First, it's not really safe to store unencrypted passwords, which is why
most unix systems don't do it. What you are trying to do is definatly
possible (you'll find just about everything is with unix), but it would
take a fair amount of scripting. Why do you need such a script? I'd
suggest hiring a unix administrator before you wreck something, if this
is at all important.

this is not a production system .Its for training purpose and every couple of weeks new person use the machines ,and also he may return after a break .thats why i wanted a easier way instead of me doing manually everytime

I found a program on Freshmeat, turns out its a whole server/client
solution, and has a few options for the type of password.
http://www.adel.nursat.kz/apg/
I hope this will do it for you. There's also a Standalone version of
APG (automated password generator) and it prints the password to
standard output, which of course can be sent to any file using the ">>
file" shell option. For example, "apg -options... >> passwordfile"