Using soft links
ian@attic4:~$ echo "another line" >> ~ian/research/lpi104-6/file ian@attic4:~$ # cat a symlink ian@attic4:~$ cat lpi104-6/file5 something else another line ian@attic4:~$ # cat a hard link ian@attic4:~$ cat lpi104-6/file1 something else another line ian@attic4:~$ # display directory contents using symlink ian@attic4:~$ ls lpidir104-6 file1 file2 file3 file4 file5 subdir |
While we're creating links, let's create a link using relative paths when our working directory is not the directory where we want the link. We'll look at what this does in the next section.
Listing 5. Creating a bad soft link
ian@attic4:~$ ln -s lpi104-6/file1 lpi104-6/file6 |
In the previous section, you saw how to create links, but not how to distinguish the links you created. Let's look at that now.
On many systems today, the ls command is aliased
to ls --color=auto, which prints different
types of file system objects in different colors. The colors are
configurable. If you use this option, hard links might show up with a dark
blue background, and symlinks with cyan text, as illustrated in Figure
1.
Figure 1. Using the --colors option of ls to identify links
While color might be convenient for sighted people who can distinguish
them, they are not much use to others, and certainly not much use to shell
scripts or programs. Without color, you need more information, such
as that provided by a long listing using ls -l.
In Listing 6 we explicitly disable color output, but you could also
explicitly call the /bin/ls command.
Listing 6. Identifying links
ian@attic4:~$ ls --color=none -lR lpi104-6 lpi104-6: total 12 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file1 -rw-r--r-- 1 ian ian 0 2010-05-26 14:11 file2 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file3 lrwxrwxrwx 1 ian ian 24 2010-05-27 17:15 file4 -> /home/ian/lpi104-6/file1 lrwxrwxrwx 1 ian ian 5 2010-05-27 17:15 file5 -> file1 lrwxrwxrwx 1 ian ian 14 2010-05-27 17:37 file6 -> lpi104-6/file1 drwxr-xr-x 2 ian ian 4096 2010-05-26 14:11 subdir lpi104-6/subdir: total 4 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file3sub ian@attic4:~$ /bin/ls -l ~ian/research/lpi104-6/file4 lrwxrwxrwx 1 ian ian 24 2010-05-25 11:51 /home/ian/research/lpi104-6/file4 -> /home/ian/ lpi104-6/file1 ian@attic4:~$ /bin/ls -l lpidir104-6 lrwxrwxrwx 1 ian ian 8 2010-05-27 17:16 lpidir104-6 -> lpi104-6 |
The second column of output is a link count showing the number of hard links to this file, so we know that file1, file3, and file3sub all have multiple hard links pointing to the object they represent, although we do not yet have enough information to know they all represent the same object. If you delete a file that has a link count greater than 1, the link count in the inode is reduced by 1, but the file is not deleted until the count goes to 0. All other hard links to the same file will show a link count that is now reduced by 1.
In the first column of output, you see the first character is an 'l' (lower-case L) for symbolic links. You also see the target of the link displayed after the -> characters. For example file4 -> /home/ian/lpi104-6/file1. Another tipoff is that the size is the number of characters in the link target's name. Note that the link counts in the directory listing are not updated for symbolic links. Deleting the link does not affect the target file. Symlinks do not prevent a file from being deleted; if the target file is moved or deleted, then the symlink will be broken. For this reason, many systems use colors in directory listings, often pale blue for a good link and red for a broken one.
You can use the -i option of the
ls command to display inode numbers for file
and directory entries. Listing 7 shows both short and long output for our
lpi104-6 directory.
Listing 7. Displaying inode information
ian@attic4:~$ ls -i lpi104-6 1680103 file1 1680103 file3 1680107 file5 1680101 subdir 1680104 file2 1680108 file4 1680110 file6 ian@attic4:~$ ls -il lpi104-6 total 12 1680103 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file1 1680104 -rw-r--r-- 1 ian ian 0 2010-05-26 14:11 file2 1680103 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file3 1680108 lrwxrwxrwx 1 ian ian 24 2010-05-27 17:15 file4 -> /home/ian/lpi104-6/file1 1680107 lrwxrwxrwx 1 ian ian 5 2010-05-27 17:15 file5 -> file1 1680110 lrwxrwxrwx 1 ian ian 14 2010-05-27 17:37 file6 -> lpi104-6/file1 1680101 drwxr-xr-x 2 ian ian 4096 2010-05-26 14:11 subdir |
You can also use the find command to search for
symbolic links using the -type l find
expression as shown in Listing 8.
Listing 8. Using find to locate symlinks
ian@attic4:~$ find lpi104-6 research/lpi104-6 -type l lpi104-6/file6 lpi104-6/file5 lpi104-6/file4 research/lpi104-6/file4 |
In Listing 5, we claimed to create a bad soft link. This is one example of a broken symlink. Since hard links always point to an inode that represents a file, they are always valid. However, symlinks can be broken for many reasons, including:
- Either the original file or the target of the link did not exist when the link was created (as in Listing 5).
- The target of a link is deleted or renamed.
- Some element in the path to the target is removed or renamed.
None of these conditions raises an error, so you need to think carefully about what might happen to your symlinks as you create them. In particular, your choice of absolute or relative paths is likely to be influenced by what you expect to happen to the objects you are linking over the life of the link.
If you are using colored output, broken symlinks are likely to show up as
red text on a black background, as is the case for file6 in Figure 1. Otherwise, you will need to use
either the -H or -L
options of ls to dereference the link and give
you information about the target. The -H option
dereferences links on the command line and the
-L option dereferences those plus links that
are part of the display. Listing 9 illustrates the difference in the
output from these two options.
Listing 9. Dereferencing links with ls -H and ls -L
ian@attic4:~$ /bin/ls -lH lpidir104-6 total 12 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file1 -rw-r--r-- 1 ian ian 0 2010-05-26 14:11 file2 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file3 lrwxrwxrwx 1 ian ian 24 2010-05-27 17:15 file4 -> /home/ian/lpi104-6/file1 lrwxrwxrwx 1 ian ian 5 2010-05-27 17:15 file5 -> file1 lrwxrwxrwx 1 ian ian 14 2010-05-27 17:37 file6 -> lpi104-6/file1 drwxr-xr-x 2 ian ian 4096 2010-05-26 14:11 subdir ian@attic4:~$ /bin/ls -lL lpidir104-6 /bin/ls: cannot access lpidir104-6/file6: No such file or directory total 20 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file1 -rw-r--r-- 1 ian ian 0 2010-05-26 14:11 file2 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file3 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file4 -rw-r--r-- 3 ian ian 28 2010-05-27 17:17 file5 l????????? ? ? ? ? ? file6 drwxr-xr-x 2 ian ian 4096 2010-05-26 14:11 subdir |
Note the error message indicating that file6 does not exist and also the output for it with all the '?' characters, again indicating that the file is not found.
One final point on our broken symbolic link. Attempts to read the file will fail as it does not exist. However, attempts to write it will work if you have the appropriate permission on the target file, as shown in Listing 10. Note that we need to create the lpi104-6/lpi104-6 before we can write the file.
Listing 10. Reading from and writing to a broken symlink
ian@attic4:~$ cat lpi104-6/file6
cat: lpi104-6/file6: No such file or directory
ian@attic4:~$ echo "Testing file6" > lpi104-6/file6
bash: lpi104-6/file6: No such file or directory
ian@attic4:~$ mkdir lpi104-6/lpi104-6
ian@attic4:~$ cat lpi104-6/file6
cat: lpi104-6/file6: No such file or directory
ian@attic4:~$ echo "Testing file6" > lpi104-6/file6
ian@attic4:~$ cat lpi104-6/file6
Testing file6
ian@attic4:~$ ls lpi104-6/lpi104-6
file1 |
To find which files are hard links to a particular inode, you can use the
find command and the
-samefile option with a filename or the
-inum option with an inode number, as shown in
Listing 11.
Listing 11. Finding hard links to the same file
ian@attic4:~$ find lpi104-6 -samefile lpi104-6/file1
lpi104-6/subdir/file3sub
lpi104-6/file3
lpi104-6/file1
ian@attic4:~$ ls -i lpi104-6/file1
1680103 lpi104-6/file1
ian@attic4:~$ find lpi104-6 -inum 1680103
lpi104-6/subdir/file3sub
lpi104-6/file3
lpi104-6/file1 |
To find which files link symbolically to a particular file, you can use the
find command and the
-lname option with a filename, as illustrated
in Listing 12. Links may use a relative or absolute path, so you probably
want a leading asterisk in the name to find all matches.
Listing 12. Finding symbolic links to a file or directory
ian@attic4:~$ find lpi104-6 research/lpi104-6 -lname "*file1" lpi104-6/file6 lpi104-6/file5 lpi104-6/file4 research/lpi104-6/file4 |
Depending on what you want to accomplish, sometimes you will use links and sometimes it may be better to make a copy of a file. The major difference is that links provide multiple names for a single file, while a copy creates two sets of identical data under two different names. You would certainly use copies for backup and also for test purposes where you want to try out a new program without putting your operational data at risk. You use links when you need an alias for a file (or directory), possibly to provide a more convenient or shorter path. In the next section, we'll look at some other uses for links.
As you have seen, when you update a file, all the links to it see the update, which is not the case if you copy a file. You have also seen that symbolic links can be broken but that subsequent write operations may create a new file. Use links with care.
Links and system administration
Links, especially symbolic links, are frequently used in Linux system administration. Commands are often aliased so the user does not have to know a version number for the current command, but can access other versions by longer names if necessary. As shown in Listing 13, the gcc command is a symlink and there are three different names for it on my system.
Listing 13. Aliasing commands to a particular version
ian@attic4:~$ which gcc /usr/bin/gcc ian@attic4:~$ ls -l /usr/bin/gcc lrwxrwxrwx 1 root root 7 2009-12-28 23:17 /usr/bin/gcc -> gcc-4.4 ian@attic4:~$ find /usr/bin -lname "*gcc-4.4" /usr/bin/x86_64-linux-gnu-gcc-4.4 /usr/bin/gcc /usr/bin/x86_64-linux-gnu-gcc |
Other uses come into play when multiple command names use the same underlying code,
such as the various commands for stopping and for restarting a system.
Sometimes, a new command name, such as genisofs, will replace an older
command name, but the old name (mkisofs) is kept as a link to the new
command. And the alternatives facility uses links extensively so you can
choose which among several alternatives to use for a command such as java.
Listing 14 shows some examples.
Listing 14. Command alias examples
ian@attic4:~$ find /sbin -lname "initctl" /sbin/restart /sbin/start /sbin/stop /sbin/status /sbin/reload ian@attic4:~$ ls -l $(which mkisofs) lrwxrwxrwx 1 root root 11 2009-12-28 23:17 /usr/bin/mkisofs -> genisoimage ian@attic4:~$ ls -l $(which java) lrwxrwxrwx 1 root root 22 2010-01-17 15:16 /usr/bin/java -> /etc/alternatives/java |
Library names are also managed extensively using symlinks, either to allow programs to link to a general name while getting the current version, or to manage systems such as 64-bit systems that are capable of running 32-bit programs. Some examples are shown in Listing 15. Notice that some use absolute paths, while some use relative paths.
Listing 15. Library links
ian@attic4:~$ ls -l /usr/lib/libm.so lrwxrwxrwx 1 root root 14 2010-05-27 11:23 /usr/lib/libm.so -> /lib/libm.so.6 ian@attic4:~$ find /usr/lib/ -lname "*libstdc++*" /usr/lib/gcc/x86_64-linux-gnu/4.4/libstdc++.so /usr/lib/libstdc++.so.6 ian@attic4:~$ ls -l /usr/lib/gcc/x86_64-linux-gnu/4.4/libstdc++.so lrwxrwxrwx 1 root root 23 2010-01-19 08:49 /usr/lib/gcc/x86_64-linux-gnu/4.4/libstdc++.s o -> ../../../libstdc++.so.6 |
For more information about linking, consult the man pages for
ln and the other commands you have seen in this
article.
Learn
- Use the developerWorks roadmap for
LPIC-1 to find the developerWorks articles to help you study for
LPIC-1 certification based on the April 2009 objectives.
- At the LPIC
Program site, find detailed objectives, task lists, and sample
questions for the three levels of the Linux Professional Institute's Linux
system administration certification. In particular, see their April 2009
objectives for LPI exam 101 and LPI exam 102. Always refer to the LPIC Program site for the
latest objectives.
- Review the entire LPI exam prep series on
developerWorks to learn Linux fundamentals and prepare for system
administrator certification based on earlier LPI exam objectives prior to
April 2009.
- For additional examples of how to use
symbolic links in Linux system administration, see:
- "Learn Linux, 101: Manage shared libraries" (developerWorks, March 2010)
- "Dissecting shared libraries" (developerWorks, January 2005)
- "Boot Linux faster" (developerWorks, September 2003)
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Ian Shields works on a multitude of Linux projects for the developerWorks Linux zone. He is a Senior Programmer at IBM at the Research Triangle Park, NC. He joined IBM in Canberra, Australia, as a Systems Engineer in 1973, and has since worked on communications systems and pervasive computing in Montreal, Canada, and RTP, NC. He has several patents and has published several papers. His undergraduate degree is in pure mathematics and philosophy from the Australian National University. He has an M.S. and Ph.D. in computer science from North Carolina State University. Learn more in Ian's profile on My developerWorks.
