I want to generate random numbers from 0 to 20 in my program, is there any function in Linux which I can use?

Thanks

Here's a link to generate random integers within a bounds: http://www.linux.com/guides/abs-guide/randomvar.shtml

1. If you're programming in C/C++:

man 3 rand

2. If you're on Linux (regardless of language), consider reading from /dev/urandom. This will give you truly random numbers.

3. Regardless of how you generate the random (or pseudo-random) number, you'll usually constrain it to a specific range with the "modulus" operator

C/C++ EXAMPLES:
a) inum = rand () % 20
<= returns a number between 0 .. 19
b) inum = rand () % 21
<= Returns a number between 0 .. 20
c) inum = (rand () % 20) + 1
<= Returns a number between 1 .. 20

I want to generate Random nos, and not pseudo-random, in that case, i cannot use rand right?

Can you define what you really require?
For most tasks pseudo-random numbers are sufficient. Why do you need true random numbers? It can be done but its isn't easy.
What language are you going to use for this?

Reading /dev/random /dev/urandom gives you random bytes, using "noise" from device interrupts etc, hence close-to-true-random bytes. Using /dev/random your program may have to wait until sufficient "noise" is gatherd by the kernel, while /dev/urandom will use the noise if available, but will mever block if there's not sufficient "noise".

See:
man 4 random
or the online man page: http://linux.about.com/od/commands/l/blcmdl4_random.htm