1
ULI101
Introduction to UNIX/Linux and the Internet
Filesystem Links
Seneca College of Applied Technology
©2005 Raymodn.chan for ULI101
2
What is a filesystem Link?
A link is a pointer to a file.
This pointer associates a file name with a number called an
i-node number
An i-node is the control structure for a file (on a
UNIX/Linux filesystem)
If two file names have the same i-node number, they are
links to the same file
©2005 Raymodn.chan for ULI101
3
Display a file's i-node number
●
Use the command “ls -i” to print i-node number
of each file:
[ray@localhost week8]$ ls -i
32764 lab3a.html
37745 lab3b.html
37740 lab3.zip
●
Use the command “ls -il” for long listing
[ray@localhost week8]$ ls -il
total 40
32764 -rw-r--r-- 1 ray ray 1097 Sep 13 08:53 lab3a.html
37745 -rw-r--r-- 1 ray ray 6582 Sep 13 08:53 lab3b.html
37740 -rw-rw-r-- 1 ray ray 6218 Sep 14 00:05 lab3.zip
i-node
©2005 Raymodn.chan for ULI101
4
Two kinds of links
Hard Links
and
Soft or Symbolic Links
©2005 Raymodn.chan for ULI101
5
Hard Links
●
Hard link is a reference to the physical data on a
filesystem
●
All named files are hard links
●
More than one name can be associated with the
same physical data
●
Hard link can only refer to data that exists on the
same file system
●
©2005 Raymodn.chan for ULI101
6
Create Hard Links
●
When you use “vi” to create a new file, you create
the first hard link (vi myfile)
●
You can also use the “touch” command to create
the first hard link (touch myfile)
●
To Create the 2nd, 3rd and etc. hard links, use the
command:
ln myfile link-name
©2005 Raymodn.chan for ULI101
7
Display Hard Links info
●
Create a new file called “myfile”
●
Run the command “ls -il” to display the i-node
number and link counter
38753 -rw-rw-r-^
|-- inode #
©2005 Raymodn.chan for ULI101
1 uli uli
29 Oct 29 08:47 myfile
^
|-- link counter (one link)
8
Display Hard Link Info
●
Create a 2nd link to the same data:
ln myfile mylink
●
Run the command “ls -il”:
38753 -rw-rw-r-- 2 uli
uli
29 Oct 29 08:47 myfile
38753 -rw-rw-r-- 2 uli
uli
29 Oct 29 08:47 mylink
^
|-- inode #
©2005 Raymodn.chan for ULI101
^
|--link counter (2 links)
9
rd
Add the 3 Link
●
Create a 3rd link to the same data:
ln myfile newlink
●
Run the command “ls -il”:
38753 -rw-rw-r-- 3 uli
uli
29 Oct 29 08:47 myfile
38753 -rw-rw-r-- 3 uli
uli
29 Oct 29 08:47 mylink
38753 -rw-rw-r-- 3 uli
uli
29 Oct 29 08:47 newlink
^
|-- inode #
©2005 Raymodn.chan for ULI101
^
|--link counter (3 links)
10
Removing a link
When a file has more than one link, you can
remove any one link and still be able to access the
file through the remaining links.
[uli@seneca links]
38753 -rw-rw-r-- 3
38753 -rw-rw-r-- 3
38753 -rw-rw-r-- 3
[uli@seneca links]
[uli@seneca links]
38753 -rw-rw-r-- 2
38753 -rw-rw-r-- 2
[uli@seneca links]
[uli@seneca links]
38753 -rw-rw-r-- 1
©2005 Raymodn.chan for ULI101
ls -li
uli uli
uli uli
uli uli
rm mylink
ls -li
uli uli
uli uli
rm myfile
ls -li
uli uli
29 Oct 29 08:47 myfile
29 Oct 29 08:47 mylink
29 Oct 29 08:47 newlink
29 Oct 29 08:47 myfile
29 Oct 29 08:47 newlink
29 Oct 29 08:47 newlink
11
Removing all links
●
Remove all the links
[uli@seneca links]
38753 -rw-rw-r-- 2
38753 -rw-rw-r-- 2
[uli@seneca links]
This is my file.
[uli@seneca links]
This is my file.
[uli@seneca links]
[uli@seneca links]
38753 -rw-rw-r-- 1
[uli@seneca links]
This is my file.
[uli@seneca links]
[uli@seneca links]
©2005 Raymodn.chan for ULI101
ls -li
uli uli
uli uli
cat myfile
29 Oct 29 08:47 myfile
29 Oct 29 08:47 newlink
cat newlink
rm myfile
ls -li
uli uli
29 Oct 29 08:47 newlink
cat newlink
rm newlink
ls -li
12
Advanced use of links
Directory
Cross-classify links
File
Courses
uli101
iso110
ica101
to_do
to_do
to_do
to_do
uli101
iso110
ica101
Links
©2005 Raymodn.chan for ULI101
13
Create Cross-classify files
[uli@seneca uli] cd courses
[uli@seneca courses] mkdir uli101 ios110 ica101
[uli@seneca courses] vi uli101/to_do ... Add to do list to each file
[uli@seneca courses] vi ios110/to_do ... under each course's
[uli@seneca courses] vi ica101/to_do ... directory
[uli@seneca courses] ls -liR
38767 drwxrwxr-x 2 uli uli
72 Oct 29 11:15 ica101
38768 drwxrwxr-x 2 uli uli
72 Oct 29 11:16 ios110
38762 drwxrwxr-x 2 uli uli
72 Oct 29 11:14 uli101
./ica101:
44309 -rw-rw-r-- 1 uli uli
45 Oct 29 11:15 to_do
./ios110:
44407 -rw-rw-r-- 1 uli uli
49 Oct 29 11:16 to_do
./uli101:
38772 -rw-rw-r-- 1 uli uli
51 Oct 29 11:14 to_do
©2005 Raymodn.chan for ULI101
14
Create Cross-classify links
[uli@seneca courses] ls -i
38767 ica101
38768 ios110
38762 uli101
[uli@seneca courses] mkdir to_do
38767 ica101
38768 ios110
38769 to_do
38762 uli101
Create links in the to_do directory to the
to_do file in each course's directory
[uli@seneca courses] ln ica101/to_do to_do/ica101
[uli@seneca courses] ln ios110/to_do to_do/ios110
[uli@seneca courses] ln uli101/to_do to_do/uli101
©2005 Raymodn.chan for ULI101
15
Display links to the same file
●
Pairs of files in different directories point to the
same physical file:
[uli@seneca courses] ls -li ica101/to_do to_do/ica101
44309 -rw-rw-r-- 2 uli uli
45 Oct 29 11:15 ica101/to_do
44309 -rw-rw-r-- 2 uli uli
45 Oct 29 11:15 to_do/ica101
[uli@seneca courses] ls -li ios110/to_do to_do/ios110
44407 -rw-rw-r-- 2 ray ray
49 Oct 29 11:16 ios110/to_do
44407 -rw-rw-r-- 2 ray ray
49 Oct 29 11:16 to_do/ios110
[uli@seneca courses] ls -li uli101/to_do to_do/uli101
38772 -rw-rw-r-- 2 ray ray
51 Oct 29 11:14 to_do/uli101
38772 -rw-rw-r-- 2 ray ray
51 Oct 29 11:14 uli101/to_do
©2005 Raymodn.chan for ULI101
16
View To_do list files
[uli@seneca courses] cat ica101/to_do
Test 3 - Hands-on Database test Nov 10, 2005
[uli@seneca courses] cat iso110/to_do
Shell Programming assignment due on Nov 14, 2005
[uli@seneca courses] cat uli101/to_do
assignment 1 due Nov 4, 2005
lab 8 due Nov 4, 2005
[uli@seneca courses] cat ica101/to_do ios110/to_do uli101/to_do
Test 3 - Hands-on Database test Nov 10, 2005
Shell Programming assignment due on Nov 14, 2005
assignment 1 due Nov 4, 2005
lab 8 due Nov 4, 2005
[uli@seneca courses] cat to_do/*
?? what would be the output ??
©2005 Raymodn.chan for ULI101
17
Recap
●
Use “ln” to create hard links
●
Files with the same i-node number refer to the
same physical file
●
Use “ls -i” to display the file's i-node number and
link counter
●
Use “rm filename” to remove a hard link
●
A physical file will be deleted from the file
system when its link counter drops to zero,
©2005 Raymodn.chan for ULI101
18
Symbolic Links
Also Know As (a.k.a.): Soft links or Symlinks
●
Symbolic link is an indirect pointer to a file – a
pointer to the hard link to the file
●
You can create a symbolic link to a directory
●
A symbolic link can point to a file on a different
filesystem
●
A symbolic link can point to a nonexistent file
©2005 Raymodn.chan for ULI101
19
Create Symbolic Links
●
To create a symboic link to the file “myfile”, use
ln -s myfile symlink
or
ln --symbolic myfile symlink
[uli@seneca courses] ls -li myfile
44418 -rw-rw-r-- 1 uli uli
49 Oct 29 14:33 myfile
[uli@seneca courses] ln -s myfile symlink
[uli@seneca courses] ls -li myfile symlink
44418 -rw-rw-r-- 1 uli uli 49 Oct 29 14:33 myfile
44410 lrwxrwxrwx 1 uli Link
uli counter
6 Oct
29 14:33 symlink -> myfile
(1 link)
File type – symboic link
Different i-node number
©2005 Raymodn.chan for ULI101
20
Properties of Symbolic Link files
●
The i-node number is different from the pointed to file
●
The link counter of the new symbolic link file is “1”
●
Symbolic link file does not affect the link counter of the
pointed to file
●
The type field of symblic file contains the letter “l”
●
The symbolic link file and the pointed to file have different
status information (e.g. file size, last modification time etc.)
©2005 Raymodn.chan for ULI101
21
Create Symbolic Link Directory
●
The syntax is the same as linking to a file
ln -s target_directory link_directory
ln --symbolic target_directory link_directory
[uli@seneca week8]$ ls -li
38766 drwxrwxr-x 7 uli uli
168 Oct 29 13:32 courses
[uli@seneca week8]$ ln courses mydir
ln: `courses': hard link not allowed for directory
[uli@seneca week8]$ ln -s courses mydir
[uli@seneca week8]$ ls -li
38766 drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses
44417 lrwxrwxrwx 1 uli uli
7 Oct 29 15:41 mydir ->
courses
©2005 Raymodn.chan for ULI101
22
Directory Listing
●
To display the contents in a directory, we normally
use the command “ls -l directory_name”
●
Compare the following to commands
[uli@seneca week8]$ ls -l mydir
lrwxrwxrwx 1 uli uli
7 Oct 29 15:41 mydir -> courses
[uli@seneca
drwxrwxr-x
drwxrwxr-x
drwxrwxr-x
drwxrwxr-x
©2005 Raymodn.chan for ULI101
week8]$ ls -l courses
2 uli uli
72 Oct 29 11:15
2 uli uli
72 Oct 29 11:16
2 uli uli
120 Oct 29 11:20
2 uli uli
72 Oct 29 11:14
ica101
ios110
to_do
uli101
23
Directory Contents of Symbolic Links
●
Must use the “-L” option in order to display
directory contents of a symbolic link
Compare the following commands:
[uli@seneca
lrwxrwxrwx
[uli@seneca
drwxrwxr-x
drwxrwxr-x
drwxrwxr-x
drwxrwxr-x
Directory
Status
week8]$ ls -l mydir
1 uli uli
7 Oct 29 15:41 mydir -> courses
week8]$ ls -lL mydir
2 uli uli
72 Oct 29 11:15 ica101
2 uli uli
72 Oct 29 11:16 ios110
2 uli uli
120 Oct 29 11:20 to_do
2 uli uli
72 Oct 29 11:14 uli101
Directory
Contents
©2005 Raymodn.chan for ULI101
24
Delete link to a directory
To delete a link to a directory, simply use the “rm”
command:
[uli@seneca week8]$ ls -l
drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses
lrwxrwxrwx 1 uli uli
7 Oct 29 15:41 mydir -> courses
[uli@seneca week8]$ rm mydir
[uli@seneca week8]$ ls -l
drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses
©2005 Raymodn.chan for ULI101
25
Properties of Symbolic Link Directory
●
The Symbolic link to a directory has a file type of
“l” (the first letter of the permission field).
●
The permissions on the link are set to “rwx” for
all.
●
chmod on the link applies to the actual directory
(or file), the permissions on the link stay the same
●
Can point to a nonexistent directory
©2005 Raymodn.chan for ULI101
26
Reference
●
Hard Links
–
●
Symbolic Links
–
●
http://en.wikipedia.org/wiki/Hard_link
http://en.wikipedia.org/wiki/Symbolic_link
I-nodes
–
http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs-7.html
–
http://en.wikipedia.org/wiki/Inode
©2005 Raymodn.chan for ULI101
27
Questions and Answers
●
What is a filesystem link?
●
What is an i-node?
●
How do you display the i-node of a file/directory?
●
How many kinds of filesystem links are there?
●
What is a hard link?
●
What is a soft link?
●
Can you create a hard link to a directory?
©2005 Raymodn.chan for ULI101
28
More Q & A
●
How do you create the first hard link to a file?
●
How do you create the 2nd (and up) hard link to a
file?
●
What is the purpose of a link counter?
●
Where can you get the link counter of a file?
●
How do you create a soft link to a file?
●
Does a soft link file has the same i-node number
©2005 Raymodn.chan for ULI101
29
More Q & A
●
How do you remove a hard link?
●
How do you delete a physical file with multiple
links?
●
How do you create a symbolic link to a file?
●
How do you create a symbolic link to a directory?
●
Could you create a symbolic to a file which does
not exist?
©2005 Raymodn.chan for ULI101
30
More Q & A
●
How do you display the contents of a regular
directory?
●
How do you display the contents of a symbolic
link to a regular directory?
●
How do you display the status information of a
regular directory?
●
More Q & A ?
©2005 Raymodn.chan for ULI101