i want to write cron job for back the mails stored in /var/mail/username , thats for 100 users at time.
can anybody give me detail steps for it
You don't really write a cron job. You would write a script, and then have cron schedule it.
Here is a sample script for backing up mailbox(es).
You can read the Linux Shell Scripting Tutorial and then modify it to suit your needs.
When you are satisfied, you can use cron to schedule it.
My crime is that of curiosity. My crime is that of judging people by what they say and think, not what they look like. My crime is that of outsmarting you, something that you will never forgive.