RHCSA Rapid Track

Objectives

After completing this section, students should be able to use hard links and soft links to make multiple names point to the same file.

Managing links between files

Creating hard links

A hard link is a new directory entry with a reference to an existing file on the file system. Every file in a file system has one hard link by default. To save space, instead of copying, a new hard link can be created to reference the same file. A new hard link either needs to have a different file name, if it is created in the same directory as the existing hard link, or it needs to reside in a different directory. All hard links pointing to the same file have the same permissions, link count, user/group ownerships, time stamps, and file content. Hard links pointing to the same file content need to be on the same file system.

The ls -l shows the hard link count after the permissions and before the owner of a file.

[root@serverX ~]# echo "Hello World" > newfile.txt
[root@serverX ~]# ls -l newfile.txt
-rw-r--r--. 1 root root 0 Mar 11 19:19 newfile.txt

The command ln creates new hard links to existing files. The command expects an existing file as the first argument, followed by one or more additional hard links. The hard links can reside anywhere as long as they are on the same file system as the existing file. After a new hard link is created, there is no way to tell which of the existing hard links is the original one.

Create a hard link newfile-link2.txt for the existing file newfile.txt in the /tmp directory.

[root@serverX ~]# ln newfile.txt /tmp/newfile-hlink2.txt
[root@serverX ~]# ls -l newfile.txt /tmp/newfile-hlink2.txt
-rw-rw-r--. 2 root root 12 Mar 11 19:19 newfile.txt
-rw-rw-r--. 2 root root 12 Mar 11 19:19 newfile-hlink2.txt

Even if the original file gets deleted, the content of the file is still available as long as at least one hard link exists.

[root@serverX ~]# rm -f newfile.txt
[root@serverX ~]# ls -l /tmp/newfile-link2.txt
-rw-rw-r--. 1 root root 12 Mar 11 19:19 /tmp/newfile-link2.txt
[root@serverX ~]# cat /tmp/newfile-link2.txt
Hello World

Creating soft links

The ln -s command creates a soft link, which is also called a "symbolic link". A soft link is not a regular file, but a special type of file that points to an existing file or directory. Unlike hard links, soft links can point to a directory, and the target to which a soft link points can be on a different file system.

[root@serverX ~]# ln -s /root/newfile-link2.txt /tmp/newfile-symlink.txt
[root@serverX ~]# ls -l newfile-link2.txt /tmp/newfile-symlink.txt
lrwxrwxrwx. 1 root root 11 Mar 11 20:59 /tmp/newfile-symlink.txt -> /root/newfile-link2.txt
-rw-rw-r--. 1 root root 12 Mar 11 19:19 newfile-link2.txt

When the original file gets deleted, the soft link is still pointing to the file but the target is gone. A soft link pointing to a missing file is called a "dangling soft link."

[root@serverX ~]# rm -f newfile-link2.txt
[root@serverX ~]# ls -l /tmp/newfile-symlink.txt
lrwxrwxrwx. 1 root root 11 Mar 11 20:59 /tmp/newfile-symlink.txt -> newfile-link2.txt
[root@serverX ~]# cat /tmp/newfile-symlink.txt
cat: /tmp/newfile-symlink.txt: No such file or directory

A soft link can point to a directory. The soft link then acts like a directory. Changing to the soft link directory with cd works as expected.

Create a soft link /root/configfiles pointing to the /etc directory.

[root@serverX ~]# ln -s /etc /root/configfiles
[root@serverX ~]# cd /root/configfiles
[root@serverX configfiles]# pwd
/root/configfiles