Unix File System.

In this post we would concentrate on the following Unix File system commands.

1. LS
2. CAT
3. CD
4. MKDIR
5. RMDIR
6. TOUCH
7. DF
8. DU
9. PWD
10. Hard link and Soft link

1.LS:- Used to display files in unix.

• Command to displaty the hidden files in unix?(Asked in many interview Questions)

>ls -a

• Command to display all the files recursively in a directory?

>ls - R

• Command to display the long listing of all the files?(Asked in many interview Questions)

>ls -lt

where l=long listing

t=sort files according to modification time.

 brw-r--r--    1 unixguy staff 64,  64 Jan 27 05:52 block         
 crw-r--r--    1 unixguy staff 64, 255 Jan 26 13:57 character     
 -rw-r--r--    1 unixguy staff     290 Jan 26 14:08 compressed.gz 
 -rw-r--r--    1 unixguy staff  331836 Jan 26 14:06 data.ppm      
 drwxrwxr-x    2 unixguy staff      48 Jan 26 11:28 directory     
 -rwxrwxr-x    1 unixguy staff      29 Jan 26 14:03 executable    
 prw-r--r--    1 unixguy staff       0 Jan 26 11:50 fifo          
 srw-rw-rw-    1 unixguy staff       0 Jan 26 12:00 socket        
 lrwxrwxrwx    1 unixguy staff       3 Jan 26 11:44 link -> dir   
 -rw-rw----    1 unixguy staff     217 Jan 26 14:08 regularfile   

 where 1st field indicates file permission
2nd field indicates number of hard links
3rd fiels indicatesowner of the file
4th field indicates the size of the file
5th field indicates the modification time 
6th and the last field indicates the file name

2.CAT:- gives the content of the file.

>cat filename

3.CD:- change directory

>cd /tmp/unix/dir1

4.MKDIR:- make directory

>mkdir dir2

5.RMDIR:- remove directory

• Command to delete a directory and all the contents inside it?Directory has many sub directories?(Asked in many interview Questions)

>rmdir -R /tmp/unix/dir1

6.TOUCH:-used to create a zero byte file

>touch filename

7.DF( Disk free space):- Displays the amount of available freespace for filesystem.

df -k

where k=shows file size in 1024 bytes instead of 512 bytes

> df -k
 Filesystem    1024-blocks      Free %Used    Iused %Iused Mounted on
 /dev/hd4            32768     16016   52%     2271    14% /
 /dev/hd2          4587520   1889420   59%    37791     4% /usr
 /dev/hd9var         65536     12032   82%      518     4% /var
 /dev/hd3           819200    637832   23%     1829     1% /tmp
 /dev/hd1           524288    395848   25%      421     1% /home
 /proc                   -         -    -         -     -  /proc
 /dev/hd10opt        65536     26004   61%      654     4% /opt

8.DU(Disk Usage):-Gives the filespace usage

>du -sk

where k=shows file size in 1024 bytes instead of 512 bytes
s= report only the sum of the usage in the current directory, not for each file

 132456  Dir1

 178645  Dir2

9.Hard link and Soft link (Asked in many interview Questions)

Firstly what is a link in Unix?

A link in UNIX is a pointer to a file. Like pointers in C , links in UNIX are pointers pointing to a file.
 > cat file1
Unix

Creating Hard link:- ln file1 file2

Creating Soft link:- ln -s file1 file 3

After links are created

>cat file 2
Unix
>cat file 3
Unix

How to list the link files ?

$ ls -li 
total 20 
9962464 -rw-r--r-- 2 guru users 8 Mar  9 file1 
9962464 -rw-r--r-- 2 guru users 8 Mar  9 file2 
9962471 lrwxrwxrwx 1 guru users 5 Mar  9 file3 -> file1

If you see the idone number 9962464 is same for both the original file and the   hard link file.The number of links are also mentioned as two.

Where as for the soft link file -> indicates its a soft link to the original file file1.

The size of the soft link is the length of the filename of the original file.The original file is "file1" length of 5 hence the size of soft link is 5.


If the original file i.e file1 is deleted the soft link i.e file3 would not be   accessible but the hard link file2 would be accessible.