Dear Friends ,

I have the following two queries :

1) what is the function of "/dev/null" in Linux . Which information contain this partition in Linux ? The same quetion also about '/dev/sda' or '/dev/sdb' ?

2) How can I measure which partition take space or allocate for the USB devices ,
/dev/sda1 or /dev/sdb1 ? I m confused about this

3)If i want to take whole backup of the Linux server , then is there any third party
tool for this purpose ? For example , in windows platform I can say , for whole
backup , a third party tool 'GHOST' is used for this purpose . Similarly is there
any tools like this for Linux ?

Your questions are unfortunately not very clear....

/dev/null is not a partition---it is a special device which can be thought of as "write only". Anything sent there is never seen again.
I don't know what you mean here...

Things like /dev/sda are not partitions, they are devices. In /dev you will find both character and block devices--If you do ls -l in /dev, you will see the difference in the first letter.

A USB device would not typically show up as /dev/sda1, etc.

The dd command is the low-level equivalent of ghost.

Quoth pixellany:

Not quite. /dev/null can also be read from, and returns an immediate end of file.