You should find the 'expect' command will do what you need it to do. Its widely available. See here for an example : http://www.thegeekstuff.com/2010/10/expect-examples/
(very rough example)
#!/usr/bin/expect
set pass "mysecret"
spawn /usr/bin/passwd
expect "password: "
send "$pass"
expect "password: "
send "$pass"