file links

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

An average computer using Linux has a file system with many hard links and symbolic links.

To understand links in a file system, you first have to understand what an inode is.

inodes

inode contents

An inode is a data structure that contains metadata about a file. When the file system stores a new file on the hard disk, it stores not only the contents (data) of the file, but also extra properties like the name of the file, the creation date, its permissions, the owner of the file, and more. All this information (except the name of the file and the contents of the file) is stored in the inode of the file.

The ls -l command will display some of the inode contents, as seen in this screenshot.

root@linux ~# ls -ld /home/project42/
drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/

inode table

The inode table contains all of the inodes and is created when you create the file system (with mkfs). You can use the df -i command to see how many inodes are used and free on mounted file systems.

root@linux ~# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/mapper/VolGroup00-LogVol00
                     4947968  115326 4832642    3% /
/dev/hda1              26104      45   26059    1% /boot
tmpfs                  64417       1   64416    1% /dev/shm
/dev/sda1             262144    2207  259937    1% /home/project42
/dev/sdb1              74400    5519   68881    8% /home/project33
/dev/sdb5                  0       0       0    -  /home/sales
/dev/sdb6             100744      11  100733    1% /home/research

In the df -i screenshot above you can see the inode usage for several mounted file systems. You don\'t see numbers for /dev/sdb5 because it is a fat file system.

inode number

Each inode has a unique number (the inode number). You can see the inode numbers with the ls -li command.

student@linux:~/test$ touch file1
student@linux:~/test$ touch file2
student@linux:~/test$ touch file3
student@linux:~/test$ ls -li
total 12
817266 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file1
817267 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file2
817268 -rw-rw-r--  1 paul paul 0 Feb  5 15:38 file3
student@linux:~/test$

These three files were created one after the other and got three different inodes (the first column). All the information you see with this ls command resides in the inode, except for the filename (which is contained in the directory).

inode and file contents

Let\'s put some data in one of the files.

student@linux:~/test$ ls -li
total 16
817266 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file1
817270 -rw-rw-r--  1 paul paul 92 Feb  5 15:42 file2
817268 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file3
student@linux:~/test$ cat file2
It is winter now and it is very cold.
We do not like the cold, we prefer hot summer nights.
student@linux:~/test$

The data that is displayed by the cat command is not in the inode, but somewhere else on the disk. The inode contains a pointer to that data.

about directories

a directory is a table

A directory is a special kind of file that contains a table which maps filenames to inodes. Listing our current directory with ls -ali will display the contents of the directory file.

student@linux:~/test$ ls -ali
total 32
817262 drwxrwxr-x   2 paul paul 4096 Feb  5 15:42 .
800768 drwx------  16 paul paul 4096 Feb  5 15:42 ..
817266 -rw-rw-r--   1 paul paul    0 Feb  5 15:38 file1
817270 -rw-rw-r--   1 paul paul   92 Feb  5 15:42 file2
817268 -rw-rw-r--   1 paul paul    0 Feb  5 15:38 file3
student@linux:~/test$

. and ..

You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself, and the dotdot .. is a mapping to the parent directory. The three other names are mappings to different inodes.

hard links

creating hard links

When we create a hard link to a file with ln, an extra entry is added in the directory. A new file name is mapped to an existing inode.

student@linux:~/test$ ln file2 hardlink_to_file2
student@linux:~/test$ ls -li
total 24
817266 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file1
817270 -rw-rw-r--  2 paul paul 92 Feb  5 15:42 file2
817268 -rw-rw-r--  1 paul paul  0 Feb  5 15:38 file3
817270 -rw-rw-r--  2 paul paul 92 Feb  5 15:42 hardlink_to_file2
student@linux:~/test$

Both files have the same inode, so they will always have the same permissions and the same owner. Both files will have the same content. Actually, both files are equal now, meaning you can safely remove the original file, the hardlinked file will remain. The inode contains a counter, counting the number of hard links to itself. When the counter drops to zero, then the inode is emptied.

finding hard links

You can use the find command to look for files with a certain inode. The screenshot below shows how to search for all filenames that point to inode 817270. Remember that an inode number is unique to its partition.

student@linux:~/test$ find / -inum 817270 2> /dev/null
/home/paul/test/file2
/home/paul/test/hardlink_to_file2

symbolic links

Symbolic links (sometimes called soft links) do not link to inodes, but create a name to name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic link gets an inode of its own.

student@linux:~/test$ ln -s file2 symlink_to_file2
student@linux:~/test$ ls -li
total 32
817273 -rw-rw-r--  1 paul paul  13 Feb  5 17:06 file1
817270 -rw-rw-r--  2 paul paul 106 Feb  5 17:04 file2
817268 -rw-rw-r--  1 paul paul   0 Feb  5 15:38 file3
817270 -rw-rw-r--  2 paul paul 106 Feb  5 17:04 hardlink_to_file2
817267 lrwxrwxrwx  1 paul paul   5 Feb  5 16:55 symlink_to_file2 -> file2
student@linux:~/test$

Permissions on a symbolic link have no meaning, since the permissions of the target apply. Hard links are limited to their own partition (because they point to an inode), symbolic links can link anywhere (other file systems, even networked).

removing links

Links can be removed with rm.

student@linux:~$ touch data.txt
student@linux:~$ ln -s data.txt sl_data.txt
student@linux:~$ ln data.txt hl_data.txt
student@linux:~$ rm sl_data.txt 
student@linux:~$ rm hl_data.txt

practice : links

1. Create two files named winter.txt and summer.txt, put some text in them.

2. Create a hard link to winter.txt named hlwinter.txt.

3. Display the inode numbers of these three files, the hard links should have the same inode.

4. Use the find command to list the two hardlinked files

5. Everything about a file is in the inode, except two things : name them!

6. Create a symbolic link to summer.txt called slsummer.txt.

7. Find all files with inode number 2. What does this information tell you ?

8. Look at the directories /etc/init.d/ /etc/rc2.d/ /etc/rc3.d/ ... do you see the links ?

9. Look in /lib with ls -l...

10. Use find to look in your home directory for regular files that have more than one hard link (hint: this is identical to all regular files that do not have exactly one hard link).

solution : links