echo thePassword | passwd theUsername --stdin

http://stackoverflow.com/questions/2150882/how-to-automatically-add-user-account-and-password-with-a-bash-script

例子：

# echo 123456 | passwd --stdin  jacky

批量添加：

for i in zhang3 li4 wang5 
  do
    useradd $i && echo 123456 | passwd --stdin $i 
  done 

echo password | passwd --stdin username