[ubuntu] Changing user account passwords in bulk?

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http://ubuntuforums.org – We are supposed to change all useraccount passwords. We have set same username and password for all the 300 systems. We are supposed to change user passwords for every 15 days. Doing it manually eats more time and man power. I hope there is some way to do this via scripting either by bash or tcl. Is is possible? If yes can someone help me pls? (Hardware)